1. 1
   A pharmacist weighs out a sample of pure sodium chloride, NaCl (relative formula mass 58.5), for use in preparing a saline solution.

   (a)

   What number of particles is contained in 1 mole of any substance?

   [1 mark]

   • A$6.02 \times 10^{23}$
   • B$1.00 \times 10^{23}$
   • C$6.02 \times 10^{20}$
   • D$3.01 \times 10^{23}$

   (b)

   The pharmacist weighs out 58.5 g of sodium chloride, given that its relative formula mass is 58.5. Using the relationship amount (mol) = mass (g) / molar mass (g/mol), how many moles of sodium chloride has she weighed out?

   [1 mark]

   • A0.5 mol
   • B58.5 mol
   • C1.0 mol
   • D2.0 mol

   (c)

   Calculate the number of formula units of sodium chloride present in the pharmacist's 58.5 g sample, using the Avogadro constant, showing your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A gas supplier fills a cylinder with a known volume of pure oxygen gas at room temperature and pressure (r.t.p.), using the molar gas volume of 24 dm3 at r.t.p.

   (a)

   What volume would 2 moles of oxygen gas occupy at r.t.p.?

   [1 mark]

   • A96 dm3
   • B24 dm3
   • C12 dm3
   • D48 dm3

   (b)

   The supplier then measures a separate cylinder and finds it contains 12 dm3 of oxygen gas at r.t.p. Using the molar gas volume of 24 dm3 at r.t.p., how many moles of oxygen gas does this cylinder contain?

   [1 mark]

   • A2.0 mol
   • B0.5 mol
   • C12 mol
   • D1.0 mol

   (c)

   State the value of the molar gas volume used for calculations involving gases at room temperature and pressure (r.t.p.), and explain what this value means in terms of the volume occupied by one mole of any gas.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A water treatment plant technician prepares a solution by dissolving 20 g of sodium hydroxide, NaOH (molar mass 40 g/mol), in water and making the total volume up to 500 cm3.

   (a)

   Calculate the number of moles of sodium hydroxide dissolved, and calculate the concentration of the resulting solution in mol/dm3.

   [3 marks]

   (b)

   The technician later needs to prepare 250 cm3 of a sodium hydroxide solution with a concentration of 2.0 mol/dm3, using solid sodium hydroxide (molar mass 40 g/mol). Calculate the mass of sodium hydroxide, in grams, that the technician must dissolve to prepare this solution, showing your working, and state the concentration of this solution in g/dm3.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A university chemistry laboratory carries out a titration to determine the concentration of an unknown hydrochloric acid solution. In the titration, 25.0 cm3 of the hydrochloric acid solution exactly neutralises 20.0 cm3 of a 0.50 mol/dm3 sodium hydroxide solution, according to the equation HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l).

   (a)

   Calculate the concentration of the hydrochloric acid solution in mol/dm3, showing all steps of your working clearly, and explain why the 1:1 ratio in the balanced equation is essential to this calculation.

   [6 marks]

   (b)

   Using the concentration of hydrochloric acid you calculated (0.400 mol/dm3), the laboratory now wants to know the mass of solid magnesium (molar mass 24 g/mol) that would react exactly with 50.0 cm3 of this hydrochloric acid solution, according to the equation Mg(s) + 2HCl(aq) $\rightarrow$ MgCl2(aq) + H2(g). Calculate the mass of magnesium required, showing full working, and calculate the volume of hydrogen gas, in dm3, that would be produced at r.t.p. (molar gas volume 24 dm3), explaining how the mole ratios from the balanced equation are used at each stage of the calculation.

   [6 marks]

   Total for question 4: 12 marks

End of questions