MYP Chemistry sample question: Moles, mass and concentration

A student dissolves 11.7 g of sodium chloride in water and makes the solution up to 500 cm³. (Relative atomic masses: Na = 23, Cl = 35.5.)

1. (a)Calculate the relative formula mass of sodium chloride.[1]
   Show the mark scheme for (a)

   • • 23 + 35.5 = 58.5 (1)
2. (b)Calculate the number of moles of sodium chloride in 11.7 g.[2]
   Show the mark scheme for (b)

   • • moles = mass ÷ Mr (1)
   • • 11.7 ÷ 58.5 = 0.20 mol (1)
3. (c)Calculate the concentration of the solution in mol/dm³.[2]
   Show the mark scheme for (c)

   • • 500 cm³ = 0.500 dm³ (1)
   • • concentration = 0.20 ÷ 0.500 = 0.40 mol/dm³ (1)

Convert cm³ to dm³ by dividing by 1000 before you use the concentration equation. It is the most common slip.