Solutions and Colligative Properties 2 Question 25
25. An organic compound $\left(\mathrm{C}{x} \mathrm{H}{2 y} \mathrm{O}{y}\right)$ was burnt with twice the amount of oxygen needed for complete combustion to $\mathrm{CO}{2}$ and $\mathrm{H}_{2} \mathrm{O}$. The hot gases when cooled to $0^{\circ} \mathrm{C}$ and $1 \mathrm{~atm}$ pressure, measured $2.24 \mathrm{~L}$. The water collected during cooling weight $0.9 \mathrm{~g}$. The vapour pressure of pure water at $20^{\circ} \mathrm{C}$ is $17.5 \mathrm{~mm} \mathrm{Hg}$ and is lowered by $0.104 \mathrm{~mm}$ when $50 \mathrm{~g}$ of the organic compound are dissolved in $1000 \mathrm{~g}$ of water. Give the molecular formula of the organic compound.
$(1983,5 \mathrm{M})$
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Solution:
- Addition of $\mathrm{HgI}_{2}$ to $\mathrm{KI}$ solution establishes the following equilibrium :
$$ \mathrm{HgI}{2}+2 \mathrm{KI} \rightleftharpoons \mathrm{K}{2}\left[\mathrm{HgI}_{4}\right] $$
The above equilibrium decreases the number of ions (4 ions on left side of reactions becomes three ions on right side), hence rises the freezing point.
