Chemical Equations — Practice

Question 1

Why should we balance a chemical equation?

Accepted Answer

Because of the law of conservation of mass. The total mass of the products must equal the total mass of the reactants consumed, and since the atom accounts for the mass of a substance, the number of atoms of each element must be the same before and after the reaction. Atoms are neither created nor destroyed in a chemical reaction.

Question 2

Balance the equation NaOH + H2SO4 -> Na2SO4 + H2O.

Accepted Answer

2NaOH + H2SO4 -> Na2SO4 + 2H2O. Sodium is 2 and 2, hydrogen is 4 and 4, sulphur is 1 and 1, oxygen is 6 and 6.

Question 3

Balance the equation KClO3 -> KCl + O2.

Accepted Answer

2KClO3 -> 2KCl + 3O2. Potassium is 2 and 2, chlorine is 2 and 2, oxygen is 6 and 6.

Question 4

What information do you get from a balanced chemical equation?

Accepted Answer

It identifies the reactants and products through symbols and formulae; it gives the ratio of molecules of reactants and products; it gives their relative masses in unified mass units; expressed in grams it gives the molar ratios; for gases it allows volumes to be calculated using the molar mass and molar volume relationship; and with molar mass and Avogadro's number it gives the number of molecules and atoms. Together these yield mass-mass, mass-volume, volume-volume and mass-volume-number relationships.

Question 5

Write the symbolic representation and balance: Calcium hydroxide (s) + Nitric acid (aq) gives Water (l) + Calcium Nitrate (aq).

Accepted Answer

Ca(OH)2(s) + 2HNO3(aq) -> 2H2O(l) + Ca(NO3)2(aq). Calcium is 1 and 1, nitrogen is 2 and 2, hydrogen is 4 and 4, and oxygen is 8 and 8.

Question 6

Write with physical states and balance: Barium chloride reacts with sodium sulphate solution to leave barium sulphate as a precipitate and form sodium chloride solution.

Accepted Answer

BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq). The barium sulphate is the precipitate, so it may also be marked with a downward arrow.

Question 7

Mention the physical states and balance: Na + H2O -> NaOH + H2.

Accepted Answer

2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g). The hydrogen is evolved as a gas, so it may carry an upward arrow.

Question 8

Two moles of zinc react with a cupric chloride solution containing 6.023 x 10^22 formula units of CuCl2. Calculate the moles of copper obtained. Zn(s) + CuCl2(aq) -> ZnCl2(aq) + Cu(s)

Accepted Answer

6.023 x 10^22 formula units is 6.023 x 10^22 / 6.023 x 10^23 = 0.1 mole of CuCl2. The balanced equation is already 1:1, so 1 mole of Zn needs 1 mole of CuCl2. Two moles of zinc would need 2 moles of CuCl2, but only 0.1 mole is present, so CuCl2 is the limiting reagent. The copper obtained is therefore 0.1 mole.

Question 9

One mole of propane on combustion at STP gives A kilojoules of heat energy. Calculate the heat liberated when 2.4 litres of propane is burnt at STP.

Accepted Answer

At STP one mole of any gas occupies 22.4 litres. So 2.4 litres is 2.4/22.4 = 0.1071 mole of propane, approximately. Since one mole liberates A kJ, the heat liberated is 0.1071 x A kJ, or (2.4A)/22.4 kJ, which is about 0.107A kJ.

Question 10

Calculate the mass and volume of oxygen required at STP to convert 2.4 kg of graphite into carbon dioxide.

Accepted Answer

The balanced equation is C(s) + O2(g) -> CO2(g). Taking C = 12 U and O = 16 U, 12 g of carbon requires 32 g of oxygen. For 2400 g of carbon, the oxygen required is (2400 x 32)/12 = 6400 g, which is 6.4 kg. For the volume, 32 g of oxygen is one gram molar mass and occupies 22.4 litres at STP, so 6400 g occupies (6400 x 22.4)/32 = 4480 litres.

Practice — Chemical Equations

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