Question

How many moles of magnesium phosphate $M{g_3}{\left( {P{O_4}} \right)_2}$ will contain 0.25 moles of oxygen atoms?
(A) $3.125 \times {10^{ - 2}}$
(B) $1.25 \times {10^{ - 2}}$
(C) $2.5 \times {10^{ - 2}}$
(D) $2 \times {10^{ - 2}}$

Answer 1

Hint: Try to recall that one mole is defined as the amount of a substance that contains as many atoms, molecules, ions, electrons or other elementary particles as there are atoms in exactly 12g of Carbon-12. Now, by using this you can easily find the correct option from the given option.

Complete step by step solution:

• It is known to you that 1 mole of magnesium phosphate $M{g_3}{\left( {P{O_4}} \right)_2}$ contains 3 mole of magnesium (Mg) atoms, 2 mole of phosphorus atoms and 8 moles of oxygen atoms and 8 mole of oxygen (O) atoms.
• Calculation:
  1 mole of magnesium phosphate ($M{g_3}{\left( {P{O_4}} \right)_2}$) contains 8 moles of oxygen atoms
  Or, we can say that 8 moles of oxygen atoms are present in 1 mole of magnesium phosphate ($M{g_3}{\left( {P{O_4}} \right)_2}$).
  So, 1 mole of oxygen atoms is present $\dfrac{1}{8} = 0.125$ mole of magnesium phosphate ($M{g_3}{\left( {P{O_4}} \right)_2}$).
  Therefore, 0.25 mole of oxygen atom is present in $0.125 \times 0.25 = 0.03125 = 3.125 \times {10^{ - 2}}$ moles of magnesium phosphate ($M{g_3}{\left( {P{O_4}} \right)_2}$).
  Therefore, from above calculations we can easily conclude that option A is the correct option to the given question.

Note:

• It should be remembered to you that Avogadro’s number and mole concept help in the chemical calculation of the number of atoms or molecules in a given mass of the element or the compound.
• Also, you should remember that it also helps in the calculation of the number of molecules present in a given volume of the gas under given conditions.