8/2/2019 Ex6. Acids and Bases (Ans)

1/3

Level

S4

Chemistry

Exercise 6

Acids and Bases

Name: ( )

Class:

Date:

(Suggested Answers)

1. (a) Add dilute hydrochloric acid to both solids.Effervescence occurs in zinc carbonate. A gas that turns lime water milky is given off.

Zinc oxide just dissolves in acid.

(b) Add dilute sodium hydroxide solution / ammonia solution to the solutions.

Magnesium sulphate gives a white precipitate.

There is no observable change in sodium sulphate.

(c) Warm the solutions with dilute sodium hydroxide separately.

A pungent gas which turns moist red litmus paper blue is given off in ammonium

chloride.

There is no observable change in sodium chloride.

2. (a) It is harmful to aquatic life if it is discharged into the sea.(b) Ca(OH)2(s) + 2HCl(aq) ----------> CaCl2(aq) + 2H2O(l)

(c) No. of moles of HCl = 0.5 20 = 10 mol

No. of moles of Ca(OH)2 = 10 / 2 = 5 mol

Mass of Ca(OH)2 = 5 (40+2+162) = 370 g

(d) The completion of neutralization can be indicated by the end of effervescence when

using sodium carbonate.Sodium carbonate is more soluble, thus faster rate of reaction.

3. (a) CuCO3(s) + H2SO4(aq) ----------> CuSO4(aq) + H2O(l) + CO2(g)(b) It is to ensure all the acid has been reacted. Otherwise, the acid left will contaminate the

crystal prepared.

(c)

(d) No. of moles of H2SO4 = 2 5010-3

= 0.1 mol

No. of moles of CuSO45H2O = 0.1 mol

Mass of CuSO45H2O = 0.1 (63.4+32.1+164+185) = 24.96 g

8/2/2019 Ex6. Acids and Bases (Ans)

2/3

4. (a) sodium hydrogencarbonate(b) pH = -log[H

+]

0.9 = -log[H+]

[H+] = 0.126 M

(c) NaHCO3(s) + HCl(aq) ----------> NaCl(aq) + H2O(l) + CO2(g)

(d) Belch is due to the accumulation of carbon dioxide formed from the reaction in

stomach.

5. (a) A weak acid is an acid that undergoes partial dissociation in water.(b) pH = -log[H

+]

1.41 = -log[H+]

[H+] = 0.039 M

(c) pH = -log[H+]

= -log (0.12)

= 0.7

(d) Any one of the following:

Measure the pH using universal indicator / pH meter.The pH of sulphuric acid is lower than that of the sulphurous acid.

This shows that sulphuric acid has a higher concentration of hydrogen ions.

Measure the electrical conductivity.The light bulb using sulphuric acid as electrolyte is brighter than that of sulphurous

acid.

This shows that sulphuric acid has a higher concentration of mobile ions, hencehigher electrical conductivity.

Test with magnesium / zinc / sodium carbonate.The rate of effervescence in sulphuric acid is faster than that in sulphurous acid.

This shows that sulphuric acid has a higher concentration of hydrogen ions.

6. (a) CaCO3(s) + 2HNO3(aq) ----------> Ca(NO3)2(aq) + H2O(l) + CO2(g)When the effervescence stops, excess acid has been added.

/ Test with a piece of pH paper. If the pH is lower than 7, excess acid has been added.

(b) Ca2+

(aq) + SO42-

(aq) ----------> CaSO4(s)

(c) No. of moles of CaSO4 =

. = 0.0773 mol

No. of moles of CaCO3 = 0.0773 mol

Mass of CaCO3 = 0.0773 (40+12+163) = 7.73 g

% by mass of CaCO3 =

= 96.87%

(d) The sample does not contain ions which form insoluble sulphate.

Calcium carbonate is the only compound in the sample containing calcium ions.

There is no loss of calcium ions during the experiment.

8/2/2019 Ex6. Acids and Bases (Ans)

3/3

7. (a) CuO(s) + H2SO4(aq) ----------> CuSO4(aq) + H2O(l)(b) It is to ensure all the acid has been reacted. Otherwise, the acid left will contaminate the

crystal prepared.

(c) During cooling, the solubility of copper(II) sulphate decreases with the drop in

temperature. The solute separates out as a crystal.

(d) Anhydrous copper(II) sulphate will be resulted.

It should be dried with blotting paper. / by putting it in a desiccator.

(e) (i) No. of moles of copper(II) sulphate = No. of moles of H2SO4 used

=1 150 10-3

= 0.15 mol

(ii) No. of moles of copper(II) sulphate crystal =

.

= 0.065 mol

(iii) The answer in (ii) should be smaller than that in (i).

It is because there was some copper(II) sulphate dissolved in the solution and did

not crystallize out. There was a loss of copper(II) sulphate during filtration.