A student wanted to make 10.0 g of zinc chloride. The equation for the reaction is: ZnO(aq) + 2HCl(aq) → ZnCl2(aq) + H2O(l) Calculate the mass of zinc oxide the student needs to react with the dilute acid to make 10.0 g of zinc chloride. [4 marks]

What do we know from the question? 1. we know the balanced equation 2. we know the mass of zinc chloride we want to form. using the equation number of moles = mass/Mr we can calculate the number of moles in 10g of zinc chloride. this would require using a periodic table to determine the Mr of zinc chloride ( 65 + (2 × 35.5) = 136 ). it is important for this equation to work that mass is in unit grams and Mr in grams per mole (g/mole). 10/136 gives 0.0735294 mol of zinc chloride. the stochiometric equation given shows us that 1 mole of zinc oxide is required to form 1 mole of zinc chloride, therefore from this ratio we know that we must need 0.0735294 moles of zinc oxide to form the same number of moles of zinc chloride. to convert this into mass we rearrange the original formula (mass = moles x Mr) and so multiple 0.0735294 mol by the Mr of zinc oxide (65 + 16 = 81) to give 6.0 g (2 significant figures).