Question 9.8: How many grams of NaCl are needed to prepare 250 mL of a 1.5...
How many grams of NaCl are needed to prepare 250 mL of a 1.5% (w/v) saline solution?
ANALYSIS We are given a concentration and a volume, and we need to find the mass of solute by rearranging the equation for (w/v)% concentration.
BALLPARK ESTIMATE The desired (w/v)% value, 1.5%, is between 1 and 2%. For a volume of 250 mL, we would need 2.5 g of solute for a 1% (w/v) solution and 5.0 g of solute for a 2% solution. Thus, for our 1.5% solution, we need a mass midway between 2.5 and 5.0 g, or about 3.8 g.
Since (w/v)\% = \frac{Mass of solute in g}{Volume of solution in mL} \times 100\%
then Mass of solute in g = \frac{(Volume of solution in mL) [(w/v)]\%}{100\%} \\ =\frac{(250)(1.5\%)}{100\%}= 3.75 g = 3.8 g NaCl \\ (2 significant figures)
BALLPARK CHECK: The calculated answer matches our estimate.