Question

What percentage of oxygen is present in washing soda?
A) 72.72%
B) 32%
C) 18%
D) 9%

Answer 1

Hint: We know that the formula of washing soda is ${\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}.10{{\rm{H}}_{\rm{2}}}{\rm{O}}$ .Here, first we have to calculate the mass of oxygen in the washing soda. Then we have to use the following formula to calculate percentage of oxygen, that is $\% \,{\rm{oxygen}} = \dfrac{{{\rm{Mass}}\,{\rm{of}}\,{\rm{oxygen}}}}{{{\rm{Mass}}\,{\rm{of}}\,{\rm{washing}}\,{\rm{soda}}}} \times 100$.

Complete step by step answer:
Let’s first calculate the mass of oxygen. We know that the atomic mass of oxygen is 16 g/mol.

Mass of oxygen=$3 \times 16 + 10 \times 16 = 48 + 160 = 208\,{\rm{g}}$


Now, we have to calculate the mass of the molar mass of the washing soda. The atomic mass of sodium (Na) is 23, atomic mass of carbon is 12 and the atomic mass of hydrogen is 1.

 ${\rm{Mass}}\,{\rm{of}}\,{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}.10{{\rm{H}}_{\rm{2}}}{\rm{O}} = {\rm{23}} \times {\rm{2 + 12 + 13}} \times {\rm{16 + 20}} \times {\rm{1 = 46 + 12 + 208 + 20 = 286}}\,{\rm{g}}$


Now, we have to calculate the percentage of hydrogen.

$\% \,{\rm{oxygen}} = \dfrac{{{\rm{Mass}}\,{\rm{of}}\,{\rm{oxygen}}}}{{{\rm{Mass}}\,{\rm{of}}\,{\rm{washing}}\,{\rm{soda}}}} \times 100$

$ \Rightarrow \% \,{\rm{of}}\,{\rm{oxygen}} = \dfrac{{208}}{{286}} \times 100 = 72.72\% $


Therefore, the oxygen percentage in washing soda is 72.72%.

So, the correct answer is Option A .

Additional Information:
Washing soda is a white crystalline solid of molecular formula ${\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}.10{{\rm{H}}_{\rm{2}}}{\rm{O}}$. It is also known by the name of ‘soda crystals’ and soda ‘ash’. It is used for cleaning purposes mainly in washing clothes. It is also none of the constituents of many dry soap powders. It is also used in removal of hardness (temporary and permanent) of water. So, washing is known as a water softening agent. Also, in manufacturing of borax washing soda is used.

Note: Always remember that, percentage composition of a compound is the relative mass of each of the constituent elements in 100 parts of it by mass. The molecular mass of a compound can be calculated by adding the atomic masses of all atoms which constitute the molecule of that compound