Question

The ratio of hydrogen and oxygen by mass in water is:
A. 1 : 8
B. 8 : 1
C. 2 : 1
D. 1 : 2

Answer 1

Consider the number of hydrogen and oxygen atoms that constitute one molecule of water and also the atomic masses of hydrogen and oxygen.

Complete answer:
We know that to calculate the ratio, we need to consider the number of hydrogen atoms, multiply them by the mass of one hydrogen atom and then divide that value by the value that we obtain after doing the same for the oxygen atoms in water
$\text{Ratio = }\dfrac{(\text{no}\text{. of hydrogen atoms}\times \text{mass of hydrogen atom})}{(\text{no}\text{. of oxygen atoms}\times \text{mass of oxygen atom})}$
We know that the molecular formula of water is ${{H}_{2}}O$. It has 2 hydrogen atoms and 1 oxygen atom.
No. of hydrogen atoms = 2
No. of oxygen atoms = 1
The relative atomic masses of both the elements are as follows:
Mass of hydrogen atom = 1
Mass of oxygen atom = 16
Now plugging these values in the equation given above, we get:
$\text{Ratio = }\dfrac{(2\times 1)}{(1\times 16)}$
$\text{Ratio = }\dfrac{1}{8}$
So, the correct answer is “Option A”.

Additional Information: The relative atomic masses are calculated relative to the mass of one hydrogen atom. So, the mass of hydrogen is used as a reference point to determine the atomic masses of other elements. Sometimes, carbon may also be used as a reference point due to its abundance in nature.

Note: Please do not get confused while considering what to take as the numerator and the denominator in the ratio. The element that is mentioned first should be considered as the numerator. Make sure that you consider the atomic masses while calculating and not the atomic numbers.