Question

Compound B contains carbon, hydrogen and oxygen only. Its mass spectrum contains a molecular ion peak at m/e = 102 (relative abundance 35%) and an M+1 peak at m/e 103 (1.5%). Calculate the number of carbon atoms in the molecule, and hence deduce the number of oxygen atoms it contains, and its molecular formula.

Answer 1

Molecular of compound B = Molecular ion peak (M⁺)= 102 g

M+1 = 103

Relative abundance of ¹²C is 98.9% and ¹³C is 1.1% in nature.

Number of carbon atom is given by the formula:

n = 100/1.1 (A_M+1 / A_M)

A_M+1 = Relative abundance of M+1 peak

A_M = Relative abundance of M⁺ peak

n = 100/1.1(1.5 / 35) = 3.89

Nearest number is 4, therefore number of C atom is 4.

Molecular mass - Mass of 4 C = 102 - 48 = 54 (mass of O and H present)

H-rule: H+ Halogen = 2C + N + 2

A molecule with four carbons and zero can have at most (2 x 4) + (0) + 2 = 10 monovalent atoms.

If we take number of O is one, then number of H will be 38. This is violation to the rule.

If we take number of O is 2, then number of H will be 22. This is violation to the rule

If we take number of O is 3, then number of H will be 6. This is reasonable

Therefore molecular formula will be C₄O₃H₆