Find Mass of element from the mass of a compound

Step 1: Find the Molar Mass of the given compound?

To do this, you must need to know its chemical formula of the compound. Now to calculate its molar mass, we add up all of the molar masses of each atom:

For example: H2O is the compound

Molar Mass [M] = (1.01 x 2 g/mol) + (1 x 16 g/mol) = 18.02 g/mol

Step 2: Find the Mass of the element in 1 mol of the compound

In each molecule of the compound (for example water H2O), there is 2 molecules of hydrogen atoms and 1 molecule of oxygen.

For example, if the question ask you to find the molecule of oxygen (element) in water (compound). This means that in 1 mol of water, there are 1 mol of oxygen. Therefore the mass of oxygen in 1 mol of adrenaline is: 1 mol×16 g/ mol=16 g.


Step 3: Calculate Percentage Composition of the element (i.e. Oxygen) by Mass

We now know that in every mole of the compound (i.e. water), there are 16 g of oxygen. We also know that 1 mol of the compound (i.e. water) weighs 18.02 g (from step 1). Therefore in 18.02 g of water, there are 16 g of oxygen. All have to do is simply divide by them by each other and multiply by 100 to get the percentage composition of oxygen. Therefore the percentage composition is 78.9%.

\(\%\ composition\ =\ \frac{mass\ of\ the\ element\ in\ g}{mass\ of\ the\ compound\ in\ g}x100\)


Step 4: Calculate Mass of the element (i.e. oxygen) in Sample

Now we know the percentage composition of oxygen which is 78.9%. This means that in every gram of  the compound (i.e. water), there is 0.789 g of the element (i.e.oxygen). Therefore to find the mass of oxygen in the sample:
m of the element, oxygen=0.789×(grma given in the question of the element) g=# g