One more that should be easy but im stuck. What is the molarity of a 15% by weight solution of HCL (Density = 1.18g/ml)?

Answer 1

density = 1.18 g/mL.
mass of 1,000 mL =
1.18 g/mL x 1000 mL = 1180 grams soln.
How much of that is HCl?
1180 g x 0.15 = xx g HCl (the remainder is water or 1180 - xx =?? but normally you never need this).
How many mols is that in the soln? xx gHCl/molar mass HCl = yy mols HCl.
yymols HCl/1 L soln = yy M.
Again, just remember the definition of molarity. It's M = mols/L. So you know to start with a L of soln and determine the mols HCl in that. I hope this helps.