The equation for the thermal dehydration of hydrated cobalt(II) chloride is:
CoCl2⋅6H2O(s)→CoCl2(s)+6H2O(g)\text{CoCl}_2\cdot 6\text{H}_2\text{O}(\text{s}) \rightarrow \text{CoCl}_2(\text{s}) + 6\text{H}_2\text{O}(\text{g})CoCl2⋅6H2O(s)→CoCl2(s)+6H2O(g)
A student investigates the dehydration of hydrated cobalt(II) chloride using a crucible and lid heated on a clay pipe triangle over a Bunsen burner.
She uses this method:
- weigh the crucible, lid, and hydrated cobalt(II) chloride
- heat the crucible, lid, and contents for 2 minutes
- allow to cool and then reweigh
- heat for a second period of 2 minutes
- allow to cool and then reweigh
- heat for a third period of 2 minutes
- allow to cool and then reweigh
The table shows the student's results:
| Experiment | Mass before heating (g) | Mass after heating for 2 minutes (g) | Mass after heating for 4 minutes (g) | Mass after heating for 6 minutes (g) |
|---|---|---|---|---|
| 1 | 31.2 | 27.5 | 27.5 | 27.5 |
| 2 | 31.6 | 27.9 | 27.9 | 27.9 |
| 3 | 31.0 | 27.8 | 26.9 | 26.0 |
| 4 | 31.4 | 27.7 | 27.7 | 27.7 |
Why does the mass decrease during heating?
State the colours of the solids in the reaction:
- CoCl2⋅6H2O(s)\text{CoCl}_2\cdot 6\text{H}_2\text{O}(\text{s})CoCl2⋅6H2O(s): __________
- CoCl2(s)\text{CoCl}_2(\text{s})CoCl2(s): __________
In which experiment might the dehydration not be complete?
Give a reason for your choice.
Which statement could explain why the dehydration might not be complete?
- A The student heated the crucible with the lid off.
- B The student used less hydrated cobalt(II) chloride than in the other experiments.
- C The student used a gentle yellow flame instead of a roaring blue Bunsen flame.
- D The student used a larger crucible.
In another experiment, the student calculates that she should obtain a mass of 6.2 g6.2\text{ g}6.2 g of CoCl2(s)\text{CoCl}_2(\text{s})CoCl2(s) after completely dehydrating a sample of CoCl2⋅6H2O(s)\text{CoCl}_2\cdot 6\text{H}_2\text{O}(\text{s})CoCl2⋅6H2O(s). She actually obtains a mass of 5.4 g5.4\text{ g}5.4 g of CoCl2(s)\text{CoCl}_2(\text{s})CoCl2(s). Calculate the percentage yield in her experiment. Give your answer to two significant figures.