What type of solid is the silica?

Which from following mixtures in water acts as a buffer?

Calculate van't Hoff factor of 0.15 M solution of electrolyte if it freezes at $-$0.5 K.

$$\left[\mathrm{K}_{\mathrm{f}}=1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right]$$

Calculate the solubility in $\mathrm{mol} \mathrm{dm}^{-3}$ of sparingly soluble salt BA if its solubility product $4.9 \times 10^{-13}$ at same temperature.