Posted by Jillian on .
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 :(

Chemistry :( 
DrBob222,
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. 
Chemistry :( 
Devron,
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. 
Chemistry :( 
Devron,
Did not know that you were posting an answer.

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

Chemistry :( 
Devron,
Excuse my typos as well.