GRAVIMETRIC ANALYSIS
Aim:
To analyse a mixture of unknown proportions, separate the components and carry out gravimetric analysis.
Risk Assessment:
When boiling the solution of salt and water, the salt may pop out of the conical flask. Wearing safety glasses and staying a suitable distance away from the setup effectively minimises risk. Handling hot objects such as the conical flask, gauze mat and tripod can also be dangerous. Using tongs and allowing the equipment before cooling down can ensure safety.
Equipment:
Dry sand, dry table salt, electronic balance, conical flask, a funnel, filter paper, bunsen burner, spoons, heat mat, tripod, beaker, safety glasses.
Setup:
Method:
1. The teacher made a mixture of shellgrit and salt, recorded the mass of the components and distributed the containers to different groups. Each group received a container had different proportions of the two.
2. The container (with the mixture inside) was weighed on an electronic balance and results taken.
3. The mixture was emptied into a beaker and the empty container was once again weighed on the electronic balance and the results were recorded. This was subtracted from the measurement of the full container to give the mass of the shellgrit and salt mixture.
4. Approximately 30mL of water was added to the beaker.
5. The beaker was stirred with a spoon to ensure that the salt had dissolved so effective filtration could take place.
6. A conical flask, filter paper and a funnel were gathered and assembled to look like the diagram above. The filter paper was folded so that there was a hole small enough for salt to pass through.
7. The contents of the beaker were slowly poured into the filter paper until the beaker was empty.
8. Once the solution had finished filtering, the funnel and filter paper were taken out.
9. A heat mat, bunsen burner, tripod, gauze mat, tongs, matches and safety glasses were gathered and setup like the diagram above.
10. The gas was turned on and the match was struck to light the bunsen burner.
11. The bunsen burners oxygen holes were opened so that the more efficient blue flame could heat the conical flask and its contents.
12. Once the water had evaporated, the teacher picked up the bunsen and heated the sides of the conical flask to evaporate any condensation on the sides and allow for an accurate reading.
13. The conical flask was allowed to cool before being weighed on the electronic scales and the results recorded.
14. The conical flask was emptied and dried, and once again weighed on the electronic balance. The result was taken and then subtracted from the previous result to get a measurement of the salt in the solution. This in turn was subtracted from the mass of the mixture that had been obtained in step 3, to give the theoretical mass of the sand.
Results:
Mass of the container with the mixture inside (g)
Mass of the empty container (g)
Mixture Mass (g)
(Mass of the empty container subtracted from the mass of the container with the mixture inside)
12.13
10.86
1.27
Mass of the conical flask with dry salt
(g)
Mass of the empty conical flask
(g)
Mass of NaCl (g)
(Mass of the empty flask subtracted from the mass of the conical flask with the dry salt)
Mass of shellgrit (g)
(Mixture mass – Mass of NaCl)
108.96
108.13
0.83
0.44
Components
Correct Values (g) Value from the experiment (g)
Salt and Shellgrit
1.24
1.27
Salt
0.92
0.83
Shellgrit
0.32
0.44
Ratio
2.9:1
1.9:1
Percentage Error
Salt;
Shellgrit;
Discussion:
The percentage of error obtained from the results of the experiment varied quite greatly from the correct values given. Although the percentage error for the salt measurements was quite low, the percentage error for the shellgrit was uncharacteristically high. This could be attributed to the smaller margin of error, as well as the time constraints which did not allow us to properly dry and weigh the shellgrit and therefore, have to dispose of it. This meant that our group had to estimate the mass
