Describe what occurs to the pH of a solution if an acid is added to it. What happens to the pH of the solution when the ratio of base is equal to the amount of acid?
As acid is added to a solution, the pH decreases. The pH at equivalence depends on the relative strengths of the acid and base in solution.
Using the simple Arrhenius definition of acids and bases, "pH" is the negative of the logarithm of the hydrogen ion concentration. Thus, when the concentration of acid (hydrogen ions) increases , the pH value decreases.
"Equal" ratios of acid and base in solution needs more careful definition. Purely volumetric ratios of different concentrations can have very different equilibrium pH values. Taking it to mean the common "equivalence point" where the acid and base components "neutralize", the resulting pH is in the "middle" - pH 7.0 - only when the acid and base are of the same relative strength in terms of dissociation, not concentration.
A strong base and weak acid will have an equivalence point at a slight higher pH value than that of a weak base and strong acid.
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