Question

How many moles of magnesium phosphate \[\text{M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\], will contain \[0.25\]moles of oxygen atoms?
A. \[0.02\]
B. \[3.125\times {{10}^{-2}}\]
C. \[1.25\times {{10}^{-2}}\]
D. \[2.5\times {{10}^{-2}}\]

Answer 1

Hint: In order to calculate mole of oxygen we first calculate the number of moles of oxygen in one mole of \[\text{M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\] then we will calculate the moles of \[\text{M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\] in 0.25 mole of oxygen.
Complete step by step solution:
Magnesium phosphate is \[\text{M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\], which means magnesium phosphate has 3 moles of magnesium (Mg), 2 moles of phosphorus (P) and 8 moles of oxygen (O).
So 1 mole of \[\text{M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\], contains & 8 moles of oxygen.
Given \[0.25\] moles of oxygen atoms
\[\text{1mole M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\to \text{8 moles oxygen}\]
\[\text{x moles M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\to 0.25\text{ moles oxygen}\]
Number of moles of \[\text{M}{{\text{g}}_{3}}{{(P{{O}_{4}})}_{2}}\] having \[0.25\] moles of oxygen atoms be x.
Therefore x can be calculated as,
\[\text{x=}\dfrac{\text{0}\text{.25 }\!\!\times\!\!\text{ 1moles oxygen}}{\text{8 moles oxygen}}\]
\[\text{x = 0}\text{.03125}\]
That means moles\[0.03125\] of magnesium phosphate contains \[0.25\] moles of oxygen atoms.

Note: By knowing the moles of any element in 1 mole of the compound, we can find the number of moles of the compound having n moles of the element.