Summary

• 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 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 (n=moles of pure substance, MM=molecular mass of the pure substance, mass in grams )
• MM = mass ÷ n (n=moles of pure substance, MM=molecular mass of the pure substance, mass in grams )

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