Lab 16: Gravimetric Determination of a Precipitate:
The goal of this experiment is to determine the molecular formula of an unknown alkali (or alkali earth) metal sulfate, via application of gravimetric analysis. We used barium nitrate, due to the very low solubility of barium sulfate. We can then react all of the sulfate ions within the HCl solution with barium (which is added to excess, to ensure that all of the sulfate reacts), filter the solution (with ashless filter paper), rinse it thoroughly, burn the paper to isolate the precipitate, then measure the mass of the precipitate. This can be applied to determine the amount of an analyte within a sample of an unknown substance.
I hypothesize that with this data gathered, and knowledge of the molar mass of barium sulfate (and its constituent ions) we should be able to calculate the mass of the sulfate, and from that, the percentage of sulfate within the unknown alkali (or alkali earth) metal sulfate. With this data, we can cross reference the unknown sulfate with all the various possible alkali and alkali earth metal sulfate, and thus determine the one that is the most likely to be the unknown metal sulfate.
- Beaker, glass, 400-mL
- Beaker, glass, 250-mL
- Watch Glass, 100-mm
- Graduated Cylinder
- Thermometer, Mercury
- Ring Stand
- Buret Stand
- Clay Triangle
- Bunsen Burner
- Hot Plate
- Mesh Pad
- Crucible with lid
- Hot Plate
- Ashless Filter Paper
- Wash bottle filled with Deionized Water
- Unknown Alkali Sulfate
- 0.5 M Barium Nitrate
- 0.1 Silver Nitrate
- 6 M Hydrochloric Acid
- Obtained equipment. Measured mass of the unknown substance by difference: 0.518g
- Added 50 mL of deionized water, then stirred. Added 5 mL of the 6 M hydrochloric acid to the solution. Covered beaker with watch glass.
- Assembled Buret stand, buret clamp, and thermometer. Set hot plate beneath stand. Placed beaker containing solution on the hot plate, and removed watch glass. Adjusted height of clamp so that the thermometer was suspended within the solution without touching the surfaces of the beaker. Heated the solution until it reached 90º C. Added 0.5 M barium nitrate, one drop at a time. Ensured that it was added in excess, so that all sulfate would be precipitated. Waited, before testing for completeness, by adding more barium nitrate.
- Heated the solution for 1 hour, removed from hot plate, placed on mesh pad, and allowed the solution to cool to room temperature. Returned beaker to heat, and heated for another hour at 90º C.
- Poured solution through funnel and ashless filter paper to separate the precipitate from the solution. The solution was drained into a 250-mL glass beaker. Washed the precipitate in the filter paper with deionized water, to wash away any remaining solution. Disposed of the solution and wash within the appropriate container.
- Assembled Bunsen Burner, ring stand, and clay triangle. Inspected crucible and lid for any defects (henceforth, assume that crucible and lid are together, unless stated otherwise). Measured mass of crucible and lid: 39.21g
- Heated crucible and lid with Bunsen Burner until glowing. Using tongs, placed crucible on mesh pad, and allowed to cool to room temperature, before measuring mass again: 39.09g
- Reheated, and re-cooled the crucible, before measuring its mass a third time: 39.09g
- Placed ashless filter paper (with precipitate) into crucible, before heating it for 30 minutes, with the lid removed. After the filter paper was incinerated, continued heating for 10 more minutes, before placing the crucible on the mesh pad, and allowing it to return to room temperature. Measured the mass of the crucible, lid, and precipitate: 39.95g
- Disposed of precipitate within the appropriate container. Cleaned crucible and lid, and set them aside to dry. Returned all equipment to its point of origin.
Calculated the mass of the dried precipitate. The mass of the crucible with dried precipitate, minus the mass of the crucible: 0.860g
Calculated the number if moles of precipitate:
Given the 1:1 ratio, there are also 0.00368 moles of sulfate ions in the precipitate.
Calculated the mass of the sulfate ions in the precipitate:
Calculated the theoretical percentage of sulfate content in the unknown sulfate sample:
Compared the properties of various sulfates of alkali and alkali earth metals, with the properties of the unknown sulfate:
The closest of these to the unknown sulfate is:
Calculated the percent error:
As such, we can conclude that the most likely molecular formula for the unknown sulfate is:
The goal of this experiment is to determine the molecular formula of an unknown alkali (or alkali earth) metal sulfate, via application of gravimetric analysis. This possible by the use of barium nitrate, due to the very low solubility of barium sulfate. All of the sulfate ions within the HCl solution reacts with the barium (which is added to excess, to ensure that all of the sulfate reacts), the solution is filtered (with ashless filter paper), rinsed thoroughly, the paper burnt to isolate the precipitate, and then the mass of the precipitate is measured.
Given the data gathered, and knowledge of the molar mass of barium sulfate (and its constituent ions) it is possible to calculate the mass of the sulfate, and from that, the percentage of sulfate within the unknown alkali (or alkali earth) metal sulfate.
The most likely molecular formula for the unknown sulfate is:
Percent error: 0.961%
Possible sources of error include:
- Incomplete precipitation (would result in lower experimental sulfate content than expected)
- Incomplete transfer of precipitate from the beaker to the funnel (would also result in lower experimental sulfate content than expected)
- Incomplete washing of the precipitate, and/or incomplete drying of the precipitate (would result in higher experimental sulfate content than expected)
- Failure to rinse the thermometer and beaker (would cause a lower experimental sulfate content, due to loss of precipitate)
- Human error is always in effect, given that the laboratory does not function under ideal conditions. As such, there is always the possibility of inaccuracies with measurement, perception of measurement, inaccuracies of equipment, and other such errors. (However, this is not likely to be the sole cause of the inaccuracies within this experiment, though it may contribute to it.)
A possible improvement to make to subsequent iterations of this experiment would be to carry out the entire experiment in a single session, and not divide it over two subsequent weekends. This reduces the possibility of human error, and the possibility that external factors can disrupt the solution.