The mass of water obtained in a combustion analysis of a 1.000 g sample of an organic compound containing carbon, hydrogen, and oxygen is 0.6662 g. What is the mass percent of hydrogen in the compound?

Question

Quizplus · Accepted Answer

First, calculate the mass of carbon and oxygen in the compound by subtracting the mass of water from the original sample mass: 1.000 g - 0.6662 g = 0.3338 g. Then, use the mass of water and the molar masses of water and hydrogen to calculate the moles of hydrogen in the compound:

moles H = (0.6662 g H2O) / (18.0153 g/mol H2O) * (2 mol H / 1 mol H2O) = 0.0740 mol H

Finally, calculate the mass percent of hydrogen in the compound:

mass % H = (mass H / total mass) x 100% 
mass H = (moles H x molar mass H) = (0.0740 mol H) x (1.0079 g/mol H) = 0.0744 g H 
total mass = 1.000 g

mass % H = (0.0744 g H / 1.000 g) x 100% = 7.456% H, which corresponds to answer choice B.

The Mass of Water Obtained in a Combustion Analysis of a 1.000