Which subatomic particles in an atom determine its mass number ?

What volume would 2.25 moles of ne has occupy at stp?

If 16.00g of hydrogen gas reacts with 126.73g of oxygen, how many grams of water are yielded? (both reactants are completely consumed in the reaction.)

Suppose that some of the compound spilled out of the crucible after it was heated. would that cause the percent by mass of water in the compound determined by the experiment to be too low, too high, or unchanged? briefly explain your answer.