Question

Acetic acid dimerises to an extent of 30% in benzene. The observed molecular mass (in g) is
a.62.5
b.70.6
c.50.6
d.80​

Answer 1

Given:

Degree of association of acetic acid, α = 30% = 0.3

To Find:

The observed molecular mass of acetic acid.

Calculation:

- For the association of acetic acid, n = 2

- For association, we have:

α = n(1 - i)/(n-1)

⇒ 0.3 = 2(1 - i)/(2-1)

⇒ i = 1 - (0.3/2)

⇒ i = 0.85

- Now, we know that Vant Hoff factor is given as:

i = Normal molar mass/observed molar mass

⇒ 0.85 = 60/Observed molar mass

⇒ Observed molar mass = 60/0.85

⇒ Observed molar mass = 70.6 gm

- So the correct answer is option (b) 70.6 gm