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MOLARITY (M)
Specifies the number of moles of solute in one litre (1000 ml) of solution. A mole of any substance contains 6.02 × 10²³ (Avagadros number) particles. Thus 1 mole of NaCl contains the same number of molecules as 1 mole of sucrose, and therefore 1M solutions of NaCl and sucrose will contain equivalent numbers of NaCl and sucrose molecules.
To determine the mass of dissolved solute from the molar concentration we use the equation
moles = M × V
where M is the molar concentration of the solution and V is the volume in litres. To determine the mass of solute from the number of moles we use the following formula which describes the relationship between moles and mass of a substance
moles = mass (in grams) ÷ formula weight
The formula weight of a substance is simply the sum of the atomic weights of all elements in the substance. Thus the formula weight of NaCl is 58.5 (23.0 for Na + 35.5 for Cl), and 1 mole of NaCl weighs 58.5 grams.
Example
A 1.5 M sucrose solution contains 1.5 moles of sucrose per litre of solution. Since the formular weight of sucrose is 342.3, this is equivalent to 1.5 × 342.3 = 513.5 grams.