Finding the limiting reagent:
Moles HCl:
N = cv = 3.0 M HCl x .01 dm³ = .03 moles HCl
Moles Mg:
.N = mass/ relative molecular mass = .0362 grams _ = .0015 mol Mg = limiting reagent
24.13 g Mg
Ratio between Mg and H2 is 1:1, therefore moles of H2 produced: .0015 mol H2
Calculating gas constant:
PV= nRT
Trial 1: 101.27( .032) = (.0015)R ( 298) = 7.25 J/mol * K
Trial 2: 101.27(.0322) = (.0015)R (298) = 7.30 J/mol * K
Trial 3: 101.27(.0323) = (.0015)R (298) = 7.32 J/mol * K
(± .001 J/mol * K)
Observations:
Almost seconds after the eudiometer tube is inverted into the water, you can see the HCl acid sinking down through the water. Soon after, tiny bubbles become visible on the Magnesium strip that starts to float to the top of the eudiometer. These bubbles are the Hydrogen gas that formed in the reaction Mg + 2HCl + H2. Over the course of the next few minutes the bubbles increase in size and quantity until bubbles were forming quite vigorously. When looked at closely the Magnesium strip is clearly reacting and decreasing in size.
Conclusion:
From this experiment, the ideal gas constant was determined using the equation PV= nRT. Using this equation should have resulted in a rather accurate constant because the pressure was retrieved from weather.com, which is as far as we know, a reliable source. The volume is measured on the eudiometer, which has an uncertainty of .0001 in dm³, which is a very low uncertainty that shouldn’t be a dominant factor in the unreliability of the data. The amount of moles are calculated by hand using the mass of the magnesium. The magnesium was weighed with an inaccurate scale because the uncertainty is .005 grams rather than .001 grams on most scales. The temperature has an uncertainty of .5 K which is also very high because the temperature is measured on a thermometer that is hard to read (which would be a random error), and imprecise. With these variables, the average calculated constant was 7.29 J/mol K. Relatively speaking, the experiment was accurate. However, the most precise measurement of R has been calculated and found it to be 8.314472(15), so there were clearly some errors in the procedure.
Evaluation:
From the results, it can be concluded that method by which the ideal gas constant was not completely accurate. One error could have occurred as the water was put into the eudiometer. The water was supposed to drizzle down the side of the eudiometer so that the acid would not mix with the water, however the dropper was squirting the water out with quite some force that I fear that the water managed to mix with the acid ever so slightly. Another error could have occurred when inverting the cylinder into the beaker of water. According to the method, the inversion had to be done quickly, as the stopper was held tight, and the eudiometer kept still after it was inverted. This however, was quite difficult and errors may have occurred in the process. The last possible error could have occurred due to an incomplete reaction. To release any trapped bubbles, you had to tap the side of the cylinder, but miniscule bubbles continuously were released for an extra few minutes. This makes it unclear whether or not, bubbles could have continued for even longer.
Improvement:
After evaluating the method, it is clear that there are a few aspects of the method that can be improved. First, the dropping of the HCl must be done more carefully next time. Secondly, the eudiometer must be inverted into the water with more precision, as well as care. Thirdly, it must be certain that the magnesium is fully reacted before results are recorded.
