Question

At a temperature of 3°C, the vapor pressure of a 500g solution of water is 5.7 mmHg. If 150g of of HCl is added, what is the pressure of the solution?

Answer 1

This task is based on Raoul’s law: $(P_A^0 -P_A)/P_A^0=X_B$ , $P_A^0$ - vapor pressure of initial solution, $P_A$ - vapor pressure of solution after addition any substance, $X_B$ - mole fraction of the subdtance.

$X_B=n_B/\sum n$, n_B - chemical amount of substance (HCl), n -total amount of all constituents in a solution.

n=m/M , n - chemical amount of substance(mol); m - mass of substance(g); M - molar mass of substance(g/mol). M(H₂O)=16G/mol+1*2g/mol=18g/mol; M(HCl)=35,5g/mol+1g/mol=36,5g/mol.

n(H₂O)=500g/18g/mol =27,8mol

n(HCl)=150g/36,5g/mol=4,1mol

X_B=4,1/(4,1+27,8)=0,13

Express the needed pressure: $P_A=P_A^0(1-X_B)$

P_A=5,7mmHg(1-0,13)=4,96mmHg

Answer: 4,96mmHg.