4.3.3 Gravimetric Analysis

Question 1

Gravimetric Analysis

Answer 1

• Gravimetric analysisis a technique for quantifying an unknown substance by converting the substance to a known product that can be isolated and weighed.

Question 2

precipitation

Answer 2

• Gravimetric analysis is a technique for quantifying an unknown substance by converting the substance to a known product that can be isolated and weighed.
• Precipitation reactions are often used in gravimetric analysis.
• For example, an unknown metalchloride can be identified using gravimetric analysis. The chloride ion can be precipitated with silver ions. This precipitate can then be dried and weighed, and the mass can be used to determine the moles of chloride ions present. This can be used to determine the moles of unknown. This data can be used, for example, to determine the molar mass of the unknown.

Question 3

example

Answer 3

• First, the mass of silver chloride is converted to moles of silver chloride. This is equal to the moles of chloride ion. Multiplying by the molar mass of chloride ion yields the mass of chloride in the unknown sample.
• Next, the percent by mass of chloride ion in the unknown is determined. The mass of chloride ion is divided by the mass of the compound, and multiplied by 100%.
• The percent by massof chloride ion in the original unknown is equal to the mass of chloride ion divided by the total mass of the compound. The total mass of the compound is the mass of chloride ion plus the mass of the unknown metal cation. Substituting the molar mass of chloride ion into this equation allows for the determination of the molar mass of the metal cation.
• The molar mass of the metal cation is found to be 39.1 g/mol. By referencing a periodic table, the metal cation is identified as potassium ion. The unknown compound is potassium chloride, a table salt substitute.

Question 4

Suppose you want to use gravimetric analysis to determine the concentration of iron(III) ions in a solution. Which of these steps would not eliminate a source of error in the analysis?

Answer 4

measuring the solution’s volume after the precipitation reaction, rather than before

Question 5

Suppose you have a solution of NaI. When you add an excess of Pb(NO3 )2, 3.50 g of PbI2 precipitate. Which sequence of steps should you follow if you want to determine the mass of NaI in the original solution?

2NaI(aq) + Pb(NO3 )2(aq) → PbI2(s) + 2NaNO3(aq)

Answer 5

(mass of PbI2 ) / (molar mass PbI2 ) → (moles of PbI2 ) (2) → (moles of NaI) (molar mass NaI) → mass of NaI

Question 6

The chloride of a metal cation has the formula MCl2, where M 2+ is the cation. Using gravimetric analysis, you determine that the compound is 52.0% chlorine by mass. What is the identity of the cation?

Answer 6

Zn2+

Question 7

A sample of water contaminated with lead nitrate (Pb(NO3 )2 ) has a volume of 30.7 mL. When an excess of sodium sulfate is added to the water, 0.0479 g of lead sulfate (PbSO4 ) precipitate. What was the original molar concentration of lead(II) ions (Pb2+ ) in the sample?

Answer 7

0.00515 M

Question 8

Suppose you have 24.6 mL sample of a potassium sulfate (K2SO4 ) solution. When you add an excess of barium chloride to the solution, 3.02 g of barium sulfate (BaSO4 ) precipitate. What was the original molar concentration of potassium ions (K+ ) in the sample?

Answer 8

1.05 M

Question 9

A sample of a barium halide has a mass of 2.34 g. When the sample is treated with an excess of sulfuric acid, 1.84 g of barium sulfate (BaSO4 ) precipitate. What is the identity of the halide in the original barium compound?

Answer 9

bromine

Question 10

An impure sample of potassium chloride (KCl) has a mass of 5.41 g. The sample is dissolved in water, and an excess of silver nitrate is added. If 2.12 g of silver chloride (AgCl) precipitate, what is the percentage of purity of the KCl sample? (Assume that all of the chlorine in the precipitate comes from KCl.)

Answer 10

20.3%

Question 11

Suppose you have 42.3 mL of a lead acetate solution. When you add an excess of potassium chromate, 3.27 g of lead chromate precipitate from the solution. What other information would you need if you wanted to determine the concentration of acetate ions (CH3COO− ) in the original solution?

Answer 11

the oxidation state of the lead ions

Question 12

A sample of a pure compound of iron and chlorine has a mass of 0.128 g. When the sample is dissolved in water and mixed with an excess of silver nitrate, 0.339 g of silver chloride (AgCl) precipitate. What is the chemical formula of the iron and chlorine compound?

Answer 12

FeCl3

Question 13

A 2.16 g sample of a silver alloy is dissolved in nitric acid. (An alloy is a solid solution of metals.) When an excess of sodium chloride is added to the solution, 1.42 g of AgCl precipitate. What is the percent by mass of silver in the alloy? (Assume that all the silver in the sample precipitates as AgCl.)

Answer 13

49.5%