Question

How to find the actual percent composition of magnesium oxide $MgO$?

Answer 1

Hint: In any compound it is the percentage of one atom through which it will be formed. We can solve it by considering an amount of one atom in total mass of the compound and then converting it in percentage by multiplying with $100$ . Use molar mass of each component like here in magnesium oxide use molar mass of magnesium to find its actual percent composition and for oxygen its molar mass.


Complete step by step answer:
We know that when different atoms combine with each other to form a molecule, this process is formed by heating, compressing, cooling etc. there are many conditions that we can apply to form a compound. Magnesium oxide is formed when we burn magnesium ribbon in excess of oxygen. It will show a white bright glow and burns wholly leaving a greyish matter that is called magnesium oxide.
We have to find out the actual percentage of this compound, so for this we have to know what actual percent composition is, it means how much amount in grams we have for $100g$ of compound. It means that in $100g$ of $MgO$ how much gram of oxygen and magnesium is present.
One mole of $MgO$ means $40.30g\,$ magnesium oxide which is further made up of one mole of magnesium and one mole of oxygen. One mole of magnesium means $24.30\,g$ of magnesium and one mole of oxygen means $15.99g$ of oxygen.
Actual percentage composition of magnesium in magnesium oxide is the amount of it in that compound so with the help of this formula, we can easily calculate the actual percentage composition.
$Percentage\,\,composition\,\,of\,A = \dfrac{{Mass\,of\,A}}{{Total\,mass\,of\,compound}}$
For magnesium lets calculate, $Percentage\,\,composition\,\,of\,Mg = \dfrac{{Mass\,of\,Mg}}{{Total\,mass\,of\,compound\,MgO}} \times 100$
$Percentage\,\,composition\,\,of\,Mg = \dfrac{{24.30g}}{{40.30g}} \times \,100$ = $60.34\% $
Similarly we can find out the actual percent composition of oxygen by dividing its mass with total mass of magnesium oxide.
$Percentage\,\,composition\,\,of\,O = \dfrac{{Mass\,of\,O}}{{Total\,mass\,of\,compound\,MgO}} \times 100$
$Percentage\,\,composition\,\,of\,Mg = \dfrac{{15.99g}}{{40.30g}} \times \,100$ = $39.69\% $
With the knowledge of actual percent composition, we can tell that in $100g$ of $MgO$ there is $60.34\% $ of magnesium and $39.69\% $ of oxygen.


Note:Take molar mass of oxygen as $15.99\,or\,16$ instead of $32$ because in the resulted compound that is in magnesium oxide only oxygen $O$ is present with magnesium and not ${O_2}$ .Don’t forget to multiply the amount with $100$ to convert it in percentage.