Homework Help: Chemistry

Posted by Jon on Friday, March 26, 2010 at 5:29pm.

A solution is made by dissolving 0.100 mol of NaCl in 5.35 mol of water. What is the mass % of NaCl?

So far, I have determined molecular weight of NaCl (58.44 g/mol) multiplied by 0.100 mol = 5.844g NaCl, and mw of H2O (18 g/mol) = 96g H20.
I thought mass solute/mass solution (5.8/102 = .057 or 5.7%) but my homework program says incorrect. Am I doing this problem wrong?

Chemistry - DrBob222, Friday, March 26, 2010 at 5:51pm
I think one problem may be that you are reporting only to two significant figures and you are allowed three.
I agree with 5.844 g NaCl. For water I have 5.35 x 18.015 = 96.38 g.
Total mass = 96.38 + 5.844 = 102.22 and
(5.844/102.22)*100 = 5.7168 which rounds to 5.72 mass percent. The other thing I do when I work these problems is (or don't do) is to round between calculations; i.e., I leave the number in the calculator with all of those dangling numbers and wait until I finish the calculation to round to the correct number of s.f.

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