‘Lawn sand’ is spread over the grass in gardens to reduce the growth of moss. Lawn sand is a mixture of sand and iron(II) sulfate crystals, FeSO₄.7H₂O. Lawn sand usually contains 6–10% FeSO₄.7H₂O by mass. To determine the exact percentage by mass of FeSO₄.7H₂O present in a sample of lawn sand, a student devises the following experiment. step 1 Use a known mass of lawn sand to prepare 250.0 cm³ of solution A containing Fe²⁺(aq) ions. Solution A must have dilute sulfuric acid, H₂SO₄(aq), added to it before it is made up to 250 cm³. step 2 To determine the concentration of Fe²⁺(aq) in solution A, titrate a 25.00 cm³ sample of solution A against 0.0200 mol dm⁻³ aqueous potassium manganate(VII), KMnO₄(aq). The reaction which takes place during the titration is shown. MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq)

1. (a)(i)The end-point of the titration is 25.00 cm³ of 0.0200 mol dm⁻³ KMnO₄(aq). Determine the concentration of Fe²⁺(aq) that was present in the 25.00 cm³ sample of solution A at the start of the titration. concentration of Fe²⁺(aq) = ........................ mol dm⁻³[1]
2. (a)(ii)If you were unable to calculate the concentration in (i), assume for (ii) and (iii) that the concentration of Fe²⁺(aq) is 0.300 mol dm⁻³. This is not the correct answer. Determine the mass of lawn sand needed to prepare the 250.0 cm³ of solution A at the concentration calculated in (i). Assume that lawn sand contains 8% FeSO₄.7H₂O by mass. [Ar: Fe, 55.8; S, 32.1; O, 16.0; H, 1.0] mass of lawn sand = ........................ g[3]
3. (a)(iii)Solution A must contain enough H⁺(aq) ions for the reaction to take place during the titration. MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq) Use the concentration of Fe²⁺(aq) from (i) to determine the minimum volume of 2.00 mol dm⁻³ H₂SO₄(aq) which must be added to prepare the 250.0 cm³ of solution A. volume = ........................ cm³[2]
4. (b)Describe a method to prepare 250.0 cm³ of solution A starting with a glass beaker which contains the known mass of lawn sand determined in (a)(ii) as shown. [Figure 2.1: Diagram of glass beaker with known mass of lawn sand] Assume that common laboratory apparatus is available. You may find it helpful to write your answer as a series of smaller steps.[5]
5. (c)State the colour change in the conical flask at the end-point of the titration. from ................................ to ................................[1]
6. (d)Aqueous potassium manganate(VII) is a powerful oxidising agent. Suggest the effect, if any, on the end-point volume if the student acidified the mixture with dilute hydrochloric acid, HCl(aq), instead of dilute sulfuric acid, H₂SO₄(aq). Explain your answer. effect, if any, on the end-point volume .......................................................................................... explanation ....................................................................................................................................................[2]

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