$5.85 g$ of $NaCl$ (molar mass $58.5 g mo l^{- 1}$ , assume complete dissociation) is dissolved in $180 g$ of water (molar mass $18 g mo l^{- 1}$ ). The pure-solvent vapour pressure is $760$ mmHg. The lowering of vapour pressure (mmHg) is closest to

Answer: C

NaCl

gives

0.100 \times 2 = 0.200

mol of particles; water

= 10.0

mol.

x_{2} = 0.200/10.2 = 0.01961

Δ P = x_{2} P^{0} = 0.01961 \times 760 = 14.9

mmHg. Ignoring dissociation gives 7.52, and 745.1 is the final pressure rather than the lowering.