How many moles of hydrogen were in the original sample that underwent combustion ?

Question

A certain compound was found to have a molar mass of 32.042 g. A 4.078 g sample of this compound was burned in pure oxygen gas. The measured amount of CO2 produced was 5.601 g, while that of H2O was 4.586 g. No other products were produced in the reaction.

Answer 1

1) determine the mass of carbon, hydrogen and oxygen.
carbon: 5.601 g x (12 / 44) = 1.528 g
hydrogen: 4.586 g x (2 / 18) = 0.510 g
oxygen: 4.078 - 1.528 - .510 = 2.040 g

2) convert to moles.
carbon: 1.528 / 12 = 0.127 mol
hydrogen: 0.510 / 1 = 0.510 mol
oxygen: 2.040 / 16 = 0.128 mol

3) divide each molar amount by the lowest value to obtain small integers
carbon: 0.127 / 0.127 = 1
hydrogen: 0.510 / 0.127 = 4
oxygen: 0.128 / 0.127 = 1

The empirical formula of the substance is (CH4O)n
The atomic masses add up to 32, which is also the molar mass of the compound. Therefore n=1 and we find CH4O

Answer 2

You need to figure out the number of Moles of H2O first. Since the atomic weight of H2O is 18, there are 4.586 / 18 = .255 Moles of H2O. Since there are two atoms of H in every molecule of H2O, double the number of Moles of H2O to .510 to get the number of Moles of H Atoms.