Question

A vitamin c tablet containing 250 mg of ascorbic acid (c6h8o6; ka=8.0×10−5) is dissolved in a 250 ml glass of water. what is the ph of the solution?

Answer 1

First, we have to get the initial [C6H8O6] = mass/molar mass
 
when the molar mass of C6H8O6 = 176.12 g/mol

∴[C6H8O6] = 0.25 g / 176.12 g/mol 
                    = 0.00142 M

when 
               C6H8O6 ⇄  H+   +  C6H7O6-
intial       0.00142 M       0           0
change    -X                   +X          +X
Equ       (0.00142-X)         X            X
 

so, Ka = [H+][C6H7O6-] / [C6H8O6]

by substitution:

8 x 10^-5 = X * X / (0.00142-X) by solving this equation for X

∴ X = 0.000299 
∴[H+] = 0.000299

∴PH = -㏒[H+]
     
        = -㏒ 0.000299
        = 3.52