Saved Given the following pK values, which of the following Bronsted-Lowry acids will yield the strongest conjugate base? Water H2O pK = 15.7 methylamine CH3NH2 pKa = 40 formic acid HCOOH pKa = 3.8 phenol C6H5OH pKa = 10.0 acetylene C2H2 pKa = 25
The Correct Answer and Explanation is:
To determine which Bronsted-Lowry acid yields the strongest conjugate base, we need to look at the acid dissociation constant (pKa) values. The lower the pKa value, the stronger the acid, and consequently, the weaker the conjugate base. On the other hand, the higher the pKa, the weaker the acid, and the stronger the conjugate base.
Here are the given compounds with their respective pKa values:
- Water (H2O): pKa = 15.7
- Methylamine (CH3NH2): pKa = 40
- Formic Acid (HCOOH): pKa = 3.8
- Phenol (C6H5OH): pKa = 10.0
- Acetylene (C2H2): pKa = 25
Concept:
The conjugate base of an acid is formed when the acid loses a proton (H+). The strength of the conjugate base is inversely related to the strength of the acid: the weaker the acid, the stronger the conjugate base. Therefore, we want to identify the compound with the highest pKa value, as this will have the weakest acid (and hence the strongest conjugate base).
Analysis:
- Water (pKa = 15.7): Water is a weak acid, so its conjugate base (OH-) is somewhat stronger but not the strongest in this set.
- Methylamine (pKa = 40): Methylamine is a very weak acid, so its conjugate base (CH3NH-) will be the strongest in this group.
- Formic Acid (pKa = 3.8): Formic acid is a relatively strong acid, meaning its conjugate base (HCOO-) is weaker.
- Phenol (pKa = 10.0): Phenol is also a weak acid, but its conjugate base (phenoxide ion) is stronger than that of formic acid but weaker than methylamine.
- Acetylene (pKa = 25): Acetylene is a weaker acid than formic acid, but its conjugate base (acetylide ion) is stronger than formate.
Conclusion:
The strongest conjugate base will be formed by methylamine (CH3NH2), as it has the highest pKa value (40), indicating it is the weakest acid in the list. Thus, its conjugate base (CH3NH-) is the strongest among the options.
