Question

The percentage of hydrogen in ${{H}_{2}}O$ is:
A. 44.45
B. 5.55
C. 88.89
D. 11.11

Answer 1

Hint: Always remember when only percentage is asked and it is not specified what percentage then we always assume that we are asked the percentage of mass of the given species. So, here in this question we basically have to find the percentage mass of hydrogen present in 1 water molecule.

Complete step by step solution:
-We know that whenever we are asked the percentage of any quantity, we have to divide the value of that particular quantity with the total value of all the quantities existing at that point. So, to find the percentage of hydrogen in water, we need to divide the mass of hydrogen by that of water.
-Also, the number of atoms forming the molecule is important because they will add the weight in the molecule. There are 2 hydrogen atoms present in 1 molecule of water and we need to consider it.
-Now, percent of hydrogen in water will be given as $\dfrac{\text{mass of hydrogen in 1 molecule of water}}{\text{total mass of 1 molecule of water}}$
There are 2 atoms in water – oxygen and hydrogen. So we need to add the masses of these atoms to get the molecular mass of water. Thus the molecular mass of water will be 1x2 + 16x1 = 18g
-The weight of hydrogen will be 1x2=2g as there are 2 hydrogen atoms present in 1 molecule of water. So now we get the percentage of hydrogen as $\dfrac{2}{18}x100=11.11%$

Therefore the correct option is D. 11.11

Additional information:
-The mass of an atom cannot be measured truly due to its very small size. So a relative term is used for it called relative atomic mass. Every atom has a mass which is related to hydrogen atom mass. Standard mass of hydrogen is 1 unit and to find the mass of any other atom, relative atomic mass is used.
\[Relative\text{ }atomic\text{ }mass\text{ }=\text{ }\dfrac{mass\text{ of 1 atom of element}}{\text{mass of 1 hydrogen atom}}\]
-Today a different unit is used which is based on the mass of 1 carbon-12 atom. It was adopted internationally in 1961 and according to it, relative atomic is given by
\[Relative\text{ }atomic\text{ }mass\text{ }=\text{ }\dfrac{mass\text{ of 1 atom of element}}{\dfrac{1}{12}\text{x mass of 1 C-12 atom}}\]


Note: If we are specified about what value we have to find, then we find the percentage of that quantity only. When nothing is specified, only then we find the
percentage of masses of the molecules or the compounds. Also we need to see the molecular formula of the compound before calculating the masses because the mass of the atoms gets multiplied by the number of the atoms present in the compound.