Question

When $20\;mL$ of $M/20$ $NaOH$ is added to $10\;mL$ of $M/10$ $HCl$ , the resulting solution will be what?
(A) Turn blue litmus red
(B) Turn phenolphthalein solution pink
(C) Turns methyl orange red
(D) Will have no effect on either red or blue litmus

Answer 1

$NaOH$ is a base and $HCl$ is an acid. When an acid and a base react, the resulting solution will either be acidic, basic, or neutral depending upon the concentration of the $H^+$ and $OH^-$ ions released by them in the solution. If the solution is acidic, it will turn blue litmus paper red or methyl orange solution red. If the solution is basic, it will turn the phenolphthalein solution pink, and if the solution is neutral, it will not affect red or blue litmus paper.

Complete step by step answer:
A base is a substance that releases $OH^-$ ions in a solution, and acid is a substance that releases $H^+$ ions in a solution. On the reaction of an acid and a base, if the number of $OH^-$ ions is more, then the solution will be basic. If the number of $H^+$ ions is more, then the solution will be acidic, and if the number of both $OH^-$ ions and $H^+$ ions are equal, then the solution will be neutral.
Given,
Volume of $NaOH = 20\ mL$

Molarity or concentration of $NaOH = \dfrac{1}{20}$

Volume of $HCl = 10\ mL$

Molarity or concentration of $HCl = \dfrac{1}{10}$

The formula for molarity is given as:

$Molarity = \dfrac{Number\ of\ moles}{Volume\ of\ the\ solution\ in\ litres}$

From the above formula, we get the number of moles of a compound as:

$Number\ of\ moles = Molarity\times Volume\ in\ litres$

Therefore, the number of moles of $NaOH = \dfrac{1}{20} \dfrac{mol}{L}\times \dfrac{20}{1000}L$

$\Rightarrow$ Number of moles of $NaOH = 0.001$

Similarly, for $HCl$:

Number of moles of $HCl = \dfrac{1}{10} \dfrac{mol}{L}\times \dfrac{10}{1000}L$

$\Rightarrow$ Number of moles of $HCl = 0.001$

Since the number of moles of both $NaOH$ and $HCl$ is equal, they will release an equal number of $OH^-$ ions and $H^+$ ions in the solution and will neutralize each other. Therefore, the solution formed will be neutral, and it will show no effect on either red litmus or blue litmus. The overall reaction can be represented as follows:

$NaOH + HCl \rightarrow NaCl + H_2O$

So, the correct answer is Option D .

Note: To save time, students can omit the conversion of $mL$ into $L$ as the volume of both $NaOH$ and $HCl$ is given in $mL$, and we are concerned only about finding whether the number of moles of both acid and base is exactly equal or not. So, we could also proceed with the solution as follows:
Number of moles of $NaOH = \dfrac{1}{20}\times 20$
$\Rightarrow$ Number of moles of $NaOH = 1$
Number of moles of $HCl = \dfrac{1}{10}\times 10$
$\Rightarrow$ Number of moles of $HCl = 1$
Here also, we find the number of moles of $NaOH$ and $HCl$ to be equal.