Question

When 2.50 mol of mg3n2 are allowed to react how many moles of h2o also react?

Answer 1

Notice that in the equation given, there is 1 mol of Mg3N2 and 6 mol of H2O. Consequently, the question is requesting, if we took 2.5 mol of Mg3N2, how many mol of H2O would be essential to complete a similar reaction -- in quintessence, multiplying everything by 2.5: 

the solution would be: 6 * 2.5 = 15 mol