# Homework Help: Percent by mass concentration

1. Jan 8, 2009

% by mass concentration

1. The problem statement, all variables and given/known data

An aqueous CaCL2 solution has a vapor pressure of 80.8mmHG at 50 degree C . The vapor pressure of pure water at this temperature is 92.6 mmHG .

What is the concentration of CaCl2 in mass percent?

2. Relevant equations

% by mass = mass solute/mass solution.

3. The attempt at a solution

I tried converting the pressure into moles then dividing the moles of CaCl2 by the total moles. Im pretty stuck..

Thanks

2. Jan 8, 2009

### Staff: Mentor

Re: % by mass concentration

Do you know Raoult's law?

3. Jan 8, 2009

Re: % by mass concentration

yeah i do. p = pa*xa+pb*xb correct? not sure how to solve for the x's....

4. Jan 8, 2009

### Staff: Mentor

Re: % by mass concentration

xa+xb=1

5. Jan 9, 2009

Re: % by mass concentration

alright knowing this, i use 'a' as the vap press for cacl2 and 'b' for vap press of water? then use molecular weights to to solve for total mass of solution..?

6. Jan 9, 2009

### Staff: Mentor

Re: % by mass concentration

Sounds OK. Don't forget CaCl2 is dissociated.

7. Jan 9, 2009