how many moles of water are produced from the combustion of 3.5 mol of hydrogen and 5.2 mol of oxygen

Answer 1

This is a limiting reagent problem. You know that when BOTH of the reactants are listed. Print these instructions and remember them.
1. Write the equation and balance it.

2a. Using the coefficients in the balanced equation, convert 3.5 moles hydrogen to moles of H2O.
2b. Same procedure, convert 5.2 moles oxygen to moles H2O.
2c. It is likely that the answer from 2a and 2b will be different which means one of them is wrong. The correct answer, in limiting reagent problems. is ALWAYS the smaller value and the reactant producing that value is the limiting reagent.