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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