Question

An acid of general molecular formula HA is \(25\%\) dissociated during the study of boiling point of its solution. If the calculated elevation of boiling point of the solution is \(1.62\,K\), find the observed value of elevation of boiling point of the solution.

• A. \(2.025\,K\)
• B. \(2.430\,K\)
• C. \(2.632\,K\)
• D. \(2.835\,K\)

Hint:

For dissociation: \[ i=1+\alpha(n-1) \] where \(n\) is the number of particles formed. For \[ HA \rightarrow H^+ + A^- \] \[ n=2 \] so, \[ i=1+\alpha \]

Answer 1

The Correct Option is A

Concept: For dissociation, \[\begin{aligned} \Delta T_b(\text{observed}) = i\,\Delta T_b(\text{calculated}) \end{aligned}\] where \(i\) is the van't Hoff factor. For \[ HA \rightleftharpoons H^+ + A^- \] \[\begin{aligned} i=1+\alpha \end{aligned}\]

Step 1: Calculate van't Hoff factor. Given, \[\begin{aligned} \alpha=25\%=0.25 \end{aligned}\] Hence, \[\begin{aligned} i=1+0.25=1.25 \end{aligned}\]

Step 2: Calculate observed elevation in boiling point. \[\begin{aligned} \Delta T_b(\text{observed}) &=i\,\Delta T_b(\text{calculated}) \\ &=1.25\times1.62 \\ &=2.025\,K \end{aligned}\] \[\begin{aligned} \boxed{2.025\,K} \end{aligned}\] Hence, option \(\mathbf{(A)}\) is correct.