David V. answered • 02/24/21
PhD in ChemE with 9+ Years of Research and Consulting Experience
Hydrogen and oxygen are combining to form water. Without any additional information, you should assume that all of the 3.5 moles of oxygen will react with an appropriate amount of hydrogen to form water. In other words, this question is asking you to apply stoichiometry to the provided chemical reaction.
The chemical equation can be used to determine the stoichiometric amount of reactants and/or products that react with/form from the 3.5 moles of oxygen reactant. The equation tells us that there are two moles of water (H2O) for every mole of oxygen (O2). Therefore, if there are three moles of oxygen supplied:
nH2O = 3.5 * 2 = 7 mol
Since the question asked for the number of grams of water that will be produced, we need to convert this molar value into mass. Each mole of water has two moles of hydrogen and one mole of oxygen. The molecular weight of water is:
MWH2O = 2 * MWH2 + MWO2 = 2 * 1.01 g/mol + 16.00 g/mol = 18.02 g/mol
We know that there are 7 mol of water formed, so we use the molecular weight of water that we just calculated to find the mass of water formed:
mH2O = nH2O * MWH2O = 7 mol *18.02 g/mol = 126.14 g
As an added challenge, can you use the same method to find the mass of hydrogen that must be reacted with 3.5 mol of oxygen to form 7 mol of water?
