pKₐ
- The range of values of Ka is very large and for weak acids, the values themselves are very small numbers
Table of Ka values
- For this reason it is easier to work with another term called pKa
- The pKa is the negative log of the Ka value, so the concept is analogous to converting [H+] into pH values
pKa = -logKa
- Looking at the pKa values for the same acids:
Table of pKa values
- The range of pKa values for most weak acids lies between 3 and 7
Worked example
Finding Ka and pKa
At 298 K, a solution of 0.100 mol dm-3 ethanoic acid has a hydrogen ion concentration of 1.32 x 10-3 mol dm-3. Calculate the Ka & pKa of the acid.
Answer
Step 1: Write down the equation for the partial dissociation of ethanoic acid
CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)
Step 2: Write down the equilibrium expression to find Ka
Step 3: Simplify the expression
The ratio of H+ to CH3COO- is 1:1
The concentration of H+ and CH3COO- is, therefore, the same
The equilibrium expression can be simplified to:
Step 4: Substitute the values into the expression to find Ka
= 1.74 x 10-5
Step 5: Determine the units of Ka
= mol dm-3
The value of Ka is therefore 1.74 x 10-5 mol dm-3
Step 6: Find pKa
pKa = - log10Ka
= - log10 (1.74 x 10-5)
= 4.76
