Question

The volume of 0.025 M ${{H}_{3}}P{{O}_{4}}$ require to neutralize 25 ml of 0.003 M $Ca{{(OH)}_{2}}$ is:
A. 40 ml
B. 20 ml
C. 10 ml
D. 80 ml

Answer 1

Hint: The balanced chemical neutralization reaction of phosphoric acid with calcium hydroxide is as follows.
\[2{{H}_{3}}P{{O}_{4}}+3Ca{{(OH)}_{2}}\to C{{a}_{3}}{{(P{{O}_{4}})}_{2}}+6{{H}_{2}}O\]
- In the above chemical reaction 2 moles of phosphoric acid reacts with three moles of calcium hydroxide and forms one mole of calcium phosphate and six moles of water as the products.

Complete Solution :
- In the question it is given that to find the volume of 0.025 M phosphoric acid to neutralize 25 ml of 0.003 M calcium hydroxide.
- The neutralization reaction of phosphoric acid with calcium hydroxide is as follows.
\[2{{H}_{3}}P{{O}_{4}}+3Ca{{(OH)}_{2}}\to C{{a}_{3}}{{(P{{O}_{4}})}_{2}}+6{{H}_{2}}O\]
- Number of moles of phosphoric acid in the above reaction = 2 moles
- Number of moles of calcium hydroxide in the above reaction = 3 moles
- There is a formula to calculate the number of moles of 0.025 M phosphoric acid and it is as follows.
Number of moles = (Volume in Liter) (Molarity)
- In the question it is given that the volume of calcium hydroxide is 25 ml = 0.025 L
- Number of moles of calcium hydroxide = (0.025 L) (0.03) = $7.5\times {{10}^{-4}}moles$
- Number of moles of phosphoric acid required to react with $7.5\times {{10}^{-4}}moles$ of calcium hydroxide= $\frac{2}{3}\times 7.5\times {{10}^{-4}}moles=5\times {{10}^{-4}}$
- So $5\times {{10}^{-4}}$ moles phosphoric required to react with calcium hydroxide.
- Then the volume of the phosphoric acid is $\begin{align}
  & \frac{5\times {{10}^{-4}}}{0.025}=0.02L=20ml \\
 & \\
\end{align}$
- Therefore 20 ml of 0.025M of phosphoric acid is required to neutralize 25 ml of 0.03 M calcium hydroxide.
So, the correct answer is “Option B”.

Note: To know the volume of the acid is required to neutralize the certain volume of base we should know the balanced chemical reaction, then only we can calculate the volume of the acid required to neutralize the certain volume of base.