Question

What is the percentage composition of hydrogen and oxygen in water $\left( {{\text{H}}_{2}}\text{O} \right)$?
(H=1, O=16)
(A) 5, 95
(B) 11.1, 88.9
(C) 15, 85
(D) 20,80

Answer 1

Find the molar mass of the water molecule. Then find the molar mass of hydrogen and oxygen present in the water molecule individually based on their no. of atoms in the molecule. Then to find percentage compositions divide the molar mass of hydrogen and oxygen respectively with the total molar mass of water molecules.

Complete Step by Step Solution
Molar mass of${{\text{H}}_{2}}\text{O}=$molar mass of hydrogen$+$molar mass of oxygen
$\Rightarrow 1\times 2+16\times 1$(2hydrogen atoms are present and 1 of oxygen)
$\Rightarrow 2+16$
$\Rightarrow 18$ g/mol
Molar mass of hydrogen in water molecule $=1\times 2=2$ g (two atoms of hydrogen are present and molar mass of one atom of hydrogen is 1)
So,
percentage composition of hydrogen in water molecule $=\dfrac{\text{molar}\,\text{mass}\,\text{of}\,\text{hydrogen}}{\text{total}\,\text{molar}\,\text{mass}\,\text{of}\,\text{water}\,\text{molecule}}\times 100$
$\Rightarrow \dfrac{2}{18}\times 100$
$\Rightarrow \dfrac{100}{9}$
$\Rightarrow 11\cdot 1$
Molar mass of oxygen in water molecule $=16\times 1=16g$ (only one oxygen atom is present)
So percentage composition of oxygen in water molecule $=\dfrac{\text{molar mass of oxygen}}{\text{total molar mass of water molecule}}\times 100$
$\Rightarrow \dfrac{16}{18}\times 100$
$\Rightarrow \dfrac{8}{9}\times 100$
$\Rightarrow \dfrac{800}{9}$
$\Rightarrow 88\cdot 9$
Therefore the percentage compositions of hydrogen and oxygen are$11\cdot 1$and$88\cdot 9$respectively.
So, the correct choice is (B).

Additional Information
Molar mass is the mass of 1 mole of the element or compound in grams. One mole contains Avogadro no. particles in a substance which can be atoms, molecules or ions. The percentage composition states how much percentage of the particular element is contained in the molecule of the compound formed from them.

Note
The molar masses of various compounds are found by adding the molar masses of their individual components or elements together. To find the molar masses one should study the atomic number of various elements from the periodic table. The atomic mass or molar mass for an element is the sum of its atomic no. and the no. of neutrons present in it.