Chemistry, Third edition

Gravimetric analysis

When two solutions react together to form an insoluble product, the concentration

of one of the solutions can be worked out from the mass of precipitate formed in the

reaction. The process is called gravimetric analysis. The precipitate is filtered off

and dried thoroughly. The precipitate is heated and weighed several times until the

mass of the product does not change (heated to constant mass); this process

ensures that the insoluble product is thoroughly dried.

VOLUMETRIC ANALYSIS 145

Problems involving the reactions of solids with solutions

Exercise 9G

Example 9.7

Barium chloride solution reacts with the sulfate ion in aqueous

solution to form insoluble barium sulfate. The ionic equation for

the reaction is as follows:

Ba^2 (aq) + SO 42 (aq)BaSO 4 (s)

If excess barium chloride solution was added to a 250.0 cm^3

sample of an unknown solution to precipitate all the sulfate ions

present as barium sulfate, and 1.2210 g of dry BaSO 4

precipitated, what was the original concentration of sulfate ion

in the unknown sample?

Answer

M(BaSO 4 )137.327 + 32.066 + (4 15.999)233.389 g mol^1

(Do not use approximate atomic masses in calculations of gravimetric

determinations; use the atomic masses in the Periodic Table.)

amount of BaSO 4 1.2210/233.3895.2316 10 ^3 mol

Since 1 mol of BaSO 4 contains 1 mol SO 42 ions:

amount of SO 42 present5.2316 10 ^3 mol

This amount of SO 42 was originally present in 250.0 cm^3 solution, therefore the

concentration of SO 42 ions in original solution is

5.2316 10 ^3  4 0.020 93 mol dm^3

(i) What is the volume of sulfuric acid of concentration

1.50 mol dm^3 which would be needed to react with

7.95 g of copper(II) oxide?

H 2 SO 4 (aq) + CuO(s) CuSO 4 (aq) + H 2 O(I)

(ii) Magnesium hydroxide is a compound used to neutralize

stomach acid (dilute hydrochloric acid) and so cure acid

indigestion. The reaction is

Mg(OH) 2 (s) + 2HCl(aq) MgCl 2 (aq) + 2H 2 O(I)

The magnesium hydroxide in an indigestion tablet was

found to react with 20 cm^3 of 0.50 mol dm^3 HCl. How

many grams of Mg(OH) 2 were in the tablet?

(iii)Carbon dioxide gas is prepared by the action of

2.00 mol dm^3 hydrochloric acid on marble:

CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + H 2 O(I) + CO 2 (g)

What volume of acid is required to give 10.0 dm^3 of

carbon dioxide at room temperature and pressure?

(iv)What mass of solid copper(II) carbonate is needed to

react completely with 50 cm^3 of sulfuric acid of

concentration 0.50 mol dm^3?