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.)
Na
23.0
Na 23.0 g/mol
Cl
+ 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