Magnesium oxide, known as magnesia, can be produced by the thermal decomposition of magnesium carbonate. The equation for the reaction is MgCO3(s) → MgO(s) + CO2(g) (a) Other Group 2 carbonates also undergo thermal decomposition. State and explain the trend in thermal stability of carbonates down Group 2. (4 marks) (b) Magnesia absorbs water from the air to form magnesium hydroxide, Mg(OH)2. MgO(s) + H2O(g) → Mg(OH)2(s) Both magnesium oxide and magnesium hydroxide react with hydrochloric acid as shown in the equations. MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l) An old bottle, labelled magnesia, contained a mixture of magnesium oxide and magnesium hydroxide. 0.180 g of this mixture was dissolved in 50.0 cm3 of hydrochloric acid, with a concentration of 0.200 mol dm−3. The excess hydrochloric acid was titrated with a solution of sodium hydroxide, with a concentration of 0.100 mol dm−3. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) 18.50 cm3 of the sodium hydroxide solution was needed to neutralize the excess acid. (i) Calculate the number of moles of hydrochloric acid that reacted with the sodium hydroxide solution. (3 marks) (ii) Calculate the number of moles of hydrochloric acid originally added to 0.180 g of the mixture. Use this answer and your answer to (b)(i) to calculate the number of moles of hydrochloric acid that have reacted with the mixture of magnesium oxide and magnesium hydroxide. (5 marks) (iii) Calculate the mass of pure magnesium oxide that would react with the number of moles of hydrochloric acid in (b)(ii). (5 marks) (iv) The difference in the mass of the impure sample and the mass of magnesium oxide in (b)(iii) is due to water absorbed from the air to form magnesium hydroxide. MgO(s) + H2O(g) → Mg(OH)2(s) Use your answer to (b)(iii) to calculate the mass of water absorbed by the sample, and hence find the mass of magnesium hydroxide present in 0.180 g of the mixture. (5 marks) (c) State why carrying out a flame test is not an appropriate way to confirm the presence of magnesium ions in the sample of magnesia.​