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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?

The reaction of solutions of acid and base can be generally represented as:


Acid + Base = Salt + Water


where


Acid contains an H that can be protonated producing the conjugate base


and 


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 `


where


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. 

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