chemistry
posted by cheri on .
How many grams of sucrose (C12H22O11)
must be dissolved in 200 g water to make
a solution in which the mole fraction of su
crose is 0.1?
Answer in units of g

200 g H2O is how many mols? That's g/molar mass.
total mol = n sucrose + n H2O
n sucrose = grams/molar mass sucrose
[n sucrose/(n sucroswe + nH2O)] = 0.1
Substitute grams/molar mass for n sucrose and solve for grams. 
i got 11.098 for the moles for the first part. so do i add .1 to that?

No. Also if I divide 200/18.015 = 11.102 which is more s.f. than allowed but I carry them and drop at the end of the problem. You can't add them. 11.1 is mols H2O. The 0.1 is not mols sucrose; that 0.1 is the mole fraction of sucrose.
You must solve this.
[(x/342)/(x/342)+(11.1)] = 0.1 and solve for x = grams sucrose (Note: x/342 = mols sucrose)