Question

How would you calculate the percentage by mass of oxygen in a compound?

Answer 1

Hint:The percentage composition of the element in the compound means how much mass of the given element is present from the total molar mass of the compound. In this question we are asked to calculate the percentage mass of oxygen in compound. We can find the percentage mass of oxygen by the formula
$%\text{ oxygen = }\dfrac{\text{mass of oxygen}}{\text{molar mass of compound}}$
 Also, we know that the molar mass of compound can easily be calculated by adding masses of individual atoms involved.

Complete step-by-step answer:Percentage composition of the compound is the relative mass of each of the constituent elements in $100$parts of it. It tells us about the mass of each element forming a respective compound.
In this question we are asked to find out the percentage composition of the oxygen in a compound.
Percentage by mass of oxygen of a compound means how much amount is oxygen in the total compound. For instance, if we say 100 g of a compound, then the percentage composition of oxygen means how much grams of oxygen is constituted in the compound out of the complete $100$gram.
We can calculate the percentage of mass of oxygen in any compound by using the above formula.
$%\text{ oxygen = }\dfrac{\text{mass of oxygen}}{\text{molar mass of compound}}$
Let us understand this with the help of an example.
Let us take an example of water $\left( {{H}_{2}}O \right)$it contains oxygen and hydrogen. If we want to calculate the percentage by mass of the oxygen in water, we can calculate it using the above relation.
We know that one mole of water contains $16$g of oxygen and molar mass of water is $18$g/mol
So, if we calculate the mass percentage of oxygen in water we get,
$%\text{ oxygen = }\dfrac{16}{18}\times 100=88.8%$

Hence, the percentage mass of oxygen in water is $88.8%$

Hence, we can find the mass percentage of oxygen using the above expression.

Note: note that the percentage composition of all the elements in the compound is always equal to $100%$. In a binary compound if the mass percentage of one element is known then the mass percentage of the other can be simply calculated by subtracting the mass percentage of the first element from $100$.