Strength of Acids and Alkalis

Which of the following solutions has the lowest electrical conductivity? (All solutions are of the same concentration.)

• Ethanoic acid

• Sulphuric acid

• Sodium hydroxide

• Sodium chloride

Which of the following statements about a dilute aqueous solution of strong acid is correct?

• It is a strong electrolyte.

• It has a pH value of 1.

• It is very sour.

• It reacts with all metals.

Answer explanation

A strong acid is one which fully (or highly) ionizes in water. And by definition, an electrolyte which fully (or highly) dissociates or ionizes in an aqueous solution (or in molten state) is a strong electrolyte. A strong acid can be very dilute and pH can be any values higher than 1. The sense of ‘sour’ is only used to describe some dilute acids (e.g. ethanoic acid), but not for the strength of acids. Dilute acid cannot react with copper and other metals below copper in the metal reactivity series.

Tags

NGSS.HS-PS1-5

Which of the following acids is a strong acid?

• 1 M CH₃COOH(aq)

• 12 M CH₃COOH(aq)

• 0.01 M HCl(aq)

• 0.5 M CH₃COOH(aq)

Answer explanation

HCl ionizes fully in aqueous solution, while CH₃COOH ionizes slightly in aqueous solution.

Which of the following particles is NOT present in a dilute aqueous solution of ammonia?

• NH₄⁺

• NH₃

• OH^-

• NH₄OH

Answer explanation

There is no such particle as NH₄OH.

Which of the following particles is in the smallest amount in a dilute aqueous solution of ethanoic acid?

• CH₃COOH

• OH^-

• H⁺

• CH₃COO^-

Answer explanation

Ethanoic acid is a weak acid with only slight ionization, so the concentration of hydrogen ion is small. An acid solution must have the hydrogen ion concentration greater than the hydroxide ion concentration. So the concentration of hydroxide ion must be the smallest.

Tags

NGSS.HS-PS1-5

Which of the following statements about a concentrated acid is correct?

• It can react with copper metal.

• It cannot react with copper(II) oxide.

• It is an acid that fully ionizes in water.

• It must contain hydrogen ions when in aqueous form.

Answer explanation

Only concentrated acids with strong oxidizing properties can react with copper (e.g. nitric and sulphuric acid). Concentrated hydrochloric acid is not an oxidizing agent, so it cannot react with copper. Reacting with a base like copper(II) oxide is a general property of acids, so concentrated acid will react with copper(II) oxide. Concentrated acids do not necessarily fully ionize in water. For example, concentrated ethanoic acid does not fully ionize. An acid should have a certain extent of ionization in water, so a concentrated acid must contain hydrogen ions when in aqueous form.

Tags

NGSS.HS-PS1-1

Which of the following solutions has the lowest pH?

• 1 M HCl(aq)

• 1 M CH₃COOH(aq)

• 1 M NH₃(aq)

• 1 M NaOH(aq)

Answer explanation

HCl(aq) is a strong acid and completely ionizes in water. It has the highest concentration of H⁺(aq).

HCl dissolved in methylbenzene reacts with magnesium to give hydrogen.

• True

• False

Pure sulphuric acid changes blue litmus paper red.

• True

• False

Answer explanation

Pure sulphuric acid will not ionize to give H⁺ ions if water is not present, so it does not show acidic properties.

12 M ammonia solution is a stronger alkali than 1 M sodium hydroxide solution.

• True

• False