• 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

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

How are?Ka?values found?

• If we neutralise a weak acid with a strong base such sodium hydroxide the equation for the reaction is:

HA (aq) + NaOH (aq) → NaA (aq) + H2O (l)

HA (aq) + OH-?(aq) → A-?(aq) + H2O (l)

• When half of the acid has been neutralised the concentration of [HA] is equal to the concentration of [A-]
• Since the acid is poorly dissociated it is assumed all the A-?comes from the product rather than HA dissociating
• This simplifies the expression to

• In practice, to find?Ka?the pH of a weak acid is measured as it is neutralised by a strong base and a graph is plotted for pH versus volume of base added
• This type of graph is known as a?pH titration curve
• A vertical tie-line is drawn to the curve at half the volume of base needed for neutralisation
• At this point??pKa?= pH, so by reading off the pH value from the y-axis, you find the?Ka?of the acid

Finding the Ka?of a weak acid from a pH titration curve