Q: A solution has a pOH of 3.25. What is the concentration of hydroxide ions, [OH$$^-$$]?

$$[\mathrm{OH}^-] = 10$$^{-3.25}$$ = 5.62 \times 10$$^{-4}$$\ \mathrm{M}$$

Q: Calculate the percent ionization of a 0.10 M acetic acid (CH$$_3$$COOH) solution if [H$$_3O$^+$$] at equilibrium is $$4.2 \times 10$$^{-3}$$ M.

$$\%\ \mathrm{ionization} = \frac{4.2 \times 10$$^{-3}$$}{0.10} \times 100\% = 4.2\%$$

Q: A 0.10 M solution of ammonia (NH$$_3) $$has a $$K_b of$$ $$1.8 \times 10$$^{-5}$$. What is the pH of the solution? (Assume x$$ \ll 0.10$$)

$$[\mathrm{OH}^-] = \sqrt{(0.10)(1.8 \times 10$$^{-5}$$)} = 1.34 \times 10$$^{-3}$$\ \mathrm{M}$$;\ pOH = -\log(1.34 \times $$10^{-3}) = 2.87$$;\ pH = 14 - 2.87 = 11.13$

Q: A 0.25 M solution of a weak acid, HA, has a pH of 2.70. What is the acid dissociation constant, $$K_a$$, for HA?

$$[H_3O^+] = 10$$^{-2.70}$$ = 2.0 \times 10$$^{-3}$$\ \mathrm{M}$$;\ K_a = \frac{(2.0 \times $$10^{-3})^2$}{0.25 - 2.0 \times 10$$^{-3}$$} \approx 1.6 \times 10$$^{-5}$$

Question 1

Which of the following statements best describes a Brønsted-Lowry acid?

Accepted Answer

A substance that donates a proton (H$$^+) to $$another species.

Question 2

A 0.040 M solution of HClO$$_4 (a $$strong acid) is prepared. What is the pH of the solution?

Accepted Answer

$$\mathrm{pH} = -\log(0.040) = 1.40$$

Question 3

Which of the following is the conjugate base of HSO$$_4^-$$?

Accepted Answer

SO$$_4^{2-}$$

Question 4

A solution has a pOH of 3.25. What is the concentration of hydroxide ions, [OH$$^-$$]?

Accepted Answer

$$[\mathrm{OH}^-] = 10$$^{-3.25}$$ = 5.62 	imes 10$$^{-4}$$\ \mathrm{M}$$

Question 5

Which of the following salts will produce a basic solution when dissolved in water?

Accepted Answer

NaF

Question 6

Calculate the percent ionization of a 0.10 M acetic acid (CH$$_3$$COOH) solution if [H$$_3O$^+$$] at equilibrium is $$4.2 	imes 10$$^{-3}$$ M.

Accepted Answer

$$\%\ \mathrm{ionization} = \frac{4.2 	imes 10$$^{-3}$$}{0.10} 	imes 100\% = 4.2\%$$

Question 7

Which of the following statements about the relationship between $$K_a$$ and $$K_b$$ for a conjugate acid-base pair is correct?

Accepted Answer

$$K_a 	imes K_b = K_w$$

Question 8

A 0.10 M solution of ammonia (NH$$_3) $$has a $$K_b of$$ $$1.8 	imes 10$$^{-5}$$. What is the pH of the solution? (Assume x$$ \ll 0.10$$)

Accepted Answer

$$[\mathrm{OH}^-] = \sqrt{(0.10)(1.8 	imes 10$$^{-5}$$)} = 1.34 	imes 10$$^{-3}$$\ \mathrm{M}$$;\ pOH = -\log(1.34 	imes $$10^{-3}) = 2.87$$;\ pH = 14 - 2.87 = 11.13$

Question 9

Which of the following is a Lewis acid?

Accepted Answer

BF$$_3$$

Question 10

Which of the following acids is the strongest, and why?

Accepted Answer

HClO$$_4$$, because it has the most oxygen atoms, which stabilize the negative charge on the conjugate base.

Question 11

A 0.25 M solution of a weak acid, HA, has a pH of 2.70. What is the acid dissociation constant, $$K_a$$, for HA?

Accepted Answer

$$[H_3O^+] = 10$$^{-2.70}$$ = 2.0 	imes 10$$^{-3}$$\ \mathrm{M}$$;\ K_a = \frac{(2.0 	imes $$10^{-3})^2$}{0.25 - 2.0 	imes 10$$^{-3}$$} \approx 1.6 	imes 10$$^{-5}$$

Question 12

Which of the following best explains why HF is a weak acid, while HCl is a strong acid?

Accepted Answer

The F$$^-$$ ion is small and highly electronegative, so it holds onto the proton more tightly, making HF less likely to dissociate.

Question 13

What is the pH of a solution formed by mixing 50.0 mL of 0.150 M HBr with 25.0 mL of 0.020 M HCl? (Assume both are strong acids.)

Accepted Answer

First, calculate total moles of H$$_3O$^+$$: $$0.0500\ \mathrm{L} 	imes 0.150\ \mathrm{M} = 0.0075\ \mathrm{mol} ($$HBr), $$0.0250\ \mathrm{L} 	imes 0.020\ \mathrm{M} = 0.0005\ \mathrm{mol} ($$HCl). Total moles = $$0.0080$$ mol. Total volume = $$0.0750 L$$. $$[H_3O^+] = \frac{0.0080}{0.0750} = 0.107\ \mathrm{M}$$. $$\mathrm{pH} = -\log(0.107) = 0.97$$

Question 14

Which of the following best describes an amphiprotic substance?

Accepted Answer

A substance that can act as either a proton donor or a proton acceptor.

Question 15

A 0.10 M solution of a weak base, B, has a $$K_b of$$ $$2.5 	imes 10$$^{-5}$$. What is the pH of the solution? (Assume x$$ \ll 0.10$$)

Accepted Answer

$$[OH^-] = \sqrt{(0.10)(2.5 	imes 10$$^{-5}$$)} = 1.58 	imes 10$$^{-3}$$\ \mathrm{M}$$;\ pOH = -\log(1.58 	imes $$10^{-3}) = 2.80$$;\ pH = 14 - 2.80 = 11.20$

Question 16

Which of the following solutions will have the lowest pH?

Accepted Answer

0.10 M HNO$$_3$$

Question 17

Which of the following best explains why the pH of a 0.10 M solution of H$$_2SO$_4 is $$not exactly 1.00?

Accepted Answer

The second ionization of H$$_2SO$_4 is $$not complete, so the total [H$$_3O$^+$$] is slightly less than 0.20 M.

Chapter 16: Acid-Base Equilibria – Study Notes

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