Question

A compound was found to have 85.7% carbon and 14.3% hydrogen. Its molecular mass is 84. What is its empirical formula? What is its molecular formula?

Answer 1

Answer:

Explanation:

empirical formula

=

C

H

2

.

molecular formula

=

C

6

H

12

Explanation:

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

100

⋅

g

of unknown compound.

And thus there are  

85.7

%

×

100

⋅

g

12.011

⋅

g

⋅

m

o

l

−

1

with respect to carbon, i.e.  

7.14

⋅

m

o

l

⋅

C

And  

14.3

%

×

100

⋅

g

1.00794

⋅

g

⋅

m

o

l

−

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

⋅

g

⋅

m

o

l

−

1

+

2

×

1.00794

⋅

g

⋅

m

o

l

−

1

)

×

n

=

84

⋅

g

⋅

m

o

l

−

1

.

So  

n

=

6

, and  

molecular formula

=

6

×

C

H

2

=

C

6

H

12