You are given
the mass of CaH2 which is 14 grams and H2O which is 28 grams. You are required
to find the maximum volume of H2 gas at STP. The balanced chemical reaction is
CaH2 + 2H2O -> Ca(OH)2 + H2. Note that for every one mole of CaH2, 2 moles
of H2O is needed to completely react and produce Ca(OH)2 + H2. So the CaH2 and
H2O are at equal proportions. At STP, temperature is at 0°C(273K) and pressure
at 1 atm. The molar mass of CaH2 is 42 grams per mole.
If a big bully and a small child are involved in a thug of war, it is clear that the bully is stronger than the child and he/she will pull the rope used in the thug of war with a greater force.
By so doing, the ball attached at the centre of the rope will naturally be drawn towards the stronger bully.