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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