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1.28 Moles and mountains


 1.28 Calculating moles

Calculating the number of moles knowing the mass and the Ar ( mass of one mole) 

The following masses of elements all contain one mole of atoms:

  • 12.0 g Carbon, 
  • 32.1 g Sulphur, 
  • 14 g Nitrogen, 
  • 24.3 g Magnesium. 

So if you have 24 g carbon you have 24/12 = 2 moles carbon atoms.

Likewise if you have 1.4 g nitrogen you have 1.4/14 = 0.1 moles nitrogen atoms.


 Rearranging things

 The mass of one mole ("MOOM")is the relative atomic number for an element. If the substance is a compound you will need to work out its relative molecular mass or relative formula mass.

The relationship between the mass in g ("MIG") of a sample, the number of moles ("NOM") of substance an the mass of  one mole ("MOOM") of a substance  can be shown as a triangle:

Cover up the quantity you want to find to give the appropriate formula for calculating that quantity.

 Activity 1. Moles from mass

Watch the short video to see how the number of moles can be calculated from the mass of the sample and the mass of one mole (Ar) . Use the same method to calculate the number of moles in the five samples given on the questions tab below. Use the answers tab to check your calculations:

How many moles of atoms are there in :
  1. 355g of chlorine 
  2. 0.69g of lithium
  3. 8.0g of oxygen
  4. 35g of copper
  5. one tonne of lead 

There are:

  1. 10
  2. 0.1
  3. 0.5
  4. 0.55
  5. 4826.25

moles of atoms

 Activity 2. Mass from moles

Watch the short video to see how the mass of a sample can be calculated from the number of moles and the mass of one mole (Ar) . Use the same method to calculate the mass in the ten samples given on the questions tab below. Use the answers tab to check your calculations: 

What is the mass of:

  1. 2.5 moles of oxygen atoms?
  2. 0.15 moles boron atoms
  3. 5 moles of aluminium atoms
  4. 0.4 moles of sulfur atoms
  5. 50 moles of oxygen atoms
  6. 1 mole of water molecules
  7. 0.5 moles of ethanol 
  8. 4 moles of glucose 
  9. 2 moles of sodium chloride
  10. 2 moles of carbon dioxide molecules

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The masses are:

  1. 2.5 x 16 = 40g
  2. 0.15 x 10.8 = 1.62g
  3. 5 x 27 = 135g
  4. 0.4 x 32.1 = 12.84g
  5. 50 x 16 = 800g
  6. 1 x 18 = 18g
  7. 0.5 x 46 =  23g
  8. 4 x 180 = 720g
  9. 2 x 58.5 = 117g
  10. 2 x 44 = 88g

           Water        
      Glucose         
       Ethanol         
  Sodium Chloride
    Carbon dioxide

 Activity 3. Matching moles and masses

When you have successfully completed the calculations in activities 1 and 2 you should be able to test your skill using the quizlet below:

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