Magnesium oxide is not very soluble in water, so it is difficult to titrate directly. Its purity can be determined by use of a back titration method. 4.08 g of impure magnesium oxide was completely dissolved in 100.0 cm3 of 2.00 mol dm-3 aqueous hydrochloric acid. The excess acid required 19.70 cm3 of 0.200 mol dm-3 aqueous sodium hydroxide for neutralization. Determine the percentage purity of the impure magnesium oxide.