$\Delta \mathrm{T}_{\mathrm{f}}=\mathrm{K}_{\mathrm{f}} \mathrm{m} \Rightarrow 2.5=1.86 \times \frac{\mathrm{n}}{0.1}$

$\Rightarrow \mathrm{n}=0.1344 \text { moles }$

$\Rightarrow \mathrm{w}=0.1344 \times 32=4.3 \mathrm{~g}$

$\text { Volume }=\frac{4.3}{0.792}=5.43 \mathrm{ml}=543 \times 10^{-2} \mathrm{ml}$