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O level Chemistry Questions and Answers(5)

Question 1

(a) A student prepared a blue solution, P, by adding black copper (II) oxide powder to nitric acid as shown below.’
Neutralisation

(i) Name two chemical substances in the blue solution.
(ii) State two observations made by the student.
(iii) Write a balanced chemical equation for the reaction.

(b) Write a balanced chemical equation for the reaction that will
occur when
(i) zinc granules are added to copper sulphate solution,
(ii) calcium carbonate is heated strongly.

Solution 1

(a)(i) Copper nitrate and water.
The reaction is a neutralisation reaction and as all neutralisation reactions go, the products are salt and water. Since the acid is nitric acid, the salt is a nitrate.

(ii) The black copper (ii) oxide dissolves and the solution turns from colourless to blue.

(iii) CuO(s) + 2HNO3 → Cu(NO3)2(aq) + H2O(l)

(b)(i) Zn(s) + CuSO4(aq) → Cu(s) + ZnSO4(aq)
This type of reaction occurs because zinc is more reactive than copper, so it displaces copper from a solution of its ions.

(ii) CaCO3(s) → CaO(s) + CO2
This is one of the reactions that occur in the blast furnace.



Question 2

(a) In a titration, 25 cm3 of aqueous ammonia required 21.50 cm3 of 0.1 mol dm -3 sulphuric acid for complete neutralisation. The equation for the reaction is:

H2SO4(aq) + 2NH4OH(aq) →(NH4)2SO4(aq) + 2H2O(aq)

(i) Calculate the number of moles of sulphuric acid in 21.50 cm3 of the solution.
(ii) Deduce the number of moles of ammonia in the 25 cm3
of ammonia.
(iii) Calculate the concentration of the ammonia solution.

(b)(i) Give one physical property in which the oxides, CO2 and SO2, are similar.
(ii) Name the industrial process by which CO2(g) is obtained
from air.

Solution 2

(a)(i)since the concentration of sulphuric acid is given in mol dm-3 we also convert the volume to dm3 for homogeneity of units.

Volume of sulphuric acid = 21.5 ÷ 1000
= 0.0215 dm2

Then Number of moles = concentration × volume
= 0.1 × 0.0215
= 0.00215 moles

(ii) according to the equation, one mole of sulphuric acid reacts with two moles of ammonium solution.

H2SO4 : NH4OH
= 1 : 2
= 0.00215 : x

x = 0.0043 moles

(iii) volume of ammonium solution = 25 ÷ 1000
= 0.025 dm3

concentration = number of moles ÷ volume
= 0.0043/0.025
= 0.172 mol dm-3



Question 3

(a) The table below shows some of the gaseous pollutants.
Complete the table by stating a use and an effect of the gases on
the environment.

gasuseeffect on the environment
CO2
SO2

(b) Explain why no chemical reaction takes place when

(i) aluminium powder that has been exposed to air is added to a solution of iron (II) sulphate,
(ii) magnesium powder is added to a solution of calcium chloride,
(iii) zinc oxide is heated in a stream of hydrogen,
(iv) carbon dioxide is bubbled through a solution of hydrochloric acid.

Solution 3

(a) Uses of CO2:

  • in fire extinguishers
  • as a refrigerant
  • in fizzy drinks.

Effects of carbon dioxide on the environment:

  • Global warming
  • Formation of acid rain

Uses of sulphur dioxide:

  • fumigation
  • food preservation
  • manufacture of sulphuric acid

Effects of sulphur dioxide on the environment:

  • Formation of acid rain

(b)(i) aluminium powder has a tough inert oxide layer that prevents it from reacting as it should.
If not for the inert oxide layer, aluminium should have displaced iron from iron (ii) sulphate because aluminium is more reactive than iron.

(ii) magnesium is less reactive than calcium, so it doesn’t displace calcium.

(iii) zinc is above hydrogen on the reactivity series so hydrogen cant reduce zinc oxide.

(iv) carbon dioxide is an acidic oxide so it doesn’t react with acids.

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