Ammonium iron(II) sulfate, (NH4)2Fe(SO4)2, has a relative formula mass, M₁, of 284.

1. (a)Define the term relative formula mass.[2]
2. (b)One of the cations in ammonium iron(II) sulfate is the ammonium ion, NH₄⁺.
3. (b)(i)Draw a 'dot-and-cross' diagram of an ammonium ion. Show outer shell electrons only. Use x to show electrons from nitrogen. Use • to show electrons from hydrogen.[2]
4. (b)(ii)Suggest the shape of an ammonium ion and predict the bond angle. shape bond angle[2]
5. (c)In aqueous solution the ammonium ion acts as a weak Brønsted-Lowry acid.
6. (c)(i)Explain the meaning of the term weak Brønsted-Lowry acid.[2]
7. (c)(ii)Write an equation to show this behaviour of the ammonium ion in water. Include state symbols.[2]
8. (d)Mohr's salt, (NH4)2Fe(SO4)2•xH2O, is the hydrated form of ammonium iron(II) sulfate. x represents the number of moles of water in 1 mole of the salt. A student wanted to determine the value of x. 0.784 g of the hydrated salt was dissolved in water and this solution was acidified. All of the solution was titrated with 0.0200 mol dm⁻³ potassium manganate(VII). 20.0 cm³ of this potassium manganate(VII) solution was required for complete reaction with the Fe²⁺ ions.
9. (d)(i)Use changes in oxidation numbers to balance the equation for the reaction taking place. MnO₄⁻(aq) + .....Fe²⁺(aq) + .....H⁺(aq) → .....Mn²⁺(aq) + .....Fe³⁺(aq) + .....H₂O(l)[1]
10. (d)(ii)State the role of the Fe²⁺ ions in this reaction. Explain your answer.[2]
11. (d)(iii)Calculate the amount, in moles, of manganate(VII) ions that reacted. amount = _________________ mol[1]
12. (d)(iv)Calculate the amount, in moles, of Fe²⁺ ions in the sample of the salt. amount = _________________ mol[1]
13. (d)(v)Calculate the relative formula mass of (NH4)2Fe(SO4)2•xH2O. relative formula mass = _________________[1]
14. (d)(vi)Calculate the value of x. x = _________________[1]

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