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

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