A teacher investigates neutralisation - OCR Gateway - GCSE Chemistry - Question 20 - 2019 - Paper 3

By inspecting the graph, find the point where the pH value reaches 10 and read the corresponding volume of hydrochloric acid added from the x-axis. The estimated volume is approximately 4 cm³.

As hydrochloric acid is added, the concentration of hydroxide ions, OH⁻, decreases because the hydroxide ions react with hydrogen ions, H⁺, from the acid to form water. This reaction continues until all the hydroxide ions are neutralised.

The balanced ionic equation for the neutralisation of hydrochloric acid with sodium hydroxide is:

A strong acid completely dissociates in solution, releasing all its hydrogen ions. For example, hydrochloric acid (HCl) dissociates fully to give H⁺ ions. In contrast, a weak acid only partially dissociates in solution, meaning not all of the acid molecules release hydrogen ions. Ethanoic acid (CH₃COOH) is an example of a weak acid, as it exists in equilibrium with its ions.

If the original pH of nitric acid is 2, the concentration of H⁺ ions is:

$ext{[H}^+] = 10^{-2} ext{ mol/dm}^3$

When the acid is diluted by a factor of 100, the new concentration of H⁺ ions is:

$ext{[H}^+] = 10^{-2} ext{ mol/dm}^3 / 100 = 10^{-4} ext{ mol/dm}^3$

The new pH is:

$pH = - ext{log} (10^{-4}) = 4$.

The balanced symbol equation for the reaction between nitric acid and sodium hydroxide is:

To obtain dry sodium nitrate crystals, heat the solution resulting from the neutralisation reaction until most of the water evaporates. Allow the remaining solution to cool, causing sodium nitrate to crystallize. Finally, filter the crystals and dry them.