Acids, bases and salts III
Francine Taylor-Campbell, Contributor

In this week's lesson, we will be examining a few questions on the topic and assessing the answers given. Comments will be provided on certain aspects of the questions.

Students sew during a clothing and textile class at Ascot High School, recently. - Anthony Minott/Freelance Photographer

Q1. You are asked to prepare a pure dry sample of hydrated copper (II) sulphate starting with copper (II) oxide and dilute sulphuric acid.

(a) Describe how you would prepare this sample.

(b) Write an equation for the reaction occurring.

(c) Write the chemical formula for hydrated copper (II) sulphate crystals.

(d) A sample of the hydrated copper (II) sulphate crystals was heated and changed to a white solid P and a neutral liquid Q was collected in a tube, cooled by cold water. Name the solid P and the liquid Q.

(e) On stronger heating, the white solid P changed to a black solid R and a gas was given off, which dissolved in liquid Q, to form a solution S, which turned indicator paper red. Addition of aqueous barium chloride to S produced a white precipitate

(i) Name the precipitate.

(ii) Write an equation for the formation of R and identify R and S.

ANSWERS

Q1. To prepare a dry sample of hydrated copper (II) sulphate, excess copper (II) oxide would be added to dilute sulphuric acid and the solution heated. When no more copper (II) oxide is dissolved, the solution is filtered hot and the filtrate is evaporated to half of its volume and left to cool in an evaporating dish until crystals appear. These crystals are filtered, washed with a little cold water and dried between filter papers.

Comment: The salt solution is not heated to dryness as you are asked to prepare a hydrated salt, that is, it has water of crystalisation in its structure.

(b) CuO (s) + H2SO4 (aq) ====== CuSO4 (s) + H2O

(c) The formula of hydrated copper (II) sulphate is CuSO4.5H2O

(d) When hydrated copper II sulphate is heated, it loses its water of crystallisation and forms the white anhydrous salt. The white solid P is anhydrous copper II sulphate, and the neutral liquid is water.

Comment: CuSO4.5H2O (s) ======= CuSO4 (s) + 5H2O (l)

blue===== white

(e) When heated strongly, copper II sulphate (anhydrous) decomposes

CuSO4 (s) ====== CuO (s) + SO3 (g)

white===== black

to form copper II oxide and produce a sulphur trioxide gas. This gas is acidic and will dissolve in water to produce sulphuric acid (H2SO4). So R is CuO and S is H2SO4. When barium ions are added to a solution containing sulphate ions, a precipitation reaction occurs and a white precipitate barium sulphate is formed. Ba2+ (aq) + SO42- (aq) ===== BaSO4 (s)

Comment: Remember this method is a way of producing insoluble salts.

Q2. Sodium phosphate is a soluble salt used as a water softener in washing powders. It is made by reacting dilute phosphoric acid with an alkali.

(a) Give the formula of the ions present in sodium phosphate.

(b) Name the alkali which reacts with phosphoric acid to make sodium phosphate.

(ii) Write an equation for this reaction.

(c) Given solutions of phosphoric acid and alkali, a suitable indicator and standard laboratory apparatus, explain how you would obtain crystals of sodium phosphate.

(d) Sodium carbonate also reacts with phosphoric acid to make sodium phosphate. Name the gas formed in this reaction and describe a test for this gas.

(e) Calcium phosphate is used to make fertilisers. Deduce the formula of calcium phosphate. What essential plant element does calcium phosphate provide?

ANSWERS

(a) The ions present are sodium ions (Na+) and phosphate ions (PO43-)

(b) The alkali is sodium hydroxide

3NaOH (aq) + H3PO4 (aq) ===== Na3PO4 (aq) + 3H2O (l)

alkali acid salt (sodium phosphate) water

(c) To produce sodium phosphate from the solutions given, the method of titrating an acid and an alkali must be used. A known volume of the alkali NaOH is pipetted into a flask and titrated against H3PO4 using the indicator given. The volume of the acid required to reach the end point is noted. The reaction is then repeated without the use of the indicator and the required volume of acid is added from a burette to the NaOH solution. The neutral solution is then heated to remove water and a small volume is left to cool for crystalisation. Crystals formed are filtered, washed and dried using filter papers.

(d) Carbon dioxide gas is produced whenever a carbonate reacts with an acid. 3Na2CO3 (s) + 2H3PO4 (aq) == 2Na3PO4 (aq) + CO2 (g) + 3H2O (l)

The test for CO2 is to pass the gas through lime water (calcium hydroxide) and a milky solution is produced due to the formation of calcium carbonate.

(e) Ca3(PO4)2 —— calcium phosphate provides the essential plant element phosphorous (P).

Francine Taylor-Campbell is an independent contributor.