1 mole of a pure substance has a mass equal to its molecular mass expressed in grams.
This is known as the molar mass, M, and has the units g mol-1
So 2 moles of a substance would have a mass = 2 x molar mass
3 moles of a substance would have a mass = 3 x molar mass etc
This leads to the formula: mass = moles x molar mass
If we let:
m = mass of substance in grams,
n = moles of pure substance,
M = molar mass of the pure substance in g mol-1
we can write the equation: m = n x M
This equation can be rearranged to give the following:
n = m ÷ M (moles = mass ÷ molar mass)
M = m ÷ n (molar mass = mass ÷ moles)