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QUANTITATIVE CHEMICAL ANALYSIS CHEM 3334

1

https://www.youtube.com/watch?v=MHhxyfgHZ9o&list=PLLilrKdbjXDEgE97_TnAZJ0-XZrLlO1Pn

Chapter 1-2

Measurements Tools of the Trade

2

Background

1.) Definition:

ANALYTICAL CHEMISTRY: The Science of Chemical Measurements.

2.) Types of Questions Asked in Analytical Chemistry a.) What is in the sample? (qualitative analysis)

b.) How much is in the sample? (quantitative analysis)

3.) Techniques used in Analytical Chemistry: a.) Wet Chemical Methods: titrations, color-forming reactions,

precipitations, etc. b.) Instrumental Methods: spectrometry, chromatography, etc.

Introduction to Analytical Chemistry

3

The Analytical Process

1.) Formulating the Question:

Translate General Question into Specific Question Is this water safe to Drink?  What is the concentration of Arsenic in the water sample?

2.) Selecting Analytical Procedures: a.) Choose procedure to measure Arsenic in water

(i) Uncertainty in measurement (ii) Limit of detection (iii) Destroy sample (iv) Availability, time, cost

b.) If necessary, develop new procedure

3.) Sampling: a.) Select representative material to analyze

(i) don’t use the entire sample (ii) consistency in sample collection

Source Caffeine (mgs per serving

Serving size (oz)

Regular coffee 106-164 5

Decaffeinated coffee

2-5 5

Tea 21-50 5

Cocoa beverage 2-8 6

Baking chocolate

35 1

Sweet chocolate 20 1

Milk chocolate 6 1

soft drinks 36-57 12

The Analytical Process

4.) Sample Preparation: a.) convert sample into form suitable for chemical analysis

(i) Dissolve sample (ii) Concentrate sample

(iii) Remove species that interfere with analysis

Introduction to Analytical Chemistry

Types of Questions Asked in Analytical Chemistry “In Bangladesh, 15–25% of the population is exposed to unsafe levels of arsenic

in drinking water from aquifers in contact with arsenic-containing minerals. The analytical problem is to reliably and cheaply identify wells in which arsenic is above 50 parts per billion (ppb). Arsenic at this level causes vascular and skin diseases and cancer.”

“Too much caffeine is harmful for many people, and even small amounts cannot be tolerated by some unlucky individuals. How much caffeine is in a chocolate bar? How does that amount compare with the quantity in coffee or soft drinks?

At Bates College in Maine, Professor Tom Wenzel teaches his students chemical problem solving through questions such as these.”

6

Units and Concentrations

Units of Measurement 1.) SI Units:

a.) international units of measurement (metric units) b.) ALL SI units are based on certain fundamental quantities

c.) To indicate multiples or fractions of units, various prefixes are used conversions to SI units

f.) Liter is commonly used for volume instead of m3

Example: 3.2x10-8 s = 32 x10-9 s = 32 ns

Quantity Unit (Symbol)

Length Meter (m)

Mass Kilogram (kg)

Time Second (s)

Electric current Ampere (A)

Temperature Kelvin (K)

Luminous intensity

Candela (cd)

Amount of substance

Mole (mol)

Plane angle Radian (rad)

Solid angle Steradian (sr)

To a large extent, analytical chemistry is a science of measurement and measurements require

minimizing errors

Prefix Symbol Factor

Mega M 106

Kilo k 103

Hecto h 102

Deca da 101

Deci d 10-1

Centi c 10-2

Milli m 10-3

Micro m 10-6

Nano n 10-9

Pico p 10-12

Femto f 10-15

Atto a 10-18 7

Angstrom 1 Å = 10-10

Chemical Concentrations:

Mole: Avogadro’s number of particles

Atomic mass: number of grams of an element containing Avogadro’s number of atoms

Molecular mass: sum of atomic masses of atoms in a molecule; number of grams of an element containing Avogadro’s number of molecules

Formula mass: “molecular mass” of a strong electrolyte Concentration: quantity per unit volume (or mass)

“how much solute is contained in a given volume or mass of solution or solvent”

Molarity (M): number of moles of a substance per liter of solution (mol/L)

Molality (m): number of moles of a substance per kilogram of solvent (mol/kg)

Find the concentration in Molarity (M) of 12.00g of benzene (C6H6) dissolved up to a total volume of 250.00 ml in hexane.

