29a. Explain why the hydrides of group 16 elements (H2O, H2S, H2Se and H2Te) are polar molecules. [2 marks]

31a. The reaction in (a) was carried out in a crucible with a lid and the following data was recorded. Calculate the amount of magnesium, in mol, that was used. [1 mark] Mass of crucible and lid = 47.372 ±0.001 g Mass of crucible, lid and magnesium ribbon before heating = 53.726 ±0.001 g Mass of crucible, lid and product after heating = 56.941 ±0.001 g