Question

Phenolphthalein has a pKa of 9.7 and is colorless in its acid form and pink in its basic form.

Answer 1

Answer:- $\frac{[In^-]}{[HIn]}=3.16*10^-^8$

Solution:- If both acid and basic form are present at the same time then it's like a buffer solution since the buffer solution also has a weak acid and its conjugate base. The Handerson equation is used for solving buffer problems. Let's use the same equation here also.

The equation is:

$pH=pKa+log(\frac{base}{acid})$

pKa is given as 9.7 and the pH is 2.2. let's plug in the values in the equation.

$2.2=9.7+log(\frac{[In^-]}{[HIn]})$

subtract 9.7 from sides:

$2.2-9.7=log(\frac{[In^-]}{[HIn]})$

$-7.5=log(\frac{[In^-]}{[HIn]})$

taking antilog:

$10^-^7^.^5=\frac{[In^-]}{[HIn]}$

$3.16*10^-^8=\frac{[In^-]}{[HIn]}$

So, the answer is $3.16*10^-^8$ .