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Molarity, Molality, or Normality? (A Quick Review)

(A Quick Review)

Molarity

Molarity (M) is defined as the number of moles of solute per liter of solution.

molarity = moles of solute / liters of solution

Molality

Molality (m) is defined as the number of moles of solute per kilogram of solvent.

molality = moles of solute / kilograms of solvent

Although their spellings are similar, molarity and molality cannot be interchanged. Compare the molar and molal volumes of 1 mol of solute dissolved in CCl4 (d = 1.59 g / mL)

By definition, a 1 M solution would contain 1 mol of solute in exactly 1.00 L of CCl4 , and a 1 m solution would contain 1 mol of solute in 629 mL of CCl4 .

1 kg of CCl4 x  (1000 g /  1 kg) x (mL / 1.59 g) = 629 mL CCl4

Normality

Normality (N) is another ratio that relates the amount of solute to the total volume of solution.

It is defined as the number of equivalents per liter of solution:

normality = number of equivalents / 1 L of solution

There is a very simple relationship between normality and molarity:

N = n × M (where n is an integer)

For an acid solution, n

is the number of H+ provided by a formula unit of acid.

example:

A 3 M H2SO4 solution is the same as a 6 N H2SO4 solution.

For a basic solution, n

is the number of OH- provided by a formula unit of base.

example:

A 1 M Ca(OH)2 solution is the same as a 2N Ca(OH)2 solution.

Remember!The normality of a solution is NEVER less

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