Examine the Ka values for the acids shown below.

Acid Name Ka
HOBr Hypobromous acid 3.3
×
10–9
CH3CO2H Acetic acid 1.8
×
10–5
HCO2H Formic acid 1.8
×
10–4
C6H5OH Phenol 1.0
×
10–10

Consider 0.10 M aqueous solutions of each acid. Arrange these solutions in order of increasing [H3O+].

Question

Examine the Ka values for the acids shown below.

Acid	Name	Ka
HOBr	Hypobromous acid	3.3 
×
 10–9
CH3CO2H	Acetic acid	1.8 
×
 10–5
HCO2H	Formic acid	1.8 
×
 10–4
C6H5OH	Phenol	1.0 
×
 10–10

Consider 0.10 M aqueous solutions of each acid.  Arrange these solutions in order of increasing [H3O+].

BlackTom AI · Accepted Answer

To compare the [H3O+] produced by 0.10 M solutions of these weak acids, we relate their strength to Ka values: higher Ka means stronger acid and larger [H3O+]. The given Ka values are: HOBr (hypobromous acid) 3.3×10^-9, CH3CO2H (acetic acid) 1.8×10^-5, HCO2H (formic acid) 1.8×10^-4, C6H5OH (phenol) 1.0×10^-10.

Option 1: Acetic acid < Formic Acid < Hypobromous acid < Phenol
Here, the sequence implies formic acid is stronger than acetic acid, and hypobromous acid is stronger than formic acid, with phenol the strongest. This contradicts the actual Ka values since phenol is far weaker (smaller Ka) than all the others, and formic acid is stronger than acetic acid, not weaker. Thus this ordering misrepresents the relative acid strengths.

Option 2: Formic acid < Acetic acid < Hypobromous acid < Phenol
This would mean formic acid is the weakest and phenol the strongest, which is incorrect because phenol has the smallest Ka (weakest) among the listed acids, not the largest. Also, hypobromous acid is far weaker than both formic and acetic acids, not in a position that would place it above them.

Option 3: Phenol < Hypobromous acid < Acetic acid < Formic acid
This ordering places phenol as the weakest (lowest Ka), followed by hypobromous acid, then acetic acid, and finally formic acid as the strongest among the set. This aligns with the Ka values since 1.0×10^-10 < 3.3×10^-9 < 1.8×10^-5 < 1.8×10^-4. In a 0.10 M solution, the approximate [H3O+] grows with sqrt(Ka·C), and the relative magnitudes of sqrt(Ka) follow the same ranking, giving the smallest [H3O+] to phenol and the largest to formic acid. The logic is consistent with standard weak-acid approximation and direct Ka comparison.

Option 4: Phenol < Hypobromous acid < Formic acid < Acetic acid
This sequence places formic acid as weaker than acetic acid, and acetic acid as the strongest among the four, which contradicts the actual Ka values where formic acid is stronger than acetic acid.

In summary, evaluating the Ka values and applying the weak-acid approximation for a 0.10 M solution shows that phenol is the weakest, hypobromous acid is stronger than phenol but weaker than the carboxylic acids, acetic acid is stronger than hypobromous acid, and formic acid is the strongest among the four acids listed. Each incorrect option either reverses relationships between pairs or misplaces the relative strengths according to the given Ka constants.

Examine the Ka values for the acids shown below. Acid Name Ka HOBr Hypobromous acid 3.3 × 10–9 CH3CO2H Acetic acid 1.8 × 10–5 HCO2H Formic acid 1.8 × 10–4 C6H5OH Phenol 1.0 × 10–10 Consider 0.10 M aqueous solutions of each acid. Arrange these solutions in order of increasing [H3O+].

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