Question

When $20mL$ of $\frac{M}{10}NaOH$ are added to $10mL$ of $\frac{M}{10}HCl$, the resulting solution will:

• A. turn blue litmus red
• B. turn phenolphthalein solution pink
• C. turn methyl orange red
• D. have no effect on either red or blue litmus

Answer 1

The correct option is B turn phenolphthalein solution pink

$20$ ml of $\frac{M}{10}NaOH=2\times {10}^{-3}$ moles of $NaOH$

$10$ ml of $\frac{M}{10}HCl=1\times {10}^{-3}$ moles of $HCl$.

Excess amount of $NaOH=(2\times {10}^{-3}-1\times {10}^{-3})$ moles

$=1\times {10}^{-3}$ moles

$\therefore$ $\left[{OH}^{-}\right]=\frac{{10}^{-3}\times 1000}{30}=0.033$

$\therefore$ $pH=14-pOH=12.49$

In this $pH$, Phenolphthalein solution turns into pink.

Answer will be $B$.