2H2(g) + O2(g) <==> 2H2O(g) ∆H = -296 kJ
I'll do (a), and hopefully you can use the same/similar procedure to do (b) and (c).
(a). How much heat is evolved when 150 g of hydrogen is burned in excess oxygen?
Explanation: There are 296 kJ of heat evolved for every TWO moles of H2 that reacts. Converting this to kJ per mole we get 296 kJ / 2 mol = 148 kJ / mole of H2. We find that we have 75 moles of H2, so we can easily calculate the heat evolved.
150 g H2 x 1 mol H2 / 2 g = 75 moles H2
-296 kJ / 2 mol H2 x 75 mol H2 = 11,100 kJ of heat evolved
(b). Find moles of water is produced and apply the same reasoning as in (a).
(c). This is a little different in that you must find which reactant (H2 or O2) is limiting. Calculate moles of H2O formed from 5 g H2 and also from 15 g of O2 (as you did above). Whichever gives the smaller answer would be limiting so multiply that by the kJ to get the answer.