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pH calculation of strong acid

A strong acid completely ionizes in water. For example, HCl ionizes completely to give the same concentration of H+ and Cl ions.
 
HCl H+ + Cl
 
Let c be the concentration of HCl. The concentration of H+ obtained from HCl would also be c. Therefore,
 
pH = –log c
 
10–1 M HCl, [H+] = 10–1 M; pH = log 10–1 = 1
 
10–2 M HCl, [H+] = 10–2 M; pH = –log 10–2 = 2
 
[H3O+] from acid is less than 10–6 M, the contribution of H3O+ from water should be taken into account for calculating pH, while if it is greater than equal to 10–6 M, then [H3O+] contribution from water can be ignored.

pH calculation of weak acid

Weak acid is that which dissociates partially in water and soon comes in equilibrium with its ions. For example,
 
Description: 21777.png
 
The equilibrium constant is called the dissociation or ionization constant (Ka).
 
Description: 21785.png
CH3COOH + H2O CH3COOH + H3O+
Description: 21794.png
∴ Description: 21801.png
Description: 21808.png

pH calculation of solution of a mixture of strong acid and weak acid (both monoprotic) in water

Let the strong acid and weak acid (both monoprotic) taken be HA and HB, respectively. Their respective concentrations are c1 and c2 M. In the presence of strong acid (HA), the degree of dissociation of weak acid (HB) would be suppressed due to common ion effect. Let the degree of dissociation of HB in the presence of HA be α and its ionization constant be Ka. The various equilibria are as follows:
 

For the equilibrium of HB,
 
Description: 21818.png
......(i)
 
Using the values of Ka, c1, and c2, α can be calculated from Eq. (i).
 
After obtaining the value of α, [H3O+] can be calculated as [H3O+]T = c1 + c2α.
 
Finally, if the total [H3O+] from acid is less than 10–6 M, the contribution of H3O+ from water should be taken into account for calculating pH, while if it is greater than equal to 10–6 M, then [H3O+] contribution from water can be ignored.
 
Using this [H3O+], pH of the solution can be calculated.

pH calculation of bases

The pH calculation of bases involves similar methods as we have done for acids. In these cases, instead of H3O+, we will first calculate OH (using Kb, concentrations, etc.) and then pOH. Now pH is calculated using pH + pOH = pKw = 14.
 
For calculating pH of bases, remember that [OH] from water needs to be considered only if the [OH] from base is less than 10–6 M and if [OH] from base is greater than 10–6 M, the [OH] from water can be ignored.
 
For weak bases, first calculate the value of α from the expression Description: 21835.png If this value of α comes out to be less than or equal to 0.1, then the assumption is valid and [OH] is calculated using the expression [OH] = Description: 21842.png. However, if the α value obtained from the given expression is greater than 1, then the assumption is not valid and one has to calculate α using the expression Description: 21850.png, and then calculate [OH] using the expression [OH] = .




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