AP Chemistry
posted by Matt on .
How many grams of sucrose (C12H22O11)
must be dissolved in 60 g water to make a
solution in which the mole fraction of sucrose is 0.4?
Answer in units of g.

60 g H2O = ? mols. mol = grams/molar mass.
mols sucrose = grams/molar mass.
Xsucrose = mols sucrose/total mols.
You know Xsucrose. Substitute that into X formula and solve for g sucrose. Post your work if you get stuck. 
I don't really understand how to set it up. I'm to figure out my moles for water but after that I'm stuck.

mols H2O = 60g/18 = 3.33
mols sucrose = n
total mol = 3.33 + n
Xsucrose = mols sucrose/(total mols)
0.4 = [n/(n+3.33)]
Solve for n = mols sucrose.
Then mols sucrose = grams sucrose/molar mass sucrose.
You know mols sucrose from the above calculation. You know molar mass sucrose (about 342), solve for grams and you have it.