Compare — 3 marks
A student is preparing two sugar solutions for a school experiment. Solution A contains 15 g of sugar dissolved in 500 cm³ of water. Solution B contains 30 g of sugar dissolved in 1000 cm³ of water.
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(a) Calculate the concentration of sugar in Solution A in g/dm³.
[1 mark]
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(b) Calculate the concentration of sugar in Solution B in g/dm³.
[1 mark]
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(c) Compare the concentrations of the two solutions. Explain your answer.
[1 mark]
Show mark scheme
- (a) 30 g/dm³
- (b) 30 g/dm³
- (c) States concentrations are equal/the same
- (c) References the proportional increase in both mass of solute and volume of solvent
Calculate — 5 marks
A student is preparing a series of sodium chloride solutions for an experiment investigating the effect of salt concentration on the boiling point of water. They need to make 500 cm³ of a 2.0 mol/dm³ sodium chloride solution, then dilute it to create a 0.5 mol/dm³ solution.
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(a) Calculate the mass of sodium chloride needed to prepare 500 cm³ of a 2.0 mol/dm³ solution. (Relative atomic masses: Na = 23, Cl = 35.5)
[2 marks]
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(b) The student dilutes 150 cm³ of the 2.0 mol/dm³ solution with distilled water to make it up to 600 cm³. Calculate the new concentration of the diluted solution.
[2 marks]
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(c) Explain why the student should use a volumetric flask rather than a measuring cylinder when preparing the original 2.0 mol/dm³ solution, and state what would be the effect on the calculated concentration if a measuring cylinder was used instead.
[1 mark]
Show mark scheme
- (a) Molar mass of NaCl = 58.5 g/mol (or 23 + 35.5) [1 mark]
- (a) Mass = concentration × volume × molar mass = 2.0 × 0.5 × 58.5 = 58.5 g [1 mark]
- (b) Use of dilution equation: C₁V₁ = C₂V₂ or correct rearrangement [1 mark]
- (b) 2.0 × 150 = C₂ × 600; C₂ = 0.5 mol/dm³ [1 mark]
- (c) Volumetric flask has greater accuracy/precision than measuring cylinder AND if measuring cylinder used, the actual concentration would be lower/less accurate because the volume would be less precise/accurate [1 mark]
Explain — 4 marks
A student is preparing salt solutions of different strengths for a chemistry experiment. They dissolve 10 g of salt in 100 cm³ of water to make solution A, and 10 g of salt in 200 cm³ of water to make solution B.
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(a) Explain why solution A has a higher concentration than solution B.
[2 marks]
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(b) The student wants to make solution A more dilute. Explain what they should do and why this would decrease the concentration.
[2 marks]
Show mark scheme
- (a) Solution A has the same mass of solute (salt) but in a smaller volume of solvent/solution
- (a) Concentration depends on the amount of solute per unit volume / concentration is inversely proportional to volume
- (b) Add more water/solvent to the solution
- (b) This increases the total volume while keeping the mass of solute the same, so there is less solute per unit volume / the concentration decreases
Calculate — 5 marks
A student is preparing a series of dilute solutions in a chemistry laboratory. They need to make a sodium chloride solution for an experiment. The student has access to a concentrated stock solution of sodium chloride with a concentration of 2.5 mol/dm³ and distilled water. They require 250 cm³ of a solution with a concentration of 0.40 mol/dm³.
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(a) Calculate the volume of concentrated sodium chloride stock solution needed to prepare 250 cm³ of a 0.40 mol/dm³ solution. Give your answer in cm³ to 2 significant figures.
[2 marks]
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(b) The student dilutes the stock solution by adding 35.0 cm³ of the concentrated sodium chloride solution to distilled water in a 500 cm³ volumetric flask. Calculate the concentration of the resulting solution in mol/dm³.
[2 marks]
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(c) Explain why using a volumetric flask and diluting to the mark is important for preparing solutions of accurate concentration.
[1 mark]
Show mark scheme
- (a) Uses correct formula: C₁V₁ = C₂V₂ or equivalent rearrangement (1 mark)
- (a) Correct calculation with answer 40 cm³ or 4.0 × 10¹ cm³ (1 mark)
- (b) Correctly identifies final volume as 500 cm³ and converts to dm³ (0.5 dm³) (1 mark)
- (b) Correct calculation: concentration = (2.5 × 0.035) / 0.5 = 0.175 mol/dm³ (1 mark)
- (c) Volumetric flask has a single calibration mark at a fixed volume, ensuring the total volume is accurate/precise and not dependent on the volume of solute added (1 mark)
Calculate — 2 marks
A student is preparing a sodium chloride solution for a food preservation experiment. The student dissolves 5.0 g of sodium chloride in water to make 250 cm³ of solution.
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(a) Calculate the mass of sodium chloride dissolved in 1 dm³ of this solution. Give your answer in grams.
[1 mark]
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(b) The concentration of the solution can also be expressed in g/dm³. Calculate the concentration of the sodium chloride solution in g/dm³.
[1 mark]
Show mark scheme
- (a) 20 (g)
- (b) 20 (g/dm³)
GCSE Perfect