MW benzene = 6 * (12.011) + 6 * (1.008) = 78.114 g/mol

Conc. C6H6 = = 0.6144 M

No. C’s at. wt. C No. H’s at. wt. H

0.2500L

) 78.114g

1mol (12.00g)(

Make Sure Units Cancel!

mol = wt (g) M =

volume (L) mw (g/mol) x volume (L)

Try example page 12 9

Molarity (moles/L, or M): (i) Most common unit of concentration

 Gives number of moles of a substance in 1 liter of the given solvent.  Recall: 1 mole (mol) of a substance = 6.022 x 1023 units (atoms,

molecules, ions, etc).

Formality (F):

Formal concentration (F): The molarity of a substance if it were not converted into other species in solution

Total number of moles in solution, regardless of any reactions that take place once dissolved in solution

(i) Concentrations expressed in M describe the actual concentration of a given chemical species in solution.

(ii) Some chemicals when placed in solution will dissociate or converte to multiple forms  Example:

(iii) Not convenient to refer to the concentrations of each individual form. (iv) Instead, concentration of total substance originally added to the solution is

used.  Formal concentration or Formality given in (mol/L)  Note: For compounds with a single form in solution, M = F

Acetic Acid:

10

Molality => m

# moles A molality => m = -------------------------

# kilograms solvent

• this concentration unit is temperature independent as the mass does not change with temperature whereas volume does

• used in freezing point depression/boiling point elevation

11

Percent Composition: (i) Weight Percent (wt/wt or w/w): Concentration expressed in terms of mass of

substance versus the total mass of the sample.

5 g / 100 g = 5%

(ii) Volume Percent (vol/vol or v/v): Concentration expressed in terms of volume of substance versus the total volume of the sample.

5 ml / 100 ml = 5%

(iii) Weight-Volume Percent (wt/vol or w/v): Concentration expressed in terms of mass of substance versus the total volume of the sample.

)100(x sampletotalorsolutiontotalofmass

cetansubsofmass percentWeight 

)100(x sampletotalorsolutiontotalofvolume

cetansubsofvolume percentVolume 

)100(x sampletotalorsolutiontotalofvolume

cetansubsofmass percentvolumeweight 

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Example Converting Weight Percent into Molarity

Find the molarity of 37.0 wt% HCl. The density of the reagent is 1.19 g/mL.

( density of a substance is the mass per unit volume, table inside the back cover of this book has density)

The density of a liter of solution is (1.19 g/ml )(1 000ml/L ) = 1.19 × 103 g/L.

- 37.0 wt% HCl  :

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mol = wt A (g) M =

volume (L) mw A (g/mol) x volume (L)

solutiong

gHCL

solutiong

gHCL

.

37.0

.100

37 

Hydrochloric acid, pure, fuming, 37 wt% solution in water,

ACROS Organics™ On the bottle

Converting Weight Percent into Molarity

14

Solution Preparation: - Dilution of a Solution: Review from general chemistry

McVc = MdVd

where: Mc = Molarity of substance in the concentrated solution Vc = volume of concentrated solution used Md = desired Molarity of the diluted solution Vd = total volume of final diluted solution

Please read section 1-3 Preparing Solutions and practice problems book and end of this chapter PP slide 22-29

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Example pg15

A solution of ammonia in water is called ”ammonium hydroxide” because of the equilibrium. The density of concentrated ammonium hydroxide, which contains 28.0 wt% NH3, is 0.899 g/mL. What volume of this reagent should be diluted to 500.0 mL to make 0.250 M NH3?

Solution To use Equation McVc = MdVd, we need to know the molarity of the concentrated reagent.

(28.0 wt%)= . d = 0.899 g/mL =the solution contains 0.899 g of solution per milliliter

Now we find the volume of 14.8 M NH3 required to prepare 500.0 mL of 0.250 M NH3:

The procedure is to place 8.45 mL of concentrated reagent in a 500-mL volumetric flask, add about 400 mL of water, and swirl to mix. Then dilute to exactly 500 mL with water and invert the flask many times to mix well.

