This question is about the laboratory preparation of salts.
A student writes this plan for preparing a sample of hydrated copper(II) sulfate crystals.
- step 1: Pour about 50 cm3 of dilute hydrochloric acid into a 100 cm3 beaker.
- step 2: Add a solution of copper(II) carbonate to the acid until there is no more effervescence.
- step 3: Heat the solution until all of the water has boiled off.
This plan will not succeed because there is one mistake in each step. Identify the mistake in each of the steps.
Another student uses the following plan to prepare a sample of ammonium sulfate, formed in this reaction between aqueous ammonia and dilute sulfuric acid:
2NH3(aq)+H2SO4(aq)→(NH4)2SO4(aq) 2\text{NH}_3(\text{aq}) + \text{H}_2\text{SO}_4(\text{aq}) \rightarrow (\text{NH}_4)_2\text{SO}_4(\text{aq}) 2NH3(aq)+H2SO4(aq)→(NH4)2SO4(aq)- use a pipette to transfer 25.0 cm3 of sulfuric acid to a conical flask
- add 3 drops of indicator
- use a burette to add aqueous ammonia until the indicator just changes colour permanently
The diagram shows the burette readings in one experiment before and after adding aqueous ammonia.
Use the readings to complete the table, entering all values to the nearest 0.05 cm3.
| Volume (cm3\text{cm}^3cm3) | |
|---|---|
| burette reading in cm3\text{cm}^3cm3 after adding aqueous ammonia | |
| burette reading in cm3\text{cm}^3cm3 before adding aqueous ammonia | |
| volume in cm3\text{cm}^3cm3 of aqueous ammonia added |
In another titration, the student made a mistake. After she filled the burette, she noticed that the space between the tap of the burette and the tip contained air. After adding the aqueous ammonia, she noticed that it now contained liquid.
Explain how, if at all, this mistake affects the calculated volume of aqueous ammonia added.
She repeats the experiment until she obtains concordant results.
The table shows the results.
| Trial 1 | Trial 2 | Trial 3 | Trial 4 | |
|---|---|---|---|---|
| burette reading in cm3\text{cm}^3cm3 after adding ammonia | 25.35 | 25.20 | 25.40 | 24.35 |
| burette reading in cm3\text{cm}^3cm3 before adding ammonia | 0.50 | 1.10 | 1.20 | 0.85 |
| volume in cm3\text{cm}^3cm3 of aqueous ammonia added | 24.85 | 24.10 | 24.20 | 23.50 |
| concordant results (✓\checkmark✓) |
Concordant results are those volumes that differ from each other by 0.20 cm3 or less.
Identify the concordant results by placing ticks (✓\checkmark✓) in the table where appropriate.
Use the concordant results to calculate the average (mean) volume of aqueous ammonia added.
The student then mixed the volumes of aqueous ammonia and sulfuric acid found in the titration (without any indicator added).
Describe how to use the method of crystallisation to obtain a pure dry sample of the salt from this mixture.