pKa
Definition
The pKa value is used to express the strength of acids. It's defined as -log10 Ka where Ka is the acid dissociation constant.
Analogy
Imagine you're judging a chili cook-off. The spicier chilis have more kick just like stronger acids have lower pKa values. So, if you're looking for something mild, go for higher pKa values just like you would choose less spicy chili!
Related terms
Strong Acid: An acid with low pKa value that completely ionizes in solution.
Weak Acid: An acid with high pKa value that only partially ionizes in solution.
Buffer Solution: A solution containing both weak acid and its conjugate base (or vice versa) capable of resisting changes in pH upon addition of small amounts of either an acid or base.
"pKa" appears in:
Study guides (3)
Practice Questions (15)
- Consider the titration of 100.0 mL of 0.200 M CH3NH2 by 0.100 M HCl. What is the pH of the solution after 65.0 mL of HCl has been added? (pKa = 10.643)
- What is the pH of a solution containing 20.0 mL of of 0.243 M KX with 27.9 mL of 0.106 M HCl? The pKa of HX is 10.39.
- Which of the following organic acids would you expect to have the lowest pKa?
- Given an unknown acid with pKa 5, what would be the pH of a 0.1 molar solution of the acid?
- Given that the pKa of butanoic acid is around 5. Which of the following would best describe the molecule when you take it from an environment of pH 4 to pH 6?
- Which of the following statements is true about pKa and pKb?
- Aspirin has a pKa of 3.4. What is the ratio of A¯ to HA in the stomach? (pH = 1.4)
- According to the Henderson-Hasselbalch equation, when the pH of a solution becomes equal to its pKa, the solution becomes a buffer. This condition is achieved when which of the following is true?
- A solution of acetic acid has a pH of 6.75. The ratio of acid to conjugate base is _______________. (pKa = 4.75)
- NaOH is added to a 500mL of 2M acetic acid. What volume of 2M NaOH must be added so that the pH of the solution is 4.8? (pKa = 4.8)
- A solution contains 500 mL of 1 M acetic acid. How many moles of sodium acetate must be added to achieve a pH of 5.75? (pKa = 4.75)
- You need to prepare a buffer solution of pH 4.178 from 25.0 mL of 0.282 M solution of a sodium salt of a weak acid, NaA where the pKa of the weak acid HA is 4.270. How many moles of 0.329 M HCl do you need to add to achieve this goal?
- You need to prepare an acetate buffer of pH 5.83 from a 0.642 M acetic acid solution and a 2.31 M KOH solution. If you have 975 mL of the acetic acid solution, how many milliliters of the KOH solution do you need to add to make a buffer of pH 5.830? (pKa = 4.752)
- pKa for phenophthalein is 9.3 at room temp. Calculate the ratio of its anionic form to acid form at a pH of 8.2.
- pKa for phenophthalein is 9.3 at room temp. Calculate the ratio of its anionic form to acid form at a pH of 10.
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