Chemistry $^{3}$ : Introducing inorganic, organic and physical chemistry

Andrew Burrows, John Holman, Andrew Parsons

Chapter 7 Acids and bases - all with Video Answers

Educators

Chapter Questions

Problem 1

Identify the acid, base, conjugate acid, and conjugate base in the following equilibria. (Section 7.1)
(a) $\mathrm{HCO}_{2} \mathrm{H}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{D}) \mathrm{HCO}_{2}{ }^{-}(\mathrm{aq})+\mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{3}{ }^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$
(c) $\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \rightleftharpoons \mathrm{HSO}_{4}^{-}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}_{2}^{+}$

Check back soon!

Problem 2

Aspirin is acetylsalicylic acid which has a $p K_{\mathrm{a}}$ of $3.49$.
(Section 7.2)
<smiles>CC(=O)Oc1ccccc1C(=O)O</smiles>
(a) Calculate the $\mathrm{pH}$ of a $0.10 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of this acid.
(b) Would you expect the sodium salt of acetylsalicylic acid to be acidic, neutral, or basic in solution? Explain your reasoning.

Check back soon!

01:22

Problem 3

$K_{w}$ for water at $40^{\circ} \mathrm{C}$ is $3.0 \times 10^{-14} \mathrm{~mol}^{2} \mathrm{dm}^{-6}$, Is $\mathrm{pH} 7$ neutral, acidic, or basic for an aqueous solution at this temperature? (Section 7.2)

David C.

Numerade Educator

05:35

Problem 4

Calculate the following (Section 7.2):
(a) $\left[\mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})\right]$ in a solution of $\mathrm{HCl}$ at $\mathrm{pH} 1.5 ;$
(b) [OH $\left.^{-}(\mathrm{aq})\right]$ in a solution of $\mathrm{KOH}$ at $\mathrm{pH} 12.8$.

Dr. Satish I.

Numerade Educator

Problem 5

Hydroxylamine $\left(\mathrm{NH}_{2} \mathrm{OH}\right)$ is a weak base. A $0.20 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of hydroxylamine has a pH of 9.7. (Section 7.2)
(a) Calculate $K_{\mathrm{b}}$ and $\mathrm{pK}_{\mathrm{b}}$ for hydroxylamine.
(b) What is the $\mathrm{pK}_{\mathrm{a}}$ of $\mathrm{NH}_{3} \mathrm{OH}^{+}$?

Check back soon!

01:12

Problem 6

Methylamine $\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)$ is a base with $\mathrm{pK}_{\mathrm{b}}$ 3.34. Identify the conjugate acid of methylamine, and determine the $\mathrm{pK}_{\mathrm{a}}$ for this at $298 \mathrm{~K}$. (Section 7.2)

David C.

Numerade Educator

Problem 7

The pH inside most cells is maintained at around $7.4$ by a phosphate buffer made up from the $\mathrm{H}_{2} \mathrm{PO}_{4}^{-}$(aq) ion and its conjugate base, $\mathrm{HPO}_{4}{ }^{2-}(\mathrm{aq})$. The $\mathrm{pK}_{\mathrm{a}}$ of the acid $\mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-}(\mathrm{aq})$ is 7.2. (Section 7.3)
$$
\mathrm{H}_{2} \mathrm{PO}_{4}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{HPO}_{4}^{2-}(\mathrm{aq})
$$
(a) Write an expression for $K_{a}$ for $\mathrm{H}_{2} \mathrm{PO}_{4}-$ (aq).
(b) Calculate the ratio of [HPO $\left._{4}^{2-}(\mathrm{aq})\right]\left[\mathrm{H}_{2} \mathrm{PO}_{4}^{-}(\mathrm{aq})\right]$ needed to give a pH of $7.4$ in the cell.
(c) A typical value for the total phosphate concentration in a cell, $\left[\mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-}(\mathrm{aq})\right]+\left[\mathrm{HPO}_{4}{ }^{2}(\mathrm{aq})\right]$, is $0.020 \mathrm{~mol} \mathrm{dm}^{-3}$. Calculate typical values of $\left[\mathrm{HPO}_{4}{ }^{2-}(\mathrm{aq})\right]$ and $\left[\mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-}(\mathrm{aq})\right]$ inside a cell.

