Question

Select the stronger acid from each pair and explain your reasoning. For each stronger acid, write a structural formula for its conjugate base. (a) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}\)

Short answer

Question: Identify the stronger acid in each pair and write the structural formula for the conjugate base of the stronger acid: (a) water vs. carbonic acid, (b) methanol vs. acetic acid, (c) ethanol vs. acetylene, (d) ethanol vs. ethanethiol. Answer: The stronger acids in each pair, along with their conjugate bases, are: (a) Carbonic acid (H2CO3) -> HCO3-, (b) Acetic acid (CH3COOH) -> CH3COO-, (c) Ethanol (CH3CH2OH) -> CH3CH2O-, (d) Ethanethiol (CH3CH2SH) -> CH3CH2S-.

Step-by-step solution

Compare Water and Carbonic Acid

To compare the acidity of water (H2O) and carbonic acid (H2CO3), we can look at their Ka values. The Ka value for water is 1 × 10^(-14), and for carbonic acid, it is 4.3 × 10^(-7). Since carbonic acid has a larger Ka value, it is the stronger acid.

Write the Conjugate Base of Carbonic Acid

The conjugate base of carbonic acid (H2CO3) can be written by removing one proton (H+) from its structure: \(\mathrm{HCO}_{3}^{-}\).

Compare Methanol and Acetic Acid

To compare the acidity of methanol (CH3OH) and acetic acid (CH3COOH), we look at their Ka values. The Ka value for methanol is 0.17 × 10^(-15), and for acetic acid, it is 1.75 × 10^(-5). Acetic acid has a larger Ka value, so it is the stronger acid.

Write the Conjugate Base of Acetic Acid

The conjugate base of acetic acid (CH3COOH) can be written by removing one proton (H+) from its structure: \(\mathrm{CH}_{3}\mathrm{COO}^{-}\).

Compare Ethanol and Acetylene

To compare the acidity of ethanol (CH3CH2OH) and acetylene (CH3C≡CH), we look at their Ka values. The Ka value for ethanol is 10^(-16), and for acetylene, it is 10^(-25). Ethanol has a larger Ka value, so it is the stronger acid.

Write the Conjugate Base of Ethanol

The conjugate base of ethanol (CH3CH2OH) can be written by removing one proton (H+) from its structure: \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\).

Compare Ethanol and Ethanethiol

To compare the acidity of ethanol (CH3CH2OH) and ethanethiol (CH3CH2SH), we look at their Ka values. We already found the Ka value for ethanol in Step 5 (10^(-16)), and for ethanethiol, it is 10^(-12). Ethanethiol has a larger Ka value, so it is the stronger acid.

Write the Conjugate Base of Ethanethiol

The conjugate base of ethanethiol (CH3CH2SH) can be written by removing one proton (H+) from its structure: \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\). In summary, the stronger acids in each pair, along with their conjugate bases, are: (a) \(\mathrm{H}_{2} \mathrm{CO}_{3}\) -> \(\mathrm{HCO}_{3}^{-}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) -> \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) -> \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}\) -> \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\)