Solutions - Fundamentals of Chemistry - page 411

Solution

3.00 g NaCl
100.0 g solution

46.6 g solution a b 1.40 g NaCl

Practice Problem 15.3 Calculate the mass of NaCl required to make
a 3.00% by mass saline solution with 46.6 g of water.

EXAMPLE 15.4

State exactly how to prepare 200.0 g of 4.000% by mass NaCl solution.

Solution

First calculate the mass of NaCl required:

4.000 g NaCl
100.0 g solution

200.0 g solution a b 8.000 g NaCl

The mass of water required is thus 200.0 g solution
water. Therefore we dissolve 8.000 g NaCl in 192.0 g water to make our
solution.

8.000 g NaCl 192.0 g

Often solutions are prepared by dilution of more concentrated solutions of
the same solute. (These dilution problems are similar to those involving molar-
ity, presented in Chapter 11.) When we dilute a solution, the mass of the solute
does not change, but the masses of the solvent and the solution do, and thus the
percentage of each component changes.

EXAMPLE 15.5

Calculate the mass of 10.00% stock solution required to prepare 0.8000 kg of
4.000% solution.

Solution

For the final solution:

a 1000 g solution
1 kg solution

4.000 g solute
100.0 g solution

0.8000 kg solution b a b 32.00 g solute

For the initial solution:

a 100.0 g solution
10.00 g solute

32.00 g solute b 320.0 g solution

Thus 320.0 g of 10.00% stock solution (containing 32.00 g of solute) is diluted
with

800.0 g final solution 320.0 g initial solution 480.0 g water