How many grams of water vapor are formed?

Question

A Hydrogen-filled balloon is ignited, and 2.50 g of hydrogen reacts with 17.50 g of oxygen. How many grams of water vapor form assuming the only product is water (use correct significant figures)?

Answer 1

Write a balanced equation for the reaction taking place:

2H₂(g) + O₂(g) ==> 2H₂O(g) ... balanced equation

moles of H2(g) initially present = 2.50 g H2 x 1 mol H2 / 2 g = 1.25 mols H2

moles O2(g) initially present = 17.50 g O2 x 1 mol O2 / 32 g = 0.5469 mols O2

Since it takes TWO moles of H2 for every ONE mole of O2, O2 will be limiting as it will run out first.

Moles of H2O formed will be determined by the limiting reactant, O2. Thus...

0.5469 mols O2 x 2 mols H2O / mole O2 = 1.0038 moles H2O formed

mass of H2O = 1.0038 mols H2O x 18 g/ mol = 19.6884 g H2O = 19.7 g H2O (3 sig. figs.)