Question 29 of 50advanced🔧 ApplyNumerical5 marks
A solution containing 30 g of non-volatile solute exactly in 90 g of water has a vapour pressure of 2.8 kPa at 298 K. Further, 18 g of water is then added to the solution and the new vapour pressure becomes 2.9 kPa at 298 K. Calculate:
(i)
molar mass of the solute
Answer
23 g mol⁻¹
(ii)
vapour pressure of water at 298 K.
Answer
3.53 kPa
Explanation
This problem applies Raoult's law for solutions containing non-volatile solutes. The vapour pressure of the solution depends on the mole fraction of the solvent. By setting up two equations for the two different solutions (before and after adding water), we can solve simultaneously for both the molar mass of the solute and the vapour pressure of pure water.