Question

The elemental analysis of an unknown compound is 57.9 % C, 3.61 % H, and 38.5 % O. The molar mass of this compound is 166 g/mol. Determine the molecular formula of this compound.

A.C 2 H 2 O 2

B.C 5 H 6 O 4

C.C 9 H 10 O 3

D.C 2 H 3 O 2

E.C 8 H 6 O 4

Answer 1

we have mass of each elements as:
C: 57.9 g
H: 3.61 g
O: 38.5 g


Divide by molar mass to get number of moles of each:
C: 57.9/12.01 = 4.821
H: 3.61/1.008 = 3.5813
O: 38.5/16.0 = 2.4062


Divide by smallest:
C: 4.821/2.4062 = 2
H: 3.5813/2.4062 = 1.5
O: 2.4062/2.4062 = 1

multiply all by 2 to get simplest whole number ratio:
C : 2*2 = 4
H:1.5*2 = 3
O:1*2 = 2


So empirical formula is:
C4H3O2

Molar mass of C4H3O2,
MM = 4*MM(C) + 3*MM(H) + 2*MM(O)
= 4*12.01 + 3*1.008 + 2*16.0
= 83.064 g/mol

Now we have:
Molar mass = 166.0 g/mol
Empirical formula mass = 83.064 g/mol
Multiplying factor = molar mass / empirical formula mass
= 166.0/83.064
= 2

Hence the molecular formula is : C8H6O4

Answer: E