Mass and amount in moles

Amount in moles (symbol n) cannot be measured directly; however the amount in moles of substance (and therefore the number of formula units) present in a given sample can be calculated if the mass (m) of the sample is known.

The mass in grams (m) can be calculated from an amount in moles (n) using the rearranged form of the equation at the left:

m = n x M
g = mol x g mol^–1

To calculate amount in moles (n) from the mass in grams (m), divide the mass in grams by the mass of one mole in grams (molar mass -  symbol M)

n	=	m
		M

mol	=	g
		g mol-1

• Note how in both cases units cancel to give the desired unit. This is a good check on whether you applied the equation correctly.
• It is usual to put the formula of the substance in brackets after each of the above symbols as a particular problem may involve the masses, amounts in moles or molar masses of several different substances.