1.19 carry out mole calculations using relative atomic mass (Ar) and relative formula mass (Mr)

Method 1: Calculating the mass from amount (i.e. the number of moles) 

Mass of substance (g) = Amount (moles) x RFM (Mr) 

Worked example: Water (H20) with a molar amount of 0.5

  1. RFM hydrogen = (1 x 2) = 2
  2. RFM oxygen = 16 
  3. RFM H20 = 16 + 2 = 18
  4. Molar amount = 0.5 
  5. Mass of substance = 0.5 x 18 = 9g

Method 2: Calculating amount from mass

Amount (moles) = mass of substance (g) / RFM (Mr or relative formula mass) 

Worked example: 80g of Sodium hydroxide (NaOH)

  1. RFM Na = 23
  2. RFM H = 1
  3. RFM O = 16 
  4. RFM NaOH = 23 + 16 + 1 = 40
  5. Mass of substance  = 80g
  6. Molar amount = 80/40 = 2 moles

Method 3: Calculations with atoms of elements 

Worked example: 32 g of Oxygen atoms (O2)

32g oxygen = 32 / 16 (RFM) = 2 mol of oxygen atoms

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