Question

A sample of ammonium phosphate ${{\left( N{{H}_{4}} \right)}_{3}}P{{O}_{4}}$, contains 3.18 mole of hydrogen atoms. The number of moles of oxygen atoms in the sample is:
(A) 0.265
(B) 0.795
(C) 1.06
(D) 3.1

Answer 1

From the molecular formula of ammonium phosphate it’s clear that one mole of ammonium phosphate has 12 moles of hydrogen and 4 moles of oxygen. In the given sample the number of mole of hydrogen is 3.18 and by finding how much grams of oxygen is present in the sample and by dividing it with molecular mass, we could find the number of moles of oxygen.

Complete step by step answer:
-The molecular weight of ammonium phosphate is 149 g and since the atomic weight of hydrogen is one g in ${{\left( N{{H}_{4}} \right)}_{3}}P{{O}_{4}}$ there is 12 hydrogen atoms and thus ammonium phosphate contains 12 g of hydrogen.
- It’s given that the sample contains 3.18 moles of hydrogen. From the concept of moles 3.18 moles of hydrogen contains 3.18 g of hydrogen. Hence we can write as follows
3.18 g of hydrogen present in $\dfrac{149\times 3.18}{12}$ = 39.5 g of ammonium phosphate

- Next we are going to see the composition of oxygen in ammonium phosphate. 149 gram of ammonium phosphate contains 64 g of oxygen. This is because in ammonium phosphate there are four oxygen atoms and the weight of oxygen is 16 g. Therefore we can write as follows
39.5 g of ${{\left( N{{H}_{4}} \right)}_{3}}P{{O}_{4}}$ will contain $\dfrac{64\times 39.5}{149}$ g of oxygen = 16.96 g of oxygen

This means that 16.96 g of oxygen is present in the sample. The number of moles in a given sample can be calculated by dividing the given mass by molar mass. Hence we can write as
$number\text{ }of\text{ }moles\text{ of Oxygen}=\dfrac{16.96}{16}=1.06 moles$
Thus, the number of moles of oxygen atoms in the sample is 1.06.
So, the correct answer is “Option C”.

Note: The answer can be found in an easier way in which the ratio of hydrogen and oxygen in ammonium phosphate is taken. There is 12 H atoms and 4 oxygen atoms in ${{\left( N{{H}_{4}} \right)}_{3}}P{{O}_{4}}$. Thus the ratio between them is $3:1$. Hence by dividing the number of moles of hydrogen 3.18 by 3 we will get the number of moles of oxygen as 1.06.