A weak acid $$HA$$ has an acid dissociation constant $$K_a = 1.8 \times 10^{-5}$$ . Which of the following statements correctly describes this acid in aqueous solution?

The equilibrium lies to the left, meaning most of the acid remains undissociated

The acid fully dissociates, producing a high concentration of $$H^+$$ ions

The acid reacts completely with water to form a neutral solution

The $$K_a$$ value indicates the acid is stronger than $$HCl$$

How does a buffer solution resist changes in pH when a small amount of strong acid is added?

The conjugate base in the buffer reacts with the added $$H^+$$ ions to form the weak acid

The buffer neutralizes the acid by releasing $$OH^-$$ ions from water

The buffer absorbs the acid by increasing the volume of the solution

The weak acid in the buffer dissociates completely to neutralize the added acid

A student prepares two solutions: Solution A contains $$0.1$$ mol/L $$HCl$$ and Solution B contains $$0.1$$ mol/L $$CH_3COOH$$ . Which solution has a lower pH and why?

Solution A, because $$HCl$$ is a strong acid and fully dissociates, producing more $$H^+$$ ions

Solution B, because acetic acid has a higher molecular weight

Solution A, because $$HCl$$ has a higher boiling point

Solution B, because weak acids always produce more $$H^+$$ ions than strong acids

A chemist needs to design a buffer that maintains a pH of approximately $$5.0$$ . Which of the following weak acid/conjugate base pairs would be most suitable, and why?

Acetic acid/acetate ( $$pK_a = 4.74$$ ), because its $$pK_a$$ is closest to the desired pH

Carbonic acid/bicarbonate ( $$pK_a = 6.35$$ ), because it has a higher $$pK_a$$

Hydrocyanic acid/cyanide ( $$pK_a = 9.21$$ ), because it is a very weak acid

Phosphoric acid/dihydrogen phosphate ( $$pK_a = 2.15$$ ), because it is a strong acid

Two acids, $$HA$$ with $$K_a = 1 \times 10^{-3}$$ and $$HB$$ with $$K_a = 1 \times 10^{-7}$$ , are each dissolved at a concentration of $$0.1$$ mol/L. Which acid produces a higher $$[H^+]$$ concentration in solution, and what does this imply about the relationship between $$K_a$$ and acid strength?

$$HA$$ , because a larger $$K_a$$ indicates greater dissociation and higher $$[H^+]$$ concentration, confirming that $$K_a$$ is a direct measure of acid strength

$$HB$$ , because a smaller $$K_a$$ means more dissociation occurs

Both acids produce the same $$[H^+]$$ because they have the same concentration

$$HA$$ , because a larger $$K_a$$ means the acid has a higher molecular weight

A patient's blood has a normal pH maintained between $$7.35$$ and $$7.45$$ by the bicarbonate buffer system: $$H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$ . If a patient hyperventilates, $$CO_2$$ is expelled rapidly, reducing $$H_2CO_3$$ concentration. Using your understanding of buffer systems and Le Chatelier's principle, predict the effect on blood pH and classify the resulting condition.

Blood pH increases, causing respiratory alkalosis, because reduced $$H_2CO_3$$ shifts the equilibrium left, decreasing $$[H^+]$$

Blood pH decreases, causing respiratory acidosis, because fewer $$H^+$$ ions are produced

Blood pH remains unchanged because the buffer fully compensates for the loss of $$CO_2$$

Blood pH decreases, causing metabolic acidosis, because $$HCO_3^-$$ concentration increases

Understanding Acid-Base Concepts

Authored by Harmanjit Singh

Chemistry

University

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Understanding Acid-Base Concepts

What is the pH of a neutral solution at 25°C?

Which of the following is the correct formula for calculating pH?

Which of the following best describes a strong acid?

Which of the following is an example of a weak acid?

What is the pH range of an acidic solution?

Which of the following pairs can form a buffer solution?

How does a buffer solution resist changes in pH when a small amount of strong acid is added?

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