What is the vapour density of ${\mathrm{PCl}}_5$ at $250°\mathrm{C}$ when it is dissociated to the extent of $80\%?$

The vapour pressure of a $0.01 \mathrm{molal}$ solution of a weak monobasic acid in water is $17.536 \mathrm{mm} \mathrm{of} \mathrm{Hg}$ at $25°\mathrm{C}$. Calculate the degree of dissociation of the acid. Aqueous tension of water at $25°\mathrm{C}$ is $17.54 \mathrm{mm}$. Represent your answer up to three decimal places.

In an ore, the only oxidizable material is ${\mathrm{Sn}}^{2+}$. This ore is titrated with a dichromate solution containing $2.5 \mathrm{g}$ of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ in $0.50 \mathrm{litre}$. A $0.40 \mathrm{g}$ sample of the ore required $10.0 {\mathrm{cm}}^3$ of titrant to reach the equivalence point. Calculate the percentage of tin in the ore.

Upon mixing $45.0 \mathrm{mL}$ of $0.25 \mathrm{M}$ lead nitrate solution with $25.0 \mathrm{mL}$ of $0.10 \mathrm{M}$ chromic sulphate solution, precipitation of lead sulphate takes place. How many moles of lead sulphate is formed? Also, calculate the molar concentrations of the species left behind in the final solution. Assume that lead sulphate is completely insoluble.