In today's lab, we dissolved three Alka Seltzer tablets and collected the gas (carbon dioxide) given off from the citric acid and sodium bicarbonate. We placed a balloon filled with crushed Alka Seltzer on top of a test tube containing water and turned the balloon over to start the reaction. After the fizzing ceased, we measured the circumference of the balloon, now filled with gas, and then used the ideal gas law learned in class today to determine the mass of the gas produced.
Analysis Questions:
1. Discuss an area in this lab where experimental error may have occurred.
Experimental error may have occurred during the transfer of Alka Seltzer to the balloon, since a little bit of powder remained in the weigh boat, and some powder was spilled. Also, when we filled the balloon with water to measure the circumference, we may have been off and the size of the balloon might not have been exactly the same as it was filled with CO2. Error could have also taken place when we mixed the water in the test tube with the Alka Seltzer if we didn't wait long enough for the fizzing to stop when we measured the balloon.
2. Choose one error from above and discuss if it would make "n" the number of moles of CO2 too big or too small.
Since some Alka Seltzer spilled, it would make the number of moles of CO2 too small, because not enough of the reaction would have occurred between the Alka Seltzer and the water, and therefore not enough carbon dioxide would have been collected in the balloon.
3. Calculate the volume of the balloon mathematically.
C = 2πr
38 = 2πr
r = 19/π
V = (4/3)π * (19/π)^3 = 926.6 cm^3= 926.6 mL
4. Are the two volumes close? Which do you feel is more accurate and why?
Yes, the two volumes are close. The volume found during the experiment is more accurate than the calculated volume, because the shape of the balloon isn't exactly a sphere, so using the formula for the volume of a sphere isn't precise. However, the volume we found during the experiment isn't 100% accurate either because of the human error possibilities listed in the previous questions.
5. Give two differences between a real gas and an ideal gas.
A real gas molecule interacts with other particles through repulsions and attractions. An ideal gas molecule (imaginary) reacts independently of others and there are no interactive forces. Another difference is that when real gas molecules collide between particles or the wall of the container, there is a net loss of energy, where as in ideal gases, there is no loss of energy during collisions.
6. Would the CO2 you collected in this lab be considered ideal? Why or why not?
No, the CO2 would not be considered ideal, because it lost energy from particle collisions and had interactive forces between its molecules. It also occupied space and mass.
Advanced Questions:
1. Calculate the mass of CO2 that should be collected per tablet.
Each tablet had 1000 mg C6H8O7 and 1916 mg NaHCO3. We used three tablets and found that the citric acid produced 2.062 g CO2, and the sodium bicarbonate produced 3.011 g CO2. Due to citric acid being the limiting reactant, the theoretical yield is 2.062 g CO2.
2. What percent is the percent yield for the CO2 collected in your sample?
We divided the experimental mass of CO2, 1.86 g, by the calculated mass, 2.062 g, and obtained 90.2% as our percent yield.
3. What effect does the fact that CO2 is water soluble have on your calculated "n" value?
The fact that CO2 is water soluble may have decreased our n value, since some of the CO2 may have dissolved in the water and so was not measured in the balloon. This is why our actual results were smaller than the theoretical yield.
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