How to calculate concentration in moles/ liter (molarity)?

In pH calculations the concentration used in any equation is in moles/ liter (molarity). However, the concentration might be given in units of mass/ volume of solution (e.g. g/ ml, g/ 100ml, mg/ ml......etc). In order to calculate molarity  we need to know the molecular weight of the dissolved compound. The molecular weight can be viewed as the weight (mass) in grams of 1 mole of the compound.

For example,  acetic acid has a molecular weight of 60.05 g/ mole. If we have a solution with a concentration of 0.01 g/ ml of acetic acid then the number of moles in each milliliter (ml) can be calculated as shown in the following equations:

So each milliliter (ml) of the above solution contains 0.000167 moles of acetic acid. To obtain molarity we need to calculate the number of moles in 1 liter (1000 ml) of solution as shown in the following equations:

Note that similar units on the nominator and on the denominator are cancelled.

Molarity is commonly used in chemistry work because it expresses the number of molecules of the compound contained in a certain volume.