Question: what is the percent composition by mass of hydrogen?
Answer: Mass Percent of Hydrogen. To find the mass percent of hydrogen in water, take the molar mass of hydrogen in the water molecule, divide by the total molar mass of water, and multiply by 100. Dividing 2.016 by 18.016 gives you 0.1119. Multiply 0.1119 by 100 to get the answer: 11.19 percent.