Question

what is the vapor pressure of an aqueous solution made up of 60.0g of urea (CH4N2O) in 180g of water? the vapor pressure of water at this temperature is 0.038atm.
please answer truthfully and explain how you got this answer because i dont know and my head hurts.
also the choices are
0.020atm
0.0025atm
0.035atm
0.00055atm

Answer 1

To get the answer you use the Law of Raoult.


Raoult's law states that the decrease of the vapor pressure of a liquid is proportional to the molar fraction of the solute.


ΔP = Pa * Xa


Here Pa = 0.038 atm


And Xa = N a / (Na + Nb), where Na is number of moles of A and Nb is number of moles of b


Na = mass of urea / molar mass of urea =  60 g / (molar mass of CH4N2O)


molar mass of CH4N2O = 12 g/mol + 4*1g/mol + 2*14 g/mol + 16 g/mol = 60 g/mol


Na = 60 g / 60 g/mol = 1 mol


Nb = mass of water / molar mass of water = 180g / 18g/mol = 10 mol


Xa = 1 mol / (10 mol + 1 mol) = 1/11 =0.09091


ΔP = Pb * Xa = 0.038 atm * 0.09091 =  0.0035 atm


Then, the final vapor pressure of water is Pb - ΔP = 0.038atm - 0.0035atm = 0.035 atm.


 Answer: 0.035 atm