Question

Explain how strong acid–strong base, weak acid–strong base, and weak base–strong acid titrations using the same concentrations differ in terms of (a) the initial pH and (b) the pH at the equivalence point. (The component in italics is in the flask.)

Short answer

(a) strong acid-strong base < weak acid-strong base < weak base-strong acid.

(b) strong acid-weak base < strong acid-strong base < weak acid-strong base.

Step-by-step solution

Define equivalence point

In a titration, an equivalence point is reached when the amount of titrant administered is sufficient to totally neutralize an analyte solution.

Explanation

(a )The initial pH:

• A strong acid with a strong base (strong acid is in the flask)As only the strong acid (with a lot of ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ions) is present in the flask, the pH value will be much lower than 7. $pH<<7$
• Weak acid-strong base (weak acid is in the flask). Because the flask contains only the weak acid (and there aren't many ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ions), the pH value will be significantly closer to 7 than in the previous case. $\text{pH}\approx \text{7.}$
• Strong acid-weak base (weak base is in the flask). The pH value is very close to 7 or maybe slightly higher than 7 because the flask contains only a weak base (more ${\text{OH}}^{\text{-}}$than ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ions) $\text{pH}\approx \text{7.}$

The following is a list of the pH levels in ascending order: strong acid-strong base < weak acid-strong base < weak base-strong acid.

Explanation

(b)The pH at equivalence point:

• A strong acid with a strong base (strong acid is in the flask). An added base always neutralizes an acid at an equivalence point, therefore, the number of moles of added ${\text{OH}}^{\text{-}}$ions equals the number of moles of $H_3{O}^{+}$. The pH value is neutral at the equivalence point because the solution contains an anion of a strong acid and a cation of a strong base (which does not react with water). pH=7.

• weak acid-strong base (weak acid is in the flask). In this example, the solution consists of the strong base's cation, which does not react with water, and the weak acid's anion, which reacts with water, takes ${\text{H}}^{\text{+}}$, and generates ${\text{OH}}^{\text{-}}$${\text{A}}^{\text{-}}{\text{+H}}_{\text{2}}{\text{O > > HA + OH}}^{\text{-}}$. The solution will be basic at the equivalence point because only ${\text{OH}}^{\text{-}}$ions are generated.

• Strong acid-weak base (weak base is in the flask). The solution in this scenario consists of an anion of a strong acid that does not react with water and a cation of the weak base that donates a proton to the water and forms ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$at an equivalence point. As a result, the solution is slightly acidic at the equivalence point.

The order in which the pH levels rises will be:

Strong acid-weak base < strong acid-strong base < weak acid-strong base.