Lab 18: Alka Seltzer and the Ideal Gas Law

Introduction: Alka Seltzer tablets contain sodium bicarbonate and citric acid, which when dropped in water, dissolve and react to produce carbon dioxide gas. In this lab we dropped 2 Alka Seltzer tablets into a test tube filled with water and then wrapped a balloon around its rim. When the Alka Seltzer stopped fizzing, we measured the circumference that the balloon blew up to. Furthermore, using the ideal gas law, we