Question

Find the pH of a solution made by dissolving 0.195 g of phenol (C6H5OH) in enough water to make 125 mL of solution.

Answer 1

Find the pH of a solution made by dissolving 0.195 g of phenol (C6H5OH) in enough water to make 125 mL of solution.

Solution :-

Lets first calculate the moles of phenol

Moles of phenol = 0.195 g / 94.111 g per mol = 0.002072 mol

Molarity of phenol = moles / volume in liter

                                  = 0.002072 mol / 0.125 L

                                  = 0.0166 M

Now lets calculate the concentration of the H3O+ using the ka of the phenol

Ka of phenol = 1.3*10^-10

C6H5OH + H2O ------ > C6H5O-   + H3O+

0.0166                                 0                 0

-x                                         +x                +x

0.0166-x                               x                    x

Ka=[H3O+][C6H5O-]/[C6H5OH]

1.3*10^-10 = [x][x]/[0.0166-x]

Since the ka is very small therefore we can neglect the x from denominator

So we get

1.3*10^-10 = x^2 / 0.0166

1.3*10^-10 * 0.0166 = x^2

2.158*10^-12 = x^2

taking square root of both sides we get

1.47*10^-6 = x=[H3O+]

pH= -log [H3O+]

pH = - log [1.47*10^-6 ]

pH= 5.83