Solution G and solution H are two acids of the same concentration. One of them is a strong acid and one is a weak acid. In this experiment you compare the t...
Question 1 Report
Solution G and solution H are two acids of the same concentration. One of them is a strong acid and one is a weak acid. In this experiment you compare the two acids first by their pH and then by how fast each one reacts with magnesium.
Measure 20 cm3 of solution G into a small beaker and stand the pH probe in the acid. Read the meter when the reading has settled. Rinse the probe with distilled water and repeat with 20 cm3 of solution H.
Now measure 20 cm3 of solution G into a boiling tube and 20 cm3 of solution H into a second boiling tube. Clean two 3 cm lengths of magnesium ribbon with emery paper. Drop one length into each tube and start the timer at once. Watch both tubes and stop timing each tube when its ribbon has completely disappeared. Record all of your results in Table 4.1.
Table 4.1
| solution | pH of the acid | time for the magnesium to disappear / s | observations in the first 30 s |
|---|---|---|---|
| G | |||
| H |
(a) Record the pH of solution G and of solution H in Table 4.1. [2]
(b) Record in Table 4.1 the time taken for the magnesium ribbon to disappear in each acid. [2]
(c) Record your observations in the two boiling tubes during the first 30 s. [2]
(d) Deduce which of the two solutions is the weak acid. Use both your pH values and your times. [2]
(e) State, in terms of the acid particles present in the solution, why this acid gives the higher pH. [2]
(f) Name the gas given off in both boiling tubes and describe the test that confirms it. [2]
(g) Plan how you would show that the two acids need the same volume of aqueous sodium hydroxide for neutralisation, even though their pH values are different. [3]
Answer Details
This experiment tests whether you can record practical results precisely and then use two independent measurements, pH and reaction rate, to tell a strong acid from a weak acid of the same concentration. Both acids contain the same number of moles of acid per dm3; they differ only in how fully those molecules ionise in water.
(a), (b) and (c) - recording the results [2 + 2 + 2]. A typical correct, self-consistent set of readings is shown below.
| solution | pH | time for Mg to disappear / s | observations in first 30 s |
|---|---|---|---|
| G | 1.0 | 45 | rapid stream of bubbles, tube warms noticeably, ribbon shrinks quickly |
| H | 3.0 | 190 | slow, gentle stream of bubbles, ribbon shrinks slowly |
For (a), both pH values must be to one decimal place and the two must differ, with one near 1.0 and one near 3.0 for acids of this concentration [1 for two values to 1 d.p.; 1 for values about 1.0 and 3.0]. For (b), both times are in seconds and the weaker acid must give the clearly longer time [1 for two times in seconds; 1 for the weaker-acid time being the longer]. For (c), record effervescence in both tubes but faster in one [1], and note that the ribbon gets smaller and the tube warms up [1].
(d) Which is the weak acid [2]. Solution H is the weak acid. It is supported by both pieces of evidence: H has the higher pH (3.0 against 1.0) and H takes the longer time for the ribbon to disappear (190 s against 45 s) [1 for naming the higher-pH acid; 1 for supporting it with both a higher pH and a longer time]. This is consequential on your own table, so the acid you name must match your own readings.
(e) Why the weak acid gives the higher pH [2]. A weak acid is only partly ionised: only some of its molecules split up to release hydrogen ions in water [1]. Because fewer molecules ionise, the concentration of hydrogen ions, \(\text{H}^+\), is lower, and a lower \([\text{H}^+]\) means a higher pH [1]. The strong acid ionises almost completely, giving a much higher \([\text{H}^+]\) and a lower pH, even though both acids started at the same concentration.
(f) The gas and its test [2]. The gas is hydrogen [1]. Hold a lighted splint at the mouth of the tube; the hydrogen burns with a squeaky pop [1]. The reaction is \(\text{Mg} + 2\text{H}^+ \rightarrow \text{Mg}^{2+} + \text{H}_2\).
(g) Showing the two acids need the same volume of alkali [3]. Equal volumes of the two acids contain the same number of moles of acid, so they need the same volume of alkali, and a titration proves it:
- Pipette 25.0 cm3 of each acid into separate conical flasks and add a few drops of a suitable indicator [1].
- Titrate each acid with the same aqueous sodium hydroxide run from a burette, swirling until the indicator just changes colour [1].
- Record both burette readings to 0.05 cm3, repeat until concordant titres agree within 0.10 cm3, and compare the two mean titres; equal titres show the same volume of alkali is needed [1].
The key idea is that pH depends on the degree of ionisation, but the volume of alkali for neutralisation depends only on the total moles of acid, which are equal for both.
Download The App On Google Playstore
Everything you need to excel in your exams
