Question

If 8 g of a non-electrolyte solute is dissolved in 114 g of n-octane to reduce its vapour pressure to 80%, the molar mass (in g mol^-1) of the solute is
[Given that the molar mass of n-octane is 114 g mol^-1.]

• A. $40$
• B. $60$
• C. $80$
• D. $20$

Hint:

Relative lowering of vapour pressure is a colligative property as it depends on the moles of the solute but is independent of its nature.

Answer 1

The Correct Option is A

Assuming the given solution is dilute.
By putting the values in the formula:

\(\dfrac{\Delta P}{ P^o_A} = \dfrac{n_B}{n_A} = \dfrac{w_B}{m_B} . \dfrac{m_A}{w_A}\)

\(\dfrac{20}{100} = \dfrac{8}{m_b}.\dfrac{114}{114}\)

\(m_B=\dfrac{8X100}{20}\)

\(=40\ gmol^{-1}\)

Hence, the correct answer is Option (A): \(40\)