Percent Composition Problems

2-32. PERCENT COMPOSITION PROBLEMS

At times it may be necessary to prepare a percent solution that requires only part

of the entire molecule to be considered in the preparation of the solution. For example,

you may wish to prepare a 2.5 g/dL sodium (Na+) solution from NaCl. Since the entire

molecule is used in the preparation of the solution, ratio and proportion will have to be

used, based on the molecular weight of the entire molecule versus the molecular weight

of the portion of the molecule being considered.

a. Example 1. How much NaCl is required to prepare 500 mL of a 2.5 g/dL

sodium solution?

Solution. Read the problem carefully and determine the desired quantity.

Grams of NaCl.

Calculate the amount of sodium needed.

1 dL

2.5 g

500 mL X -------- X ------= 12.5 g

100 mL

1 dL

Calculate the gram molecular weight of each substance. (See Appendix C.)

23.0

Na 23.0 g/mol

+ 35.5

58.5 g/mol

Use ratio and proportion to determine the amount of available form needed.

23.0 g/mol Na

12.5 g Na

-------------- = ----------

58.5 g/mol NaCl

x g NaCl

(12.5 g Na) (58.5 g/mol NaCl)

x g NaCl = --------------------------

23.0 g/mol Na

x g NaCl = 31.8 g

MD0837

2-17