Before you can solve Normality and Molarity problems, you have to understand the definitions of these terms. Molarity is the gram molecular weight (GMW) per liter (L) of solution, while Normality is the gram equivalent weight per liter of solution. The difference between the two is that valence is involved in Normality but not in Molarity.
You have also to remember these principles:
· When the valence is one, Molarity is equal to Normality.
· Molarity may be equal but is always lesser than Normality.
· Normality may be equal but is always greater than Molarity.
· Convert your volume to liters before using in the formula.
Here are ANSWERS to the sample problems:
1. If you weigh 60 grams of NaCl (sodium chloride) and dissolved it in 1 liter of solution:
a. What is the Molarity of the solution?
M= GMW/L ; since GMW = W/MW,
W = 60 grams and MW = 23 + 35.5 = 58.5,
Substitute now the values:
60 = is your weight in grams
58.5 = is your Molecular Weight (MW)
1 = is your volume in liters
M = 1.026
b. What is the Normality of the solution?
Since the valence is one the Normality is equal to the Molarity.
Hence, Normality = 1.026
c. What is the percent solution?
The formula for percent is:
% = [W (in grams of solute)/total volume of solution (milliliters)] X 100,
% = (60/1,000) X 100
% = 6 %
2. Compute for the Normality of a 2M H2SO4 solution.
Use the short cut formula N = M X Valence
N = 2 (Molar) X 2 (Valence)
N = 2 X 2
N = 4
3. What is the Molarity of a 10% CaCl2 solution?
Use the formula:
M = % X 10/GMW
M = 10 X 10/111
M = 0.90
4. Compute for the Molarity of a 0.5N NaCl solution.
Since the Normality is given, use the short cut formula, which is:
M = N/Valence
M = 0.5/1
M = 0.5
You could also use the principle that if the valence is 1, the Molarity is equivalent to Normality, hence, since the Normality is = 0.5, the Molarity is also 0.5 because the valence is 1.
5. What is the Normality of a 1.5 M HCl solution?
You can use the same formula and principle above.
Hence, N = 1.5 also because the valence is 1.