Question

What is the percent composition by mass of oxygen in magnesium oxide?

Answer 1

Hint : As we know the formula for atomic composition of magnesium oxide i.e., MgO and then it would be easy for us to find the mass composition with the help of atomic masses of the element.

Complete Step By Step Answer:
As we know that the magnesium oxide has formula MgO which tells us that every sample of substance there is the same number of magnesium and oxygen atoms.
We know the atomic masses of the magnesium and oxygen from periodic tables i.e., $24\cdot 3$ grams and $16$ grams respectively. From their atomic weight we came to know that Mg atoms are heavier i.e., having more mass composition than O atoms.
When we take just one atom of Mg and O it combines to form MgO molecule having Molecular weight of MgO is $16+24\cdot 3=40\cdot 3$ gram which represents the minimum amount of magnesium oxide (as we take least amount i.e., one atom).
Percentage composition by mass of any compound =molar mass of atom $\times 100$ /molar mass of the complete molecule.
Percentage composition by mass of oxygen (O)= molar mass of oxygen $\times 100$ /molecular mass of the molecule (MgO). $\Rightarrow$ $16\times 100/40\cdot 3=39\cdot 70$ % in mass of oxygen(O).
Therefore, $39\cdot 70$ by mass oxygen is present in magnesium oxide (MgO).

Note :
To find the percentage composition you may know a few things like: molecular mass of the atoms/elements and how to calculate the molecular mass of unknown compounds, you have to memorize the formula of percentage composition and how to apply it in the question. By this information you can calculate percentage composition by mass easily.