Question

Calculate the mass of urea that should be dissolved in 221 g water at 35 C to produce a solution with the vapor pressure of 37.5 mmhg. At 35 C, Pnull water is 42.2 mmg.

Answer 1

According to Raoult’s law:

P = Po*X(solvent)

37.5 = 42.2*X(solvent)

X(solvent) = 0.889

This is mole fraction of water

mass(water)= 221 g

number of mol of water,

n = mass of water/molar mass of water

=(221.0 g)/(18 g/mol)

= 12.28 mol

X(water) = n(water)/( n(water) + n(urea))

0.889 = 12.28 / ( 12.28+n(urea))

10.91+0.889*n(urea) = 12.28

0.889*n(urea) = 1.367

n(urea) = 1.539 mol

molar mass of urea = 60.06 g/mol

mass of urea = molar mass * number of moles

= 60.06 g/mol * 1.539 mol

= 92.4 g

Answer: 92.4 g