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Mass-Mole Calculations (n = mass ÷ MM)

Key Concepts

1 mole of a pure substance has a mass equal to its molecular mass (MM)
So 2 moles would have a mass = 2 x MM
3 moles would have a mass = 3 x MM etc
This leads to the formula: mass = n x MM
mass is in grams,
n = moles of pure substance,
MM = molecular mass of the pure substance
This formula can be rearranged to give the following:
n = mass ÷ MM
MM = mass ÷ n

Examples

1. mass = n x MM

Calculate the mass of 0.25 moles of water

mass (in grams) = n x MM
n = 0.25mol
MM of water (H₂O) = (2 x 1.008) + 16.00 = 18.016g mol^-1
mass = 0.25 x 18.016 = 4.504g

2. n = mass ÷ MM

Calculate the moles in 124.5g of oxygen gas.

n = mass ÷ MM
mass = 124.5g
MM of O₂ = 2 x 16.00 =32.00g mol^-1
n = 124.5 ÷ 32.00 = 3.89mol

3. MM = mass ÷ n

Calculate the molecular mass of a pure substance if 1.75 moles of the substance has a mass of 29.792g

MM = mass ÷ n
mass = 29.792g
n = 1.75mol
MM = 29.792 ÷ 1.75 = 17.024g mol^-1

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