Chemistry :(
posted by Jillian .
Calculate the molality, % percent composition by weight, and the mole fraction of HCl in an aqueous 12M HCl solution that has a density of 1.20 g?mL
Please help :(

12M HCl means 12 mols HCl/L solution.
Use density to convert L solution to grams. That is approximately 1200 grams (water + HCl). You know how much the HCl weighs; it is 12 mols or 12*36.5 = about 440g. Therefore, the water must have a mass of 1200440 = about 762 or 0.762 kg.
molality = mols/kg solvent.
I'm confused about what you want in % composition. If you mean % HCl it is
(mass HCl/mass 1L)*100 = ? . However, when one asks for %composition one USUALLY means %H, %Cl.
For mols fraction,
You have mols HCl. Convert g solvent to mols. Then XHCl = nHCl/total mols.
Post your work if you get stuck. You need to confirm each step in the above since I've estimated here and there. 
Assume 12 moles of HCL per liter:
To solve for the molarity first, compute the total weight of the solution. Since, molarity is equal to moles/liter, assume that you have a total volume of 1L. 1L converted to mL is equal to 1.00 x10 ^3 mL. This multiplied by the density is equal to a 1.20 x10^3 g. Since you now know the amount of moles in your solution and the total weight, you can figure out the molality (moles/weight in Kg); convert g to kg before computing the answer.
Assume 12 moles of HCL per liter:
12 moles*36.46g/mole = the number of grams of HCl per liter.
Divide your answer for grams obtained for HCl by the total weight. Make sure that you are dividing g/g or Kg/Kg to obtain %w/w.
Assuming the 12M HCl solution is made with H2O, subtract the weight of HCl that you obtained from the total weight; this gives you the number of grams of H20. Since the molecular weight of H2O is 18g/mole, divide the weight that you obtained for H20 by the molecular weight for H2O (g of H2O/18g) to obtain the number of moles of H20. To get the mole fraction divide the moles of HCl by the moles of HCL plus the moles of H2O.
moles of HCL/(moles of HCL +moles of H20) = mole fraction. 
Did not know that you were posting an answer.

Correction, you have to subtract the grams of HCL from the total weight before calculating molality.

Excuse my typos as well.