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gsolution

gNH 328.0

Percent Composition: Instead of expressing concentrations as a percentage, express in terms of:

parts per thousand (ppt) 1/103 =1 x10-3 g/mL

parts per million (ppm) 1/ 106 =1 x10-6 g/mL Parts per billion (ppb) 1/ 109 =1 x10-9 g/mL

which mean grams of substance per million or billion grams of total solution or mixture.

Because the density of a dilute aqueous solution is close to 1.00 g/mL, we frequently equate 1 g of water with 1 mL of water, although this equivalence is only approximate.

1 ppm corresponds to 1 x10-6 g/mL= 1 μg/mL = (= 1 mg/L)

1 ppb is 1 x10-9 g/mL= 1 ng/mL= (= 1 μg/L).

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L

1000ml

mL

9g-10 X

L

1000ml

mL

g 6-10 X

Molarity to ppm Convert molar concentration to grams per liter (Molarity x Atomic mass of solute), then convert to milligrams per liter (ppm) by multiplying by 1000.

- Example Converting ppb( Parts per Billion) into Molarity

Normal alkanes are hydrocarbons with the formula CnH2n+2. Plants selectively synthesize alkanes with an odd number of carbon atoms. The concentration of C29H60 in summer rainwater collected in Hannover, Germany, is 34 ppb. Find the molarity of C29H60 and express the answer with a prefix from Table 1-3.

Solution : A concentration of 34 ppb  means there are 34 ng of C29H60 per gram of

rainwater, a value that we equate to

(Multiplying nanograms and milliliters by 1 000 gives 34 μg of C29H60 per liter of rainwater. (MW C29H60)= 408.8 g/mol,

1 ppb is 1 x10-9 g/mL= 1 ng/mL= (= 1 μg/L).

:

An appropriate prefix from Table 1-3 would be nano (n), which is a multiple of 10−9

18

L

g 6-10 x 34

mL

g 9-10 x 34

mL

ng 34 

L

1000ml

mL

9g- x101 X

= 34 μg/L

Examples

What is the maximum volume of 0.25M sodium hypochlorite solution (NaOCl, laundry bleach) that can be prepared by dilution of 1.00 L of 0.80 M NaOCl?

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Percent Composition:

1--Weight Percent (wt/wt or w/w): Concentration expressed in terms of mass of substance versus the total mass of the sample.

2- Instead of expressing concentrations as a percentage, express in terms of: parts per thousand (ppt) 1/103 =1 x10-3 g/mL

parts per million (ppm) 1/ 106 =1 x10-6 g/mL Parts per billion (ppb) 1/ 109 =1 x10-9 g/mL

Formality (F): (i) Concentrations expressed in M describe the actual concentration of a given chemical species in solution.

Expressions of concentration Molarity (moles/L, or M):

(i) Most common unit of concentration Gives number of moles of a substance in 1 liter of the given solvent.

Summary

Molarity to ppm Convert molar concentration to grams per liter (Molarity x Atomic mass of solute), then convert to milligrams per liter (ppm) by multiplying by 1000.

21 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Section 1-3 Preparing Solutions

22 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Preparing a Solution with a Desired Molarity

• Use pure solid or liquid. • Weigh out the correct mass of reagent. • Dissolve in volumetric flask using distilled or deionized water.

Distillation • Water is boiled to separate it from less volatile impurities. • Vapor is condensed and collected in a clean container.

Deionization • Water is passed through a column that removes ionic

impurities. • Nonionic impurities remain in the water.

Figure 1-4

23 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Example: Preparing a Solution with a Desired Molarity (1 of 3)

Copper(II) sulfate pentahydrate, CuSO4 · 5H2O, has 5 moles of H2O for each mole of CuSO4 in the solid crystal. The formula mass of CuSO4 · 5H2O (= CuSO9H10) is 249.68 g/mol. (Copper(II) sulfate without water in the crystal has the formula CuSO4 and is said to be anhydrous.) How many grams of CuSO4 · 5H2O should be dissolved in a volume of 500.0 mL to make 8.00 mM Cu2+?

24 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Example: Preparing a Solution with a Desired Molarity (2 of 3)

Solution: An 8.00 mM solution contains 8.00 × 10−3 mol/L. We need

3 3 4 2

mol 8.00 10 0.500 0 L 4.00 10 mol CuSO 5H O

L      

The mass of reagent is 3(4.00 10 mol g

) 249.68 mol

 0.999 g.     

