Exam Questions

Prelim Questions on Acids

Try out challenging questions from prelim papers to apply what you have learned about acids: dissociation of hydrogen ions, acid formation, and reactions.

Acid Strength

Citric acid, C6H8O7 is a weak acid. Explain what is meant by a weak acid.

Citric acid partially ionises in water to form hydrogen ions.

Which do you think conducts electricity better, 1.0 mol/dm3 dilute hydrochloric acid or 1.0 mol/dm3 aqueous ethanoic acid? Explain your answer, including the use of appropriate equations in your explanation.

Hydrochloric acid conducts electricity better.

HCl (aq) ⟶ H+ (aq) + Cl (aq)
CH3COOH (aq) ⇄ CH3COO (aq) + H+ (aq)

Hydrochloric acid is a strong acid. All molecules completely ionise in water to form a higher concentration of mobile ions. However, ethanoic acid is a weak acid. Only some molecules partially ionise in water to form a lower concentration of mobile ions.

Marker’s note: Use double arrow to show the partial dissociation of ethanoic acid. Write state symbols to highlight that the dissociation takes place under aqueous condition.

A few drops of Universal Indicator were added to four separate test tubes containing aqueous citric acid, aqueous ethanoic acid, aqueous sulfuric acid and aqueous hydrochloric acid.

Which solutions turned the indicator orange and which turned the indicator red?
The colours and corresponding pH of the Universal Indicator.

Citric acid and ethanoic acid turned the indicator orange. They are weak acids that dissociate partially to give a lower concentration of hydrogen ions. Therefore, the pH is slightly higher (though still less than 7).

Hydrochloric acid and sulfuric acid turned the indicator red. They are strong acids that dissociate fully to give a higher concentration of hydrogen ions. Therefore, the pH is much lower.

Marker’s note: For us to relate acid strength directly with concentration of hydrogen ions, we must assume that the initial concentration of the four acids are similar. Also, concentration of hydrogen ions is inversely proportional to pH. In other words, the higher the concentration of hydrogen ions, the lower the pH.

Formation of Acids from Acidic Oxides

Arsenic oxide, As2O3, reacts slowly with water to form the acid H3AsO3. This acid reacts with dilute lithium hydroxide to form a lithium salt. From this information, deduce the nature of the element arsenic. Predict the chemical formula of the lithium salt.

Arsenic is a non-metal, because acidic oxides are non-metal oxides. The formula of lithium salt is Li3AsO3, as there are three hydrogen ions that can be dissociated and replaced by three singly charged lithium ions.

Marker’s note: This is an application question, in which you have to infer that arsenic oxide is an acidic oxide based on its reaction with water, and that all the hydrogen atoms can be dissociated. Do note that this is not true for organic acids. Ethanoic acid, CH3COOH has 4 hydrogen atoms, but only the one at the “end” can be dissociated.

Reactions of Acids

A certain fruit salt contains the following compounds: sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), anhydrous citric acid (C6H8O7), sodium saccharin (C7H4NNaO3S). When added to water, the citric acid reacts with the sodium bicarbonate and sodium carbonate to form carbon dioxide gas.

State two observations when fruit salt is added to water. Explain why the compounds do not react until water is added.

Solid decreases in size (as the compounds are soluble in water and react away). Effervescence of colourless, odourless gas is observed (as carbon dioxide is produced).

Water is needed for the anhydrous (a cheem word for dry) citric acid to dissociate to form hydrogen ions. The hydrogen ions can then react with bicarbonate ions and carbonate ions to form carbon dioxide.

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