Normality
The concentration of a solution could also be expressed in terms of Normality. It is based on an alternate chemical unit of mass called the equivalent weight. The normality of a solution is the concentration expressed as the number of equivalent weights (equivalents) of solute per liter of solution. In a chemical mixture 1 normal (1 N) solution contains 1 equivalent weight of solute per liter of solution. Since normality simplifies the calculations required for chemical concentration, it is widely used in analytical chemistry. Every substance may be assigned an equivalent weight. The equivalent weight may be equal to the formula weight (molecular weight, mole weight) of the substance or equal to an integral fraction of the formula weight (i.e., molecular weight divided by 2, 3, 4, and so on). The above phenomenon could be better explained with the following example to gain an understanding of the meaning of equivalent weight:
HCl(aq) + NaOH(aq) ⎯→ NaCl(aq) + H2O
1 mole 1 mole
(36.5 grams) (40.0 grams)
H2SO4(aq) + 2NaOH(aq) ⎯→ Na2SO4(aq) + 2H2O
1 mole 1 mole
(98.1 grams) (80.0 grams)
In the above chemical reaction 1 mole of hydrochloric acid (HCl) reacts with 1 mole of sodium hydroxide (NaOH) and 1 mole of sulfuric acid (H2SO4) reacts with 2 moles of NaOH. If you made 1 molar solutions of these substances, 1 liter of 1 M HCl will react with 1 liter of 1 M NaOH and 1 liter of 1 M H2SO4 will react with 2 liters of 1 M NaOH. Therefore, H2SO4 has twice the chemical capacity of HCl when reacting with NaOH. The equivalent weight of HCl is equal to its molecular weight, but that of H2SO4 is ½ its molecular weight. Expressions for normality are shown below. Notice the similarity to molar solution definition.
Normality (N) = Number of equivalents of solute / 1 liter of solution = Equivalents / liter
where Number of equivalents of solute = grams of solute / equivalent weight of solute
finally N = grams of solute /eq wt solute × L = grams / solution eq wt × L
So, 1 liter of solution containing 36.5 grams of HCl would be 1 N, and 1 liter of solution containing 49.0 grams of H2SO4 would also be 1 N. A solution containing 98.1 grams of H2SO4 (1 mole) per liter would be 2 N.
QUESTIONS
1. …………………. based on an alternate chemical unit of mass called the equivalent weight
(a) Normality (b) Molarity (c) Percent (d) Molality
2. One mole of …………………. reacts with 1 mole of sodium hydroxide (NaOH)
3. One mole of sulfuric acid (H2SO4) reacts with …………………. moles of NaOH
4. The equivalent weight of HCl is equal to its ………………….
