Question

A hydrocarbon was found to contain 85.7% by mass of carbon and 14.3% by mass of hydrogen. The molar mass of hydrocarbon is 56 g mol-1. The formula for hydrocarbon is

Answer 1

AS with all these problems, we assume (for simplicity)

100g of unknown compound.

And thus there are

85.7%×100g

12.011gmol−1

with respect to carbon, i.e.

7.14mol C

And

14.3%

×

100g

1.00794g.ol

−

1

with respect to hydrogen, i.e.

14.1

⋅

m

o

l

⋅

H

.

And thus the

empirical formula

, the simplest whole number ratio that defines constituent elements in a species is

C

H

2

.

Now the

molecular formula

is alway a whole number multiple of the

empirical formula

:empirical formula×n=molecular formula

So

(12.011⋅gmol−1+2×1.00794gmol−1)×n

=84gmol−1

.

So n=6, and

molecular formula=6×CH2=C6H12