So $m =\frac{0.5}{74.6} \times \frac{1}{0.1}$

$\Delta T _{ f }= i \times m _{ f }$

$0.24=i \cdot \frac{0.5}{74.6} \times \frac{1.80}{0.1}$

$i=\frac{0.24 \times 74.6}{0.5 \times 1.80} \times 0.1$

$=1.989$

$1.989=1+\alpha( n -1)$

$1.989=1+\alpha$

$\alpha=.989$

$\% \alpha=98.9 \%$

Ans $99 \,\%$

If mass of $H _{2} O =99.5$

$m =\frac{0.5}{74.5} \times \frac{1}{.0995}$

$i =\frac{0.24 \times 74.6 \times .0995}{.5 \times 1.80}$

$=1.979$

$1.979=1+\alpha( n -1)$

$1.979=1+\alpha$

$\alpha=.979$

$\% \alpha=97.9\, \%$

$98\, \%$