Understanding Acids and pH Levels

Which of the following is a characteristic of a strong acid?

• It completely dissociates in water.

• It partially dissociates in water.

• It has a high pH value.

• It is always a solid at room temperature.

Answer explanation

A strong acid is characterized by its ability to completely dissociate in water, releasing all of its hydrogen ions. This is why the correct choice is 'It completely dissociates in water.' The other options do not accurately describe strong acids.

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

• 0

• 7

• 14

• 1

Answer explanation

A neutral solution at 25°C has a pH of 7, which indicates equal concentrations of hydrogen and hydroxide ions. Values below 7 are acidic, while values above 7 are basic.

Which of the following acids is considered a weak acid?

• Hydrochloric acid (HCl)

• Sulphuric acid (H₂SO₄)

• Ethanoic acid (CH₃COOH)

• Nitric acid (HNO₃)

Answer explanation

Ethanoic acid (CH₃COOH) is a weak acid because it partially dissociates in solution, unlike hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃), which are strong acids that fully dissociate.

• 3

• 10

• 7

• 1

Answer explanation

To find the pH, use the formula pH = -log[H+]. For a hydrogen ion concentration of 1 x 10^-3 mol/L, pH = -log(1 x 10^-3) = 3. Therefore, the correct answer is 3.

Which of the following statements is true about the pH scale?

• It ranges from 0 to 7.

• It ranges from 0 to 14.

• It ranges from 7 to 14.

• It ranges from 1 to 10.

Answer explanation

The pH scale measures acidity and alkalinity, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. Therefore, the correct statement is that it ranges from 0 to 14.

What is the pH of a 0.01 M solution of hydrochloric acid (HCl)?

• 2

• 4

• 12

• 7

Answer explanation

The pH of a strong acid like HCl is calculated using the formula pH = -log[H+]. For a 0.01 M HCl solution, [H+] = 0.01 M, so pH = -log(0.01) = 2. Therefore, the correct answer is 2.

Which of the following is a property of weak acids?

• They have a high degree of ionisation.

• They have a low degree of ionisation.

• They are always colourless.

• They have a pH greater than 7.

Answer explanation

Weak acids have a low degree of ionisation, meaning they do not fully dissociate in solution. This is why the correct choice is 'They have a low degree of ionisation.' The other options do not accurately describe weak acids.

If the pH of a solution decreases from 5 to 3, how does the hydrogen ion concentration change?

• It decreases by a factor of 100.

• It increases by a factor of 100.

• It remains the same.

• It increases by a factor of 10.

Answer explanation

The pH scale is logarithmic, meaning each unit change represents a tenfold change in hydrogen ion concentration. A decrease from pH 5 to pH 3 is a change of 2 units, resulting in an increase in hydrogen ion concentration by a factor of 100.

Which of the following is a strong acid?

• Citric acid

• Hydrochloric acid

• Acetic acid

• Carbonic acid

Answer explanation

Hydrochloric acid is a strong acid, fully dissociating in water to release hydrogen ions. In contrast, citric, acetic, and carbonic acids are weak acids, only partially dissociating.

• 4

• 10

• 14

• 7

Answer explanation

To find the pH, first calculate the pOH using pOH = -log[OH-]. For [OH-] = 1 x 10^-4 mol/L, pOH = 4. Then, use pH + pOH = 14. Thus, pH = 14 - 4 = 10. The correct answer is 10.