Since many problems are stated in terms of the weight of solute and require you to
determine the number of gram molecular weights (moles), the following formula will be
of benefit:
weight of solute
No. of GMWs =
GMW
(3) Example. What is the molarity of a solution containing 29.25 grams of
sodium chloride in 500 ml. of total solution?
Step 1. Find the number of GMWs.
GMW of NaCl = 58.4 grams
weight of solute
No. of GMWs =
GMW
29.25
No. of GMWs =
58.4
= 0.5
Step 2.
Find the molarity.
no. of GMWs of solute
Molarity =
no. of liters of solution
500 ml = 0.5 liter
Molarity =
0.5 =
1 molar or 1M
0.5
c. Milligram Equivalent Weight (Milliequivalent Weight). Sometimes we are
interested in more than just the weight ratios of reacting compounds. Since the valence
of an element is a measure of that element's combining power, the valences in a
compound should be indicative of their reactivity. Therefore, chemists have modified the
milligram formula weight to include the positive or negative valence of a compound.
This value is called the milligram equivalent weight and is defined as the milligram
molecular weight divided by the total positive or negative valence. Consider, for
example, sodium hydroxide:
Milligram molecular weight = 40 mg
Total positive valence = 1
Milligram equivalent weight = 40 mg = 40 mg
1
MD0803
2-9