Using a volumetric flask: The procedure is to place 0.999 g of solid CuSO4 · 5H2O into a 500-mL volumetric flask, add about 400 mL of distilled water, and swirl to dissolve the reagent. Then dilute with distilled water up to the 500-mL mark and invert the flask many times to ensure complete mixing.

25 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Example: Preparing a Solution with a Desired Molarity (3 of 3) Test Yourself: Find the formula mass of anhydrous CuSO4. How many grams should be dissolved in 250.0 mL to make a 16.0 mM solution?

26 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Preparing a Solution by Dilution

To prepare a solution of low concentration from a more concentrated solution: • Transfer a calculated volume of concentrated solution to volumetric flask. • Dilute to final volume.

The number of moles taken from the concentrated solution determines the number of moles delivered to the dilute solution.

𝐜𝐨𝐧𝐜 · 𝐜𝐨𝐧𝐜 = 𝐝𝐢𝐥𝐮𝐭𝐞 · 𝐝𝐢𝐥𝐮𝐭𝐞 **You can use any units for concentration or volume**

as long as you use the same units on both sides.

27 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Example: Preparing 0.100 M HCl (1 of 3)

The molarity of “concentrated” HCl purchased for laboratory use is approximately 12.1 M. How many milliliters of this reagent should be diluted to 1.000 L to make 0.100 M HCl?

28 Quantitative Chemical Analysis, Daniel C. Harris and Charles A. Lucy, © 2020 W. H. Freeman and Company

Example: Preparing 0.100 M HCl (2 of 3)

Solution: The dilution formula tells us how many mL to withdraw from the concentrated solution to obtain 0.100 mol HCl:

conc conc dil dilM M (12.1 M)( mL) (0.100 M)(1 000 mL) 8.26 m

L

V V x x

     

To make 0.100 M HCl, place about 900 mL of water in a 1-L volumetric flask, add 8.26 mL of concentrated HCl, and swirl to mix. Then dilute to 1.000 L with water and invert many times to mix well. The concentration will not be exactly 0.100 M because the reagent is not exactly 12.1 M.

The table after the periodic table at the front of the textbook lists volumes of common reagents required to make 1.0 M solutions.

Test Yourself: How many mL of 15.8 M nitric acid should be diluted to 0.250 L to make 3.00 M HNO3?

Chapter 2

Tools of the Trade

29

https://www.youtube.com/watch?v=rCiQQ5YSuN0

Laboratory Notebook Objectives of a Good Lab Notebook (a) State what was done (b) State what was observed (c) Be easily understandable to someone

else

 Include Complete Description of Experiment:

 Purpose  Methods  Results

 Include Balanced Chemical Equations for Every Reaction Used

 Paste Hardcopies of Important Data in Notebook

 Include locations Where other Data is stored (computer files)

 Notebooks are Legal Documents and Routinely Used for Patent

Litigation

 Laboratory Notebook should be bound (not spiral), use one with grids

instead of lined pages for graphs. 30

31

http://www.youtube.com/watch?v=xrgmrqRFZ8k&amp Please watch before going to the lab –next week

Methods of Weighing: Basic operational rules

 Chemicals should never be placed directly on the weighing pan - corrode and damage the pan may affect accuracy - not able to recover all of the sample

Weight by difference:  Useful for samples that change weight upon exposure to the atmosphere

- hygroscopic samples (readily absorb water from the air)

Weight of sample = ( weight of sample + weight of container)initial – (weight of container)final

Taring:

 Done on many modern electronic balances  Container is set on balance before sample is added  Container’s weight is set automatically to read “0”

Errors in Weighing: Sources

(i) Any factor that will change the apparent mass of the sample  Dirty or moist sample container

- also may contaminate sample  Sample not at room temperature

- avoid convection air currents (push/lift pan)  Adsorption of water, etc. from air by sample  Vibrations or wind currents around balance  Non-level balance

Tolerance (mg) Tolerance (mg)

Grams Class 1 Class 2 Milligrams Class 1 Class 2

500 1.2 2.5 500 0.010 0.025

200 0.5 1.0 200 0.010 0.025

100 0.25 0.50 100 0.010 0.025

50 0.12 0.25 50 0.010 0.014

20 0.074 0.10 20 0.010 0.014

10 0.050 0.074 10 0.010 0.014

5 0.034 0.054 5 0.010 0.014

2 0.034 0.054 2 0.010 0.014

1 0.034 0.054 1 0.010 0.014

?

