Question

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

For pH= 2.2 calculate [In−]/[HIn].

Answer 1

Answer:-
`([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((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(([In^-])/([HIn]))`

subtract 9.7 from sides:

`2.2-9.7=log(([In^-])/([HIn]))`

`-7.5=log(([In^-])/([HIn]))`

taking antilog:

`10^-^7^.^5=([In^-])/([HIn])`

`3.16*10^-^8=([In^-])/([HIn])`

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