Question

A compound has a molar mass of 92.02 grams/mole, and its percent composition is 30.4% nitrogen (N) and 69.6% oxygen (O). What is its molecular formula?

Answer 1

Answer:

Empirical formula is

N

O

2

, and the molecular formula is

N

2

O

4

, dinitrogen tetroxide.

Explanation:

In

100

⋅

g

of this compounds there are

30.4

⋅

g

N

, and the balance

O

. We divide thru by the atomic masses in order to approach the empirical formula:

30.4

⋅

g

14.01

⋅

g

⋅

m

o

l

−

1

=

2.17

⋅

m

o

l

⋅

N

;

69.6

⋅

g

16.0

⋅

g

⋅

m

o

l

−

1

=

4.35

⋅

m

o

l

⋅

O

.

After we divide thru by 2.17, we get an empirical formula of

N

O

2

. And as I (and many others) have said before, the empirical formula

is the simplest whole number ratio defining constituent atoms

in a species.

Now the molecular formula is always a multiple of the empirical formula; i.e.

M

F

=

(

E

F

)

n

, or

92.4

⋅

g

⋅

m

o

l

−

1

=

2

×

(

14.01

+

2

×

16.00

)

(

g

⋅

m

o

l

−

1

)

. So

n

=

2

, and the molecular formula is

N

2

O

4

, i.e. dinitrogen tetroxide. the dimer of

N

O

2

radical.