CHEMISTRY 59-320 ANALYTICAL CHEMISTRY

Fall - 2010

Lecture 10 Chapter 27: Gravimetric and combustion analysis

Midterm exam

• Time: Oct. 21 between 8:30 and 9:40 AM.

• Location: BB121 or CHS 51, depending on your surname (A – J write in room CHS51) • The exam covers chapters 0-7 and chapter 27.

• It consists of (1) multiple choice (~ 25%), (2) short answers and analysis (~15%), and (3) calculations (~60%).

Gravimetric Analysis

• Gravimetric Analysis – one of the most accurate and precise methods of macro-quantitative analysis.

• Analyte selectively converted to an insoluble form.

• Measurement of mass of material • Correlate with chemical composition • Why choose this technique?

-- Simple -- Often required for high precision

How to Perform a Successful Gravimetric Analysis •

What steps are needed?

1. Preparation of the solution 2. Precipitation 3. Digestion 4. Filtration 5. Washing 6. Drying or igniting 7. Weighing 8. Calculation

How to Perform a Successful Gravimetric Analysis • Precipitation Techniques – Add precipitating reagent to sample solution – Reacts with analyte to form insoluble material – Precipitate has known composition or can be converted to known composition • Precipitate handling involves – Quantitative collection (no losses) – Isolation of pure product • Measure mass of precipitate (Direct or By difference) • Calculation of original analyte content (concentration). It requires the knowledge of Chemistry, Stoichiometry, and Composition of precipitate

How to Perform a Successful Gravimetric Analysis • Desirable properties of analytical precipitates: – Readily filtered and purified – Low solubility, preventing losses during filtration and washing – Stable final form (unreactive) – Known composition after drying or ignition

Filterability of Precipitates

• Colloidal suspensions – 10 -7 to 10 -4 cm diameter – Normally remain suspended – Very difficult to filter • Crystalline suspensions – > tenths of mm diameter – Normally settle out spontaneously – Readily filterable

27-1 An example of gravimetric analysis

• The determination of Cl Ag + by precipitating with : A 10.00 ml solution containing Cl was treated with excess AgNO 3 to precipitate 0.4368 g of AgCl. What was the molarity of Cl in the unknown. • Solution: The formula mass of AgCl is 143.321

0.4368 g/ (143.321 g/mol) = 3.048 x 10 -3 mol Based on 1:1 stoichiometric relationship: [Cl ] = 3.048 x 10 -3 mol /(0.01000 L) = 0.3048 M

27.2 Precipitation

The ideal product of a gravimetric analysis should be: very pure insoluble easily filterable known composite

Nucleation

: molecules in solution come together randomly and form small aggregates.

Supersaturated

: a solution contains more Solute than should be present at equilibrium.

• Techniques that promote particle growth: -- raising the temperature to increase solubility and thereby decrease supersaturation.

-- Adding precipitant slowly with vigorous mixing -- Using a large volume of solution so that the concentrations of analyte and precipitant low • Homogeneous precipitation • Precipitation in the presence of electrolyte.

27.3 Examples of Gravimetric calculations

• Problem 27-12: Marie Cure dissolved 0.09192 g of RaCl 2 treated it with excess AgNO 3 and to precipitate 0.08890 g of AgCl. In her time (1900), the atomic mass of Ag was known to be 107.8 and that of Cl was 35.4. From these values, find the atomic mass of Ra that Marie Curie would have calculated.

• Answer: The formula mass of AgCl is (35.4 + 107.8) = 143.2 0.08890 g/(143.2 g/mol) = 6.2081 x10 -4 mol.

Cl is 6.2081 x10 -4 mol RaCl 2 is (6.2081 x10 -4 )/2 mol the formula mass of RaCl 2 is 0.09192/(3.10405x10

-4 ) = 296.1292

the atomic mass of Ra = 296.1292 – 2x35.4 = 225.329

= 225.3

Example: A problem with two components

• Problem 27-22:

27.4 Combustion analysis

Combustion analysis (2)

Thermal analysis

Lab 2:Gravimetric determination of calcium

• Prep time for Experiment 2 will begin the

week of October 11, 2010

Experiment 2.

experiment.

.

• For students in Lab Sections • For students in Lab Sections

53 - 60

, please attend on your regularly scheduled lab day between 12:30 p.m. to 4:30 p.m. during the week of October 11th to prep for • Your GA will be in the lab during these hours to assist and answer any questions you may have regarding the

51

and 1:30 p.m. to prep for Experiment 2.

52

, please attend Friday, October 15th between the hours of 9:30 a.m. to • If you have a problem attending the prep session on your regularly scheduled lab day during the hours indicated, please contact your GA for alternative arrangements.

A few tips for Lab Preparation

• Steps that have great impacts on your final accuracy.

• Label your glassware in a way that your partner can easily recognize, such as 52 – 2 -1 ; 52-2-2 … (e.g. section # -- group # -- crucible #).

• Standard deviation and Q test?

• Discuss with your partner. In case you have a busy week, one can come in to clean and place the three crucibles into the oven, while the other weights the crucibles an hour late.

Data analysis

• (H • Ca 2 N) 2+ 2 CO + 3H + C 2 O 4 2 2 O + heat + H 2 O ⇌ ⇌ CaC CO 2 2 O 4 • The dried precipitate is CaC 2 O 4 mass of 128 + 2NH 4 + · H 2 O + 2OH with a formula • # mol CaC 2 O 4 = ?? g/(128 g/mol) = yy mol • # mol CaC 2 O 4 = # mol CaO • Mass of CaO = yy mol x 56.0790 g mol -1 = yy2 g • % calcium = yy2/?? =