Question

Find the $pH$ of the resulting solution and then mark the option in which $pH$ exists between color transition range of an indicator.

50 ml of 0.2M HA solution $(K_a={10}^{-5})$ + 50ml of 0.1M HCl solution + 100 ml of 0.13 M NaOH solution.

• A. Phenol red (6.8 to 8.4)
• B. Propyl red (4.6 to 6.4)
• C. Phenolphthalein (8.3 to 10.1)
• D. Malachite green (11.4 to 13)

Answer 1

The correct option is B Propyl red (4.6 to 6.4)

Milimoles of HCl = Volume (ml) $\times$ Molarity = $50\times 0.1=5$ milimoles.
Milimoles of NaOH $=100\times 0.13=13$ milimoles.

Some of the NaOH will be neutralized by HCl.
Milimoles of NaOH remaining unreacted $13-5=8$ milimoles.

Milimoles of HA$=50\times 0.2=10$ milimoles.
Out of this 8 milimoles will be neutralized to form a salt.

Thus the solution now contains 2 milimoles of weak acid HA and 8 milimoles of its salt with strong base.

It is an acidic buffer solution.

The expression for the pH of the acidic buffer solution is as given below.

$pH=p{K}_a+log\frac{\left[salt\right]}{\left[acid\right]}$

$p{K}_a=-log{10}^{-5}=5$

$pH=5+log\frac{8}{2}=5.6$.

Thus the suitable indicator is propyl red with pH range 4.6 to 6.4.