Fig. 1 shows a fuel-cell laboratory preparing sulfuric acid electrolyte. A student needs 2.00 dm3 of 0.0750 mol/dm3 sulfuric acid, H2SO4. The Mr of H2SO4 is...
Question 1 Report
Fig. 1 shows a fuel-cell laboratory preparing sulfuric acid electrolyte. A student needs 2.00 dm3 of 0.0750 mol/dm3 sulfuric acid, H2SO4. The Mr of H2SO4 is 98.0. The acid is added to water slowly while the beaker is cooled.
(a) Calculate the number of moles of H2SO4 required. [1]
(b) Use the Mr to calculate the mass of H2SO4 required. [2]
(c) Give the correct method for mixing the acid and water. [1]
(d) Suggest why this method is safer. [1]
Fig. 1 shows a student using a colorimeter to compare coloured potassium manganate(VII) solutions. The student prepares solution P by dissolving 0.0150 mol in 250 cm3 of water. Solution Q has concentration 0.0200 mol/dm3.
(a) Use the data to calculate the concentration of solution P. [2]
(b) What volume of solution Q contains 0.0150 mol of solute? [2]
Answer Details
Sulfuric acid electrolyte
- (a) Use \(n=cV\): \[n=0.0750\text{ mol/dm}^3\times2.00\text{ dm}^3=0.150\text{ mol}\] [1 mark]
- (b) Use \(m=nM_r\): \[m=0.150\times98.0=14.7\text{ g}\] [2 marks]
- (c) Add the acid slowly to the water. [1 mark]
- (d) Diluting acid releases heat. Adding acid slowly releases this heat gradually, reducing the risk of rapid boiling and splashing. [1 mark]
Potassium manganate(VII) solutions
- (a) Convert the volume: \(250\text{ cm}^3=0.250\text{ dm}^3\). Then \[c=\frac{n}{V}=\frac{0.0150}{0.250}=0.0600\text{ mol/dm}^3\] [2 marks]
- (b) Rearrange \(c=\frac{n}{V}\) to \(V=\frac{n}{c}\): \[V=\frac{0.0150}{0.0200}=0.750\text{ dm}^3\] [2 marks]
Exam reminder: for acid dilution, the safe rule is always acid into water, never water into concentrated acid.
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