You are given two beakers containing separately an aqueous solution of strong acid (HA) and an aqueous solution of weak acid (HB) of the same concentration. Describe how you would compare the strengths of these two acids by (a) measuring the \(\mathrm{pH},\) (b) measuring electrical conductance, (c) studying the rate of hydrogen gas evolution when these solutions are reacted with an active metal such as \(\mathrm{Mg}\) or \(\mathrm{Zn}\).

Understanding the pH of a solution offers great insight into its acid strength. The pH scale, ranging from 0 (most acidic) to 14 (most basic), is a logarithmic scale, which means a difference of one pH unit reflects a tenfold change in hydrogen ion concentration (H^+). In the scenario with strong acid (HA) and weak acid (HB), measuring pH becomes a reliable indicator of their relative strengths because a strong acid fully dissociates in water, releasing more H^+ ions, resulting in a lower pH. Conversely, a weak acid only partially dissociates, so it contributes fewer H^+ ions and hence has a higher pH.

For students, it's essential to remember that the pH meter should be calibrated properly before use. When comparing the pH of the strong and weak acids, be sure to rinse the electrode with distilled water between measurements to avoid cross-contamination. This will ensure accurate and meaningful comparisons of the acid strengths.

When it comes to the electrical conductance of a solution, the prime contributors are the ions present. The degree of ionization is key — a strong acid like HA dissociates completely, producing a large number of free ions, thereby increasing the solution's ability to conduct electricity. We measure this as a high electrical conductance. On the other hand, a weak acid like HB only partially dissociates, resulting in fewer free ions and subsequently lower electrical conductance.

The laboratory practice of measuring conductance involves using a conductivity meter. While performing this test, it's vital to maintain consistency in parameters such as temperature, as conductivity is temperature-dependent. Comparing the electrical conductance values gives us a clear comparison of the acid strengths revealing which acid ionizes more effectively and is therefore stronger.

The reaction between acids and active metals like magnesium (Mg) or zinc (Zn) serves as a dynamic demonstration of acid reactivity through the evolution of hydrogen gas (H_2). The rate at which H_2 gas is produced reflects the acid's tendency to donate H^+ ions to the metal, which is a measure of the acid's strength. A strong acid such as HA will react more vigorously and release hydrogen gas at a faster rate compared to the weak acid HB, which generates H_2 more slowly due to its limited ionization capability.

In a classroom setting, observing the bubbles of H_2 gas form can be particularly engaging, and quantifying this evolution over time can solidify the concept of acid strength in students' minds. However, it's crucial to conduct this experiment under controlled conditions and with appropriate safety measures to manage the potentially exothermic reaction.