Question

Vapour density of PCI, at 400 K is found to be 90.
Degree of dissociation of PCI, will be
(1) 15.8%
(2) 13.6%
(3) 14.25%
(4) 90%​

Answer 1

answer : option (1) 15.8 %

using formula,

α = (M - m)/M

where α is degree of dissociation, M is theoretical molecular weight and m is observed molecular weight.

theoretical molecular weight of PCl5 = 208.25 g/mol

observed molecular weight = 2 × vapor density

= 2 × 90 = 180 g/mol

now, α = (208.24 - 180)/180 = 28.25/180

= 0.156944

hence, percentage degree of dissociation = α × 100 = 15.69 % it is nearest value of 15.8%

hence, option (1) is correct choice.

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Answer 2

Given:

1. Vapour density of PCl₅ = 90
2. Temperature = 400K

To find:

1. Degree of dissociation of PCl₅

Solution:

1. To calculate the degree of dissociation of PCl₅ in the given specific conditions, we will need to compare it to its theoretical vapor density.

Theoretical vapor density of PCl₅ can be obtained as follows:

1. Vapour density = Molecular weight / 2
2. We know that molecular weight of PCl₅ = 208.24 g
3. Vapour density of PCl = Molecular weight of PCl₅ / 2 = 208.24 / 2 = 104.12
4. The vapour density observed at 400K is = 90

Thus,

Degree of dissociation (α) will be = (calculated observation - actual observation) / actual observation

Degree of dissociation (α) = (104.12 - 90)/90 = 0.1569 = 15.69 % Ξ 15.8%

Thus, at 400K with a vapor density of 90, the degree of dissociation of PCl₅ will be (Option 1) 15.8%.