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(allowable deviations, Uncertainty / error)

Errors in Weighing: Sources

- Any factor that will change the apparent mass of the sample We know that the weight of an object in air is much different than the weight in water, due to

buoyancy (Archimedes Principle)

In very accurate weight measurements, the buoyant force of air must also be accounted for

 Buoyancy errors – failure to correct for weight difference due to displacement of air by the sample.

 Correction for buoyancy to give true mass of sample

Different displacement of ice and wood in water

ice

wood

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- Any factor that will change the apparent mass of the sample

 Buoyancy errors – failure to correct for weight difference due to displacement of air by the sample.

 Correction for buoyancy to give true mass of sample

m = true mass of sample m’ = mass read from balance d = density of sample da = density of air (0.0012 g/ml at 1 atm &

25oC) dw = density of calibration weights (~ 8.0 g/ml)

) d d

1(

) d d

1('m

m a

w

a

 

Figure 2-5 shows buoyancy corrections for several substances. When you weigh water with a density of 1.00 g/mL, the true mass is 1.001 1 g when the balance reads 1.000 0 g. The error is 0.11%. For NaCl with a density of 2.16 g/mL, the error is 0.04%; and for AgNO3 with a density of 4.45 g/mL, the error is only 0.01%.

Errors in Weighing: Sources

34

Example Buoyancy Correction A pure compound called “tris” is used as a primary standard to measure

concentrations of acids. The volume of acid required to react with a known mass of tris tells us the concentration of the acid. Find the true mass of tris (density = 1.33 g/mL) if the apparent mass weighed in air is 100.00 g.

Solution Assuming that the balance weights have a density of 8.0 g/mL and the density of air is 0.001 2 g/mL, we find the true mass by using Equation 2-1:

:

Unless we correct for buoyancy, we would think that the mass of tris is 0.08% less than the actual mass and we would think that the molarity of acid reacting with the tris is 0.08% less than the actual molarity.

)1(

)1('

d d d d

m

m a

w

a

 

m’ = mass read from balance d = density of sample da = density of air (0.0012 g/ml at 1 atm

& 25oC) dw = density of calibration weights (~ 8.0 g/ml)

35

Preventing Weighing Errors

-Use a paper towel or tissue to handle the vessel you are weighing.

-Samples should be at ambient temperature (the temperature of the surroundings) to prevent errors due to air currents.

-A sample that has been dried in an oven takes about 30 min to cool to room temperature.

-Place the sample in a desiccator during cooling to prevent accumulation of moisture.

- Close the glass doors of the balance .

36

http://www.youtube.com/watch?v=3wOPthcJkRg&feature=related

Please watch before going to the lab –next week

Volume Measurements Burets

(i) Purpose: used to deliver multiple aliquots of a liquid in known volumes

(ii) Correct use of buret  Read buret at the bottom of a concave meniscus

Buret volume (ml) Smallest graduation (ml) Tolerance (mL)

5 0.01 ± 0.01

10 0.05 or 0.02 ± 0.02

25 0.1 ± 0.03

50 0.1 ± 0.05

100 0.2 ± 0.10

Meniscus at 9.68 mL

32.50ml 32.45- 32.55ml

37

(allowable deviations/error)

Volume Measurements

1- Burets (iii) always read the buret at the same eyelevel as the liquid

 Avoids parallax errors  The error that occurs when your eye is not at the same height as the

liquid is called parallax

(iv) Consistently read all levels versus a given position on the nearest mark When reading a buret, it is important that your line of sight be in a direction perpendicular

to the buret column.

15.46 mL 15.31 mL 1% error

eyelevel View from above

38

http://www.stthomas.edu/chemistry/Videos.html

MUST watch

How to Read and Use a Buret

A constant dark reflection against a white background enables higher precision in determining relative titrant volumes.

Read volume associated with bottom of “meniscus”.

