Question

Elemental analysis of a compound showed that it consisted of 81.82% carbon and 18.18% hydrogen by mass. How many hydrogen atoms appear in the empirical formula of the compound?

Answer 1

The empirical formula is `"C"_3"H"_8"`.

Explanation:

Since the percentages add up to 100, we can assume that we have a 100.0 g sample of the compound, and the percentages become grams.

Determine the Moles of Each Element
Divide the mass of each element by its molar mass. The molar mass of an element is its atomic weight on the periodic table in grams/ mole (g/mol).

`81.82cancel"g C"xx(1"mol C")/(12.011cancel"g C")="6.812 mol C"`

`18.18cancel"g H"xx(1"mol H")/(1.00794cancel"g H")="18.04 mol H"`

Determine the Mole Ratios
Divide the number of moles of each element by the least number of moles.

`"C":` `(6.812)/(6.812)="1.000"`

`"H":` `(18.04)/(6.812)="2.648"`

Since the mole ratio for H cannot be rounded to a whole number, we must multiply it times a factor that will result in a whole number.

`"H":` `2.648xx3=7.944~~8`

We must multiply both mole ratios times `3`.

`"C":` `1.000xx3=3.000"`

The empirical formula is `"C"_3"H"_8"`.