Question 1

What is the difference between acid strength and pH?

Accepted Answer

pH measures the concentration of hydrogen ions in a solution on a logarithmic scale from 0 to 14, where values below 7 are acidic and above 7 are basic. Acid strength, however, refers to how readily hydrogen ions dissociate from the acid molecule. Two acids can have different pH values but similar strength, or vice versa. Strength is quantified using the acid dissociation constant, Ka, or its logarithmic form, pKa.

Question 2

How does pKa relate to acid strength?

Accepted Answer

pKa is the negative logarithm of the acid dissociation constant, Ka. Lower pKa values indicate stronger acids because the hydrogen ion dissociates more readily from the conjugate base. Higher pKa values correspond to weaker acids that dissociate less completely. This inverse relationship makes pKa a convenient way to compare acid strength across different compounds.

Question 3

What is the difference between monoprotic and polyprotic acids?

Accepted Answer

Monoprotic acids, like hydrochloric acid, can dissociate only one hydrogen ion and have a single pKa value. Polyprotic acids, such as phosphoric acid, can dissociate multiple hydrogen ions sequentially, with each dissociation having its own distinct pKa value. During titration, polyprotic acids produce multiple equivalence points, one for each hydrogen ion released.

Question 4

Why must a base be standardized before titration?

Accepted Answer

A standardized base solution has a precisely known concentration, which is essential for accurate titration calculations. Sodium hydroxide, commonly used as a titrant, is hygroscopic and absorbs water from the atmosphere, making its actual concentration lower than expected. To determine the exact concentration, the base is first titrated against a non-hygroscopic acid like potassium hydrogen phthalate with a known concentration.

Question 5

What does the equivalence point represent in a titration curve?

Accepted Answer

The equivalence point occurs when the moles of hydrogen ions from the acid equal the moles of hydroxide ions added from the base, resulting in a neutralization reaction that forms salt and water. On a titration curve, it appears at the inflection point of the steepest section. For polyprotic acids, multiple equivalence points exist, one for each hydrogen ion dissociation.

Question 6

How can you determine pKa from a titration curve?

Accepted Answer

The pH at the halfway point to the equivalence point equals the pKa of the acid. At this midpoint, the concentrations of the dissociated and non-dissociated acid forms are equal. By identifying the equivalence point volume on the titration curve and measuring the pH at half that volume, you can directly read the pKa value without additional calculations.

Question 7

How does phenolphthalein indicate the endpoint of a titration?

Accepted Answer

Phenolphthalein is a pH indicator that remains colorless in acidic solutions (pH 0-8) and turns pink in basic solutions. During titration, as the base neutralizes the acid and pH increases past the equivalence point, phenolphthalein changes from colorless to light pink. This permanent color change signals the endpoint, indicating a slight excess of hydroxide ions and completion of the titration.

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