A 50 mL buret can be read to ±0.01 ml.

Buret reading tips: 1. Allow time for draining.

2. Read the bottom of the concave meniscus.

3. Avoid parallax. 4. Account for the thickness of the marking

lines in your readings

Upper limit of the black streak ought to be placed just under the meniscus, so that the

bottom of the meniscus can be seen distinctly against a narrow zone of white.

39

How to Read and Use a Buret

•A bubble in the nozzle of a buret will produce an inaccurate volume reading if the bubble escapes during a titration

•The quickest way to get rid of bubbles is to fill the buret with titrant and open the valve.

•Some bubbles may require “light” tapping to dislodge them.

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-Estimate the buret reading to the nearest 1/10 of a division - expel all air bubbles from the stopcock prior to use - rinse the buret with a solution 2-3x before filling the buret for a titration

-Near the end of a titration, volume of 1 drop or less per delivery should be used with the buret.

Volume Measurements

2.) Volumetric Flasks (i) Purpose: used to prepare a solution of a single known volume (ii) Types of volumetric flasks

Flask Capacity (mL) Tolerance (mL)

1 ± 0.02

2 ± 0.02

5 ± 0.02

10 ± 0.02

25 ± 0.03

50 ± 0.05

100 ± 0.08

200 ± 0.10

250 ± 0.12

500 ± 0.20

1000 ± 0.30

2000 ± 0.50 41

(allowable deviations)

2.) Volumetric Flasks (iii) Correct use of volumetric flask

 Add reagent or solution to flask and dissolve in volume of solvent less than the total capacity of the flask

 Slowly add more solvent until the meniscus bottom is level with the calibration line.

 Stopper the flask and mix solution by inversion (40 or more times)  (for later use) Remix by inverting the flask if the solution has been sitting

unused for more than several hours  Glass adsorbs trace amount of chemicalsclean using acid wash

- adhere to surface

stopper

42

3.) Pipets and Syringes (i) Use to deliver a given volume of liquid (ii) Types of pipets

 Transfer pipet - similar to volumetric flask - transfers a single volume  fill to calibration mark - last drop does not drain out of the pipet  do not blow out - more accurate than measuring pipet

 Measuring pipet - calibrated similar to buret -

Transfer Pipet

Volume (mL) Tolerance (mL)

0.5 ±0.006

1 ±0.006

2 ±0.006

3 ±0.01

4 ±0.01

5 ±0.01

10 ±0.02

15 ±0.03

20 ±0.03

25 ±0.03

50 ±0.05

100 ±0.08

Figure 2-11 (a) Transfer pipet and (b) measuring (Mohr) pipet.

43

use to delivery a variable volume

http://www.youtube.com/watch?v=X2qsFNVrVLM&feature=BFa&list=SP8AFE4671EB3EE233&lf=list_related

Please watch before going to the lab –next week

Uncertainty

Filtration

1.) Mechanical separation of a liquid from the undissolved particles floating in it.

2.) Purpose: used in gravimetric analysis for analysis of a substance by the mass of a precipitate it produces (i) Solid collected in paper or fritted-glass filters

3.) Process: (i) pour slurry of precipitate down a glass rod to

prevent splattering. (ii) dislodge solid from beaker/rod with rubber

policeman (iii) use wash liquid (squirt bottle) to transfer particles to

filter paper (iv) dry sample

Rubber policeman

44

Drying

1.) Purpose: (i) to remove moisture from reagents or samples (ii) to convert sample to a more readily analyzable form

2.) Oven Drying: commonly used for reagent or sample preparation (i) Typically 110 oC for H2O removal (ii) Use loose covers to prevent contamination from dust

3.) Dessicator: used to cool and store reagent or sample over long periods of time.

(i) Contains a drying agent to absorb water from the atmosphere (ii) Airtight seal

45

Drying

46

Calibration of Volumetric Glassware

All instruments have a scale of some sort to measure a quantity such as mass, volume, force, or electric current.

Calibration is the process of measuring the actual quantity that corresponds to an indicated quantity on the scale of the instrument.

Manufacturers certify that the indicated quantity lies within a certain tolerance. o Class A transfer pipets are certified to deliver 10.00  0.02 mL when

used properly.

A specific pipet might average better. o Deliver 10.016  0.004 mL when used properly.

