If you added an equal volume of a strong alkali to a weak acid (e.g. vinegar pH=4) what pH would you expect to get?
Vinegar is basically a dilute solution of acetic acid in water. Acetic acid has the chemical formula CH3COOH and the terminal hydrogen atom is weakly acidic. You stated that the pH of vinegar is 4 which is in the mildly acidic realm. If we add a solution of strong alkali (basically a strong base like NaOH), it will neutralize the acetic acid according to the chemical equation below:
CH3COOH + NaOH --> CH3COONa + H2O
If by equal volumes you mean an equal number of moles of both acetic acid and alkali, then all of the acetic acid and base will be neutralized. If the mixture were to stay like this, then the pH would be 7, or completely neutral. But the sodium acetate conjugate base of acetic acid (CH3COONa) will establish an equilibrium with water to produce a small amount of acetic acid, thus the final pH of the resulting solution will be higher than 4 but slightly less than 7.
The reaction of solutions of acid and base can be generally represented as:
Acid + Base = Salt + Water
Acid contains an H that can be protonated producing the conjugate base
Base as an alkali base, has an alkali metal combined with a hydroxide.
In your question, the reaction of equal volume of vinegar with an alkali base can be represented as:
`CH_3 COOH + MOH = CH_3 COO^(-) M^(+) + H_2 O `
MOH is the Alkali base and;
CH3COOH is Acetic acid, the weak acid present in vinegars.
The concentration of the product CH3COO-M+ is dependent on the concentrations of the alkali base and the acid. 1 mole of the base reacting with 1 mole of the acid will produce 1 mole of CH3COO-M+ in the solution. The pH of the resulting solution will be greater than pH = 4 since the acetic acid will be consumed in the reaction.
Since acetic acid is a weak acid, the neutralization reaction of 1 mole of the acid and 1 mole of the base will not give a neutral solution. The reason is because the salt CH3COO-M+ will dissociate in the same reaction as:
`CH_3 COO^(-) M^(+) = CH_3 COO^(-) + M^(+) `
where CH3COO- is the acetate ion and M+ is the alkali metal ion.
The acetate ion being a conjugate base of a weak acid will react with the water in the solution as:
`CH_3 COO^(-) + H_2 O =CH_3 COOH + OH^(-). `
The production of OH- in the solution will increase the pH of the solution compared with the pH of vinegar.
Therefore, mixing equal volumes of alkali acid and acetic acid will increase the pH of the resulting solution.