Check back soon!

05:59

Problem 8

In a titration, a solution of $0.200 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaOH}$ solution is added from a burette to $30.0 \mathrm{~cm}^{3}$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HNO}_{3}$ solution in a conical flask. Calculate the $\mathrm{pH}$ of the solution (Section 7.4):
(a) before any $\mathrm{NaOH}$ is added;
(b) after addition of $5.0 \mathrm{~cm}^{3}$ of the $\mathrm{NaOH}$ solution;
(c) after addition of $10.0 \mathrm{~cm}^{3}$ of the $\mathrm{NaOH}$ solution;
(d) at the equivalence point;
(e) after addition of $20.0 \mathrm{~cm}^{3}$ of the $\mathrm{NaOH}$ solution.

David C.

Numerade Educator

Problem 9

For the following titrations, select which of the two indicators given is more appropriate. (Section 7.5)
(a) Propanoic acid titrated against sodium hydroxide solution using either phenolphthalein $\left(\mathrm{pK}_{\mathrm{n}} 9.4\right)$ or methyl orange $\left(\mathrm{pK}_{\text {in }} 3.4\right)$
(b) Hydrochloric acid titrated against methylamine solution using either thymol blue $\left(\mathrm{p} K_{\mathrm{h}} 9.0\right)$ or bromophenol blue $\left(\mathrm{pK}_{\text {in }} 3.9\right)$

Check back soon!

Problem 10

Estimate $\mathbf{p K}_{\mathrm{a} 1}$ for the following oxoacids: (a) $\mathrm{HBrO}_{3}$; (b) $\mathrm{H}_{2} \mathrm{SeO}_{3}$; (c) $\mathrm{H}_{2} \mathrm{CO}_{3}$. The actual values of $\mathrm{p} K_{\mathrm{a}}$ and $\mathrm{pK}_{\mathrm{a} 2}$ for $\mathrm{H}_{2} \mathrm{CO}_{3}$ are $6.35$ and $10.33$, respectively. Suggest a reason for the large difference between these and your calculated values.
(Section 7.6)

Check back soon!

01:57

Problem 11

The structure of diphosphoric acid $\left(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{7}\right)$ is shown below. Its four $\mathrm{pK}_{\mathrm{a}}$ values are $1.0,1.8,6.6$, and $9.6$. Suggest reasons for the relative values. (Section 7.6)

David C.

Numerade Educator

Problem 12

Citric acid (see Box 7.6, p.334) has the speciation curve shown below in water at $298 \mathrm{~K}$. Use this to estimate values for $\mathrm{pK}_{\mathrm{a} 1}$, $\mathrm{p} \mathrm{K}_{\mathrm{a} 2}$, and $\mathrm{pK}_{\mathrm{a} 3}$. (Section 7.6)

Check back soon!

Problem 13

Identify the Lewis acid and Lewis base in the following reactions. (Section 7.8)
(a) $\mathrm{I}_{2}+\mathrm{I}^{-} \rightarrow \mathrm{I}_{3}^{-}$
(b) $\mathrm{NH}_{3}+\mathrm{HBr} \rightarrow \mathrm{NH}_{4} \mathrm{Br}$
(c) $\mathrm{SiF}_{4}+2 \mathrm{py} \rightarrow \mathrm{SiF}_{4}(\mathrm{py})_{2}$
(d) $\mathrm{CO}_{2}+\mathrm{OH}^{-} \rightarrow \mathrm{HCO}_{3}^{-}$ Which, if any, of the reactions are also Bronsted-Lowry acidbase reactions.

Check back soon!

Problem 14

Sulfur dioxide can act as either a Lewis acid or a Lewis base. Suggest how it reacts with (a) $\mathrm{NMe}_{3}$ and (b) $\mathrm{SbF}_{5}$, in each case identifying the product and stating whether $\mathrm{SO}_{2}$ is acting as a Lewis acid or a Lewis base. (Section 7.8)

Check back soon!