You see wheather an aqueous solution is acidic,neutral, or basic depends on the hydrogen ion concentration.The hydrogen ion concentration in aqueous solution ranges from enxtremely high to extremely low.Because these concentration values may be very small,it is often more conveinent to give acidity in terms of pH.
pH is defined as the negative of the logarithum of the molar ion concentration:
pH = - log [H+] or pH = -log[H3O+]
pH of the solution is obtained byh taking the logarithum of the hydronium ion concentration and then then changing the sign .For example,the pH of the pure water is
pH = -log[H+] = - log (1.0 x 10^-7 )
= - (- 7.00) = 7.00
The reverse conversation from pH to H+ or H3O+ uses the power of 10.For example, lemon juice has a pH of about 2.0 :
[H+] = 10^-pH = 10 ^-2.0 = 1 x 10-2 M
A logarithum carries no units, but the result of this calculation is a concentration in mol/L or M
Lets see few solved examples:
Sample problem 1 : A sample of orange juice has a hydrogen ion concentration of 2.9 x 10^-4 M.What is the pH ?
Step 1 : pH is the negative log of the molar hydrogen ion concentration .Determine the log by using either an electronic calculator or log tables.
Step 2 : On a calculator ,enter the H+ concentration , press log, and change the sign of the result.You should write the pH with the same number of decimal places as there are significant figures in [H+].
Step 3 :Now, using the formula solve the
pH = - log [H+] = -log (2.9 x 10^- 4) = 3.54