Molar solutions (M)

A Molar solution (M) is a solution that contains 1 mole of solute in each litre of solution. A mole is the molecular weight (MW) expressed as grams (sometimes referred to as the ‘gram molecular weight’ (gMW)). Therefore, 1 M = 1 gMW of solute per litre of solution.

For example, how much sodium chloride is needed to make 1 litre of a 1 M solution?

First, we find out the molecular weight (MW) of sodium chloride....

The chemical formula for sodium chloride is NaCl. In simple terms, this means that a ‘molecule’ of sodium chloride contains one sodium atom and one chlorine atom.

The atomic weight of sodium (Na) is 23
The atomic weight of chlorine (Cl) is 35.5

So the molecular weight of sodium chloride (NaCl) is:
Na (23) + Cl (35.5) = NaCl (58.5)

Therefore, a 1 M solution of sodium chloride contains 58.5 grams of sodium chloride in 1 litre of solution.

Similarly, a 2M solution contains 117 grams of sodium chloride per litre (MW x 2).

And, a 0.1M solution contains 5.85 grams/litre of sodium chloride (MW x 0.1).

Normal solutions (N)

A Normal solution (N) is a solution that contains 1 ‘gram equivalent weight’ (gEW) of solute per litre of solution. The gram equivalent weight is equal to the molecular weight expressed as grams divided by the ‘valency’ of the solute.

To understand valency, consider the following acids:

Hydrochloric acid (HCl) has one replaceable hydrogen ion (H), sulphuric acid (H2SO4) has two replaceable hydrogen ions (H2) and phosphoric acid (H3PO4) has three replaceable hydrogen ions (H3). The valencies of these acids are determined by their respective replaceable hydrogen ions:

HCl, Valency = 1
H2SO4, Valency = 2
H3PO4, Valency = 3

So, for 1N HCl the MW is 36.5, the EW is 36.5 and therefore 1N would correspond to 36.5 grams/litre.

For 1N H2SO4 the MW is 98, the EW is 98/2 = 49 (that is, valency = 2) and so a 1 N solution would be 49 grams/litre.

Similarly, for 1N H3PO4 the MW is 98, the EW is 98/3 = 32.7 and 1N would be 32.7 grams/litre.

The relationship between Molar and Normal solutions is therefore determined by the number of replaceable ions in the solute molecule, that is;

for 1 replaceable ion M = N
for 2 replaceable ions M = 2N
for 3 replaceable ions M = 3N

In other words, M divided by valency = N

1N hydrochloric acid = 1M
1N sulphuric acid = 0.5M
1N phosphoric acid = 0.33M

If this looks complicated don’t worry. Normal solutions are little used these days. The general practice in chemical analysis, such as titrimetry, is to use the simpler Molar solutions. And just to help you, we print the Normal equivalent on the labels of our ROMIL-TvR™ Traceable Volumetric Reagents.