For greatest accuracy, we calibrate volumetric glassware to measure volumes associated with a particular piece of equipment.

Calibration of Volumetric Glassware ( first week lab)

- Class A transfer pipet is certified to deliver 10.00 ± 0.02 mL when you use it properly.

- Measure volume that is actually contained in or delivered by a particular piece of equipment.

- Measure mass of water contained in or delivered, then convert mass into volume

True volume = grams of water x volume of water (at the given temperature, table 2-7)

48

Is : the process of measuring the actual physical quantity (such as mass, volume, force, or electric current) corresponding to an indicated quantity on the scale of an instrument.

Transfer Pipet

Volume (mL) Tolerance (mL)

0.5 ±0.006

1 ±0.006

2 ±0.006

3 ±0.01

4 ±0.01

5 ±0.01

10 ±0.02

15 ±0.03

20 ±0.03

25 ±0.03

50 ±0.05

100 ±0.08

Calibration of Volumetric Glassware

True volume= grams of water x volume of water (at the given temperature, table 2-7)

Example :

An empty weighing bottle had a mass of 10.313 g. After adding water from a 25-ml pipet, the mass was 35.225 g. If the lab temp was 27 ºC, find the true volume that was delivered by the pipet?

Mass of water= 35.225-10.313 = 24.912 g

From eq 2.3 and table 2-7 (temp 27 ºC)

True volume= grams of water x volume of water in table 2-7

= 24.912 g x (1.0046 ml/g) = 25.027 ml at 27 ºC

49

• Calibration of Volumetric Glassware ( first week lab)

A- a 25-ml and or 10ml/5ml pipet

B- 5ml increment from 50ml Buret •

50

Trial

Mass of both Flask and Water

(g)

Mas s of Flas k (g)

Mass of Water

Delivere d (g)

Buoyan cy

Correcte d Mass

of Water (g)

True Volume of Water Delivere

d

1 66.003 41.0

53 24.95 24.9761 25.0635

2 66.001 41.0

54 24.947 24.9731 25.0605 3

Mean: Standard Deviation: Relative Standard Deviation:

Initial Buret Read ing

Diffe renc e in Read ings

Mass of Wate r and Flask (g)

Mas s of Wat er (g)

Buoya ncy

Correc ted

Mass of

Water (g)

True Volume of Total Water in Flask at T=23°C

True Volume of Water Delivere d Between Intervals (ml)

Corr ectio n

0 0 41.02

8 0 0 0 0 0

5.1 5.1 46.03

4 5.00

6 5.011 5.029 5.029 0.071

10.0 4.9 51.05

1 10.0

23 10.033 10.069 5.040 -

0.140 15.0 5.0 20.0 5.0 25.0 5.0 30.0 5.0 35.0 5.0 40.0 5.0 45.0 5.0 50.0 5.0

Total 50.0 mean (ml) stdev

True volume= grams of water x volume of water (at the given temperature, table 2-7)

Example Buret Calibration page 38 When draining the buret at 24°C, you observe the following values:

To calculate the actual volume delivered when 9.984 g of water is delivered at 24°C, look at the column of Table 2-7 headed “Corrected to 20°C.” In the row for 24°C, you find that 1.000 0 g of water occupies 1.003 8 mL. Therefore, 9.984 g occupies (9.984 g)(1.003 8 mL/g) = 10.02 mL. The average correction for both sets of data is +0.045 mL.

To obtain the correction for a volume greater than 10 mL, add successive masses of water collected in the flask. Suppose that the following masses were measured: The total volume of water delivered is (29.890 g)(1.003 8 mL/g) = 30.00 mL. Because the indicated volume is 30.03 mL, the buret correction at 30 mL is 20.03 mL. What does this mean? Suppose that Figure 3-3 applies to your buret. If you begin a titration at 0.04 mL and end at 29.00 mL, you would deliver 28.96 mL if the buret were perfect. Figure 3-3 tells you that the buret delivers 0.03 mL less than the indicated amount, so only 28.93 mL were actually delivered.

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= 10.02 – 9.98 =0.04

Practice before coming to the lab next week

Actual V – buret reading

Please read Sec 2-10 Introduction to Microsoft Excel - practice learn how to use

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