Concentrations of solutions
4 marks · NormalGive the formula that links concentration in mol/dm³, amount in mol, and volume in dm³.
A student dissolves 2.5 g of magnesium sulfate (MgSO₄, Ar: Mg = 24, S = 32, O = 16) in 200 cm³ of water.
i. Calculate the number of moles of MgSO₄ in the solution.
ii. Calculate the concentration of MgSO₄ in mol/dm³ in the solution.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) concentration (mol/dm³) = amount (mol) / volume (dm³)
(b) 0.0208 mol
(c) 0.104 mol/dm³
Common mistakes: Not converting 200 cm³ to 0.2 dm³; Calculation uses grams instead of moles
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States or writes the correct formula: concentration = amount / volume (mol/dm³ = mol / dm³)
M1 (b) · 1 mark · Calculates moles of MgSO₄ correctly (mass / Mr: 2.5/120 = 0.0208 mol)
M2 (c) · 1 mark · Converts 200 cm³ to 0.2 dm³ or shows correct method for concentration
A1 (c) · 1 mark · Correct concentration (0.104 mol/dm³, allow 0.10 – 0.105 with correct sig. fig.), with correct units (follow-through)
Worked solution
a. concentration (mol/dm³) = amount (mol) / volume (dm³)
b. Relative formula mass (Mr) of MgSO₄ = 24 + 32 + (16 × 4) = 24 + 32 + 64 = 120
c. Volume = 200 cm³ = 0.2 dm³