Concentrations of solutions
4 marks · NormalA solution has a concentration of 1.5 mol/dm³. What is the amount (in mol) of solute in 750 cm³ of this solution?
Describe how you would prepare 750 cm³ of this solution from a 3.0 mol/dm³ solution.
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(a) 1.13 mol
(b) Take 375 cm³ of the 3.0 mol/dm³ solution and add water until the total volume is 750 cm³. This dilutes it by half to give 1.5 mol/dm³.
Common mistakes: Not converting 750 cm³ to 0.75 dm³; Mixing up concentrations in dilution
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Marking points
M1 (a) · 1 mark · Converts 750 cm³ to 0.75 dm³
A1 (a) · 1 mark · Calculates correct amount: 1.5 × 0.75 = 1.13 mol (accept 1.125) (follow-through)
M2 (b) · 1 mark · Correctly calculates that half-strength (1.5 from 3.0 mol/dm³) requires solution to be diluted by half
A2 (b) · 1 mark · Describes process: mix equal parts 3.0 mol/dm³ solution and water to make up to 750 cm³, or equivalent method (follow-through)
Worked solution
a. Volume = 750 cm³ = 0.75 dm³
b. To dilute from 3.0 to 1.5 mol/dm³, mix 375 cm³ of 3.0 mol/dm³ solution with 375 cm³ of water to reach 750 cm³. This halves the concentration.