Show — 5 marks
A chemistry student is preparing a solution for an experiment. They need to dissolve sodium chloride (NaCl) in water to create a standard solution. The student has a bottle containing 250 cm³ of distilled water and needs to dissolve 58.5 g of sodium chloride to make a solution with a concentration of 4.0 mol/dm³.
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(a) Calculate the number of moles of sodium chloride (NaCl) in 58.5 g. (The relative atomic masses are: Na = 23, Cl = 35.5)
[2 marks]
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(b) The student dissolves all 58.5 g of sodium chloride in the 250 cm³ of water. Show that the concentration of the resulting solution is 4.0 mol/dm³.
[2 marks]
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(c) If the student had only used 150 cm³ of water instead, calculate the concentration of sodium chloride in this new solution. Give your answer to 2 significant figures.
[1 mark]
Show mark scheme
- (a) Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol (1 mark)
- (a) Number of moles = mass ÷ molar mass = 58.5 ÷ 58.5 = 1.0 mol (1 mark)
- (b) Convert 250 cm³ to dm³: 250 cm³ = 0.25 dm³ (1 mark)
- (b) Concentration = moles ÷ volume in dm³ = 1.0 ÷ 0.25 = 4.0 mol/dm³ (1 mark)
- (c) Convert 150 cm³ to dm³: 150 cm³ = 0.15 dm³; Concentration = 1.0 ÷ 0.15 = 6.67 mol/dm³ = 6.7 mol/dm³ (to 2 s.f.) (1 mark)
Describe — 5 marks
A pharmaceutical company is developing a new antibiotic medication. They need to prepare a solution by dissolving 2.5 g of the active ingredient in 500 cm³ of distilled water. The company must ensure the concentration is correct for clinical trials. The molar mass of the active ingredient is 250 g/mol.
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(a) Describe how you would calculate the number of moles of the active ingredient in the 2.5 g sample.
[2 marks]
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(b) Describe the steps you would follow to determine the concentration of the solution in mol/dm³ using the mass and volume data provided.
[2 marks]
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(c) Describe why it is important to use precise measurements of both mass and volume when preparing this pharmaceutical solution, making reference to concentration calculations.
[1 mark]
Show mark scheme
- (a) Divide the mass by the molar mass (2.5 ÷ 250)
- (a) This gives 0.01 mol (or equivalent working)
- (b) Convert volume from cm³ to dm³ (500 cm³ = 0.5 dm³)
- (b) Divide number of moles by volume in dm³ (0.01 ÷ 0.5 = 0.02 mol/dm³)
- (c) Errors in mass or volume measurements will lead to incorrect concentration calculations / inaccurate dosing in clinical trials / unreliable results
Calculate — 2 marks
A student is investigating the mass of water that can be held in different containers. They have a large cylindrical tank and want to determine how much water it can hold. The tank has a mass of 2.5 kg when empty. When filled completely with water, the total mass is 42.5 kg. (The density of water is 1000 kg/m³)
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(a) Calculate the mass of water that the tank can hold.
[1 mark]
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(b) Calculate the volume of water in the tank in cubic metres (m³). Show your working.
[1 mark]
Show mark scheme
- (a) Subtracts the empty tank mass from the total mass: 42.5 - 2.5 = 40 kg (or equivalent correct calculation showing subtraction of the two masses)
- (b) Uses the correct formula rearranged from density = mass/volume to give volume = mass/density, and substitutes correctly: 40 ÷ 1000 = 0.04 m³ (or shows working that demonstrates correct use of the density relationship)
Calculate — 2 marks
Calcium oxide (quicklime) is produced industrially by heating calcium carbonate in a kiln. This process is essential for manufacturing cement. The equation for the reaction is: CaCO₃ → CaO + CO₂. The relative atomic masses (Aᵣ) are: Ca = 40, O = 16, C = 12.
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(a) Calculate the relative formula mass (Mᵣ) of calcium carbonate, CaCO₃.
[1 mark]
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(b) A factory heats 200 kg of calcium carbonate. Calculate the maximum mass of calcium oxide that can be produced. Give your answer in kilograms.
[1 mark]
Show mark scheme
- (a) 40 + 12 + (16 × 3) = 100 (no penalty for missing or incorrect working)
- (b) Mᵣ of CaO = 56
- (b) 200 × (56/100) = 112 kg
Describe — 2 marks
A student is carrying out a laboratory experiment to make magnesium oxide by heating magnesium ribbon in a crucible. The student wants to calculate how much magnesium oxide should be produced.
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(a) Describe how to calculate the relative formula mass of magnesium oxide, MgO.
(Relative atomic masses: Mg = 24, O = 16)
[1 mark]
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(b) The student starts with 2.4 g of magnesium. Describe how to calculate the maximum mass of magnesium oxide that can be made.
[1 mark]
Show mark scheme
- (a) Add the relative atomic masses (24 + 16 = 40) OR Add the mass numbers of magnesium and oxygen
- (b) Use the ratio of masses (Mg:MgO = 24:40) to scale from 2.4 g magnesium OR Divide mass of magnesium by 24 then multiply by 40 OR Use Mr from 01.1 to calculate mass of product
GCSE Perfect