finding mass or volume from percent concentration

Question

To reduce dangerous swelling in the brain, physicians sometimes administer intravenous hypertonic saline, a 7.0 % (w/v) solution of salt NaCl in water. Calculate the mass of NaCl in 35.0 mL of this solution. Be sure your answer has a unit symbol and is rounded to the correct number of significant digits.

Answer 1

7.0% (w/v) means 7.0 g NaCl per 100 mls of solution.

To make 35.0 mls of this solution, we calculate the mass of NaCl as follows:

7.0 g NaCl / 100 mls soln x 35.0 mls soln = 245 g NaCl

Since 7.0 g has 2 sig. figs., we should report our answer as 250 g NaCl