Percentage Mass

We found out about the Relative Atomic Mass and Relative Formula Mass last lesson.

Remember, the number at the top of the symbol is the Relative Atomic Mass (A_r ):

Hydrogen	Carbon	Oxygen	Sodium	Magnesium
Sulphur	Chlorine	Potassium	Calcium	Copper

 

What are the Relative Atomic Masses of these elements:

oxygen ?

copper ?

hydrogen ?

calcium ?

sodium ?

sulphur ?

chlorine ?

carbon ?

potassium ?

magnesium ?

You scored out of 10 on that test

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Calculating Percentage Mass

To work out the percentage mass of an element in a molecule you need to divide the relative atomic mass of the element by the relative formula mass of the molecule.
You then multiply the answer by 100.

 

What is the percentage mass of Mg in MgO ?

the relative atomic mass of Mg is 24
the relative formula mass of MgO is 40

24/40 x 100 = 60 %

 

Use a calculator to answer these questions:

What is the Relative Atomic Mass of oxygen ?

What is the Relative Formula Mass of MgO ?

What is the Percentage Mass of O in MgO ?

What is the Relative Atomic Mass of sodium ?

What is the Relative Formula Mass of NaCl ?

What is the Percentage Mass of Na in NaCl ?

What is the Relative Atomic Mass of hydrogen ?

What is the Relative Formula Mass of NaOH ?

What is the Percentage Mass of O in NaOH ?

What is the Percentage Mass of H in NaOH ?

You scored out of 10 on that test