question_answer 31)

A solution containing 30 g of a non-volatile solute exactly in 90 g water has a vapour pressure of 2.8 kPa at 298 K. Further 18 g of water is then added to the solution, the new vapour pressure becomes 2.9 kPa at 298 K. Calculate (i) molecular mass of the solute (ii) vapour pressure of water at 298 K.  

Answer:

Using Raoult's law for dilute solutions. In 1st case;                ?. (i) In 2nd case                ?.. (ii) Dividing (i) by (ii)   V.P. of pure water at 298 K, P° = (2.9 × 6 ? 2.8 × 5) k Pa = (17.4 ? 14) k Pa = 3.4 k Pa Substituting the value of p° in (i),