Question

For each conjugate acid-base pair, identify the first species as an acid or a base and the second species as its conjugate acid or base. In addition, draw Lewis structures for each species, showing all valence electrons and any formal charge. (a) \(\mathrm{HCOOH} \mathrm{HCOO}^{-}\) (b) \(\mathrm{NH}_{4}{ }^{+} \mathrm{NH}_{3}\) (d) \(\mathrm{HCO}_{3}^{-} \quad \mathrm{CO}_{3}^{2-}\) (e) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-} \quad \mathrm{HPO}_{4}{ }^{2-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (g) \(\mathrm{CH}_{3} \mathrm{~S}^{-} \mathrm{CH}_{3} \mathrm{SH}\) (f) \(\mathrm{CH}_{3} \mathrm{CH}_{3} \mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\)

Short answer

Question: Identify whether the first species in each pair is an acid or a base and whether the second species is its conjugate acid or base. Draw Lewis structures for each species, showing all valence electrons and any formal charges. 1. HCOOH and HCOO- - HCOOH is an acid; HCOO- is its conjugate base - Lewis structures: HCOOH - O=C(OH)O-H (no formal charges); HCOO- - O=C-O- (formal charge of -1 on O) 2. NH4+ and NH3 - NH4+ is an acid; NH3 is its conjugate base - Lewis structures: NH4+ - N(+)-(H)(H)(H)(H) (formal charge +1 on N); NH3 - N(H)(H)(H) (no formal charges) 3. CH3CH2O- and CH3CH2OH - CH3CH2O- is a base; CH3CH2OH is its conjugate acid - Lewis structures: CH3CH2O- - C-C-O- (formal charge of -1 on O); CH3CH2OH - C-C-OH (no formal charges) 4. HCO3- and CO32- - HCO3- is an acid; CO32- is its conjugate base - Lewis structures: HCO3- - O=C-O- (formal charge of -1 on O); CO32- - O=C=O (formal charge of -2 on O) 5. H2PO4- and HPO42- - H2PO4- is an acid; HPO42- is its conjugate base - Lewis structures: H2PO4- - O=P(OH) (formal charge of -1 on O); HPO42- - O=P=O (formal charge of -2 on O) 6. CH3S- and CH3SH - CH3S- is a base; CH3SH is its conjugate acid - Lewis structures: CH3S- - S-(CH3) (formal charge of -1 on S); CH3SH - S-CH3 (no formal charges) 7. CH3CH3CH3CH2- and CH3CH3CH3CH3 - CH3CH3CH3CH2- is a base; CH3CH3CH3CH3 is its conjugate acid - Lewis structures: CH3CH3CH3CH2- - C-C-C (formal charge of -1 on the last C); CH3CH3CH3CH3 - C-C-C-C (no formal charges)

Step-by-step solution

Identify as acid or base and its conjugate acid or base

We can use the Brønsted-Lowry theory to identify acids and bases. An acid is a proton (H+) donor, while a base is a proton (H+) acceptor. (a) \(\mathrm{HCOOH}\) donates a proton to form \(\mathrm{HCOO}^{-}\), which means it is an acid, and \(\mathrm{HCOO}^-\) is its conjugate base. (b) \(\mathrm{NH}_{4}^{+}\) donates a proton and becomes \(\mathrm{NH}_{3}\), so it is an acid and \(\mathrm{NH}_{3}\) is its conjugate base. (d) \(\mathrm{HCO}_{3}^{-}\) donates a proton to form \(\mathrm{CO}_{3}^{2-}\), which means it is an acid, and \(\mathrm{CO}_{3}^{2-}\) is its conjugate base. (e) \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) donates a proton and becomes \(\mathrm{HPO}_{4}^{2-}\), so it is an acid and \(\mathrm{HPO}_{4}^{2-}\) is its conjugate base. (c) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) accepts a proton and becomes \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\), which means it is a base, and \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) is its conjugate acid. (g) \(\mathrm{CH}_{3}\mathrm{S}^{-}\) accepts a proton and becomes \(\mathrm{CH}_{3}\mathrm{SH}\), so it is a base and \(\mathrm{CH}_{3}\mathrm{SH}\) is its conjugate acid. (f) \(\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{2}^{-}\) accepts a proton and becomes \(\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\), which means it is a base, and \(\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\) is its conjugate acid.

Draw Lewis structures

Now, we'll draw the Lewis structures for each species, showing all valence electrons and any formal charge. (a) \(\mathrm{HCOOH}:\) O=C(OH)O-H, with a formal charge of 0 on each atom. \(\mathrm{HCOO}^{-}:\) O=C-O^{-}, with O-H, and a formal charge of -1 on O. (b) \(\mathrm{NH}_{4}^{+}:\) N(+)-(H)(H)(H)(H), with a formal charge +1 on N. \(\mathrm{NH}_{3}:\) N(H)(H)(H), with a formal charge of 0 on each atom. (d) \(\mathrm{HCO}_{3}^{-}:\) O=C-O^{-}, with O-H and a formal charge of -1 on O. \(\mathrm{CO}_{3}^{2-}:\) O=C=O, with O^{2-} and a formal charge of -2 on O. (e) \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}:\) O=P(OH), with a formal charge of -1 on O. \(\mathrm{HPO}_{4}^{2-}:\) O=P=O, with a formal charge of -2 on O. (c) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}:\) + formal charge of -1 on O. \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}:\) No formal charges. (g) \(\mathrm{CH}_{3}\mathrm{S}^{-}:\) S-(CH3), with a formal charge of -1 on S. \(\mathrm{CH}_{3}\mathrm{SH}:\) No formal charges. (f) \(\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{2}^{-}:\) C-C-C, with a formal charge of -1 on the last C. \(\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}\mathrm{CH}_{3}:\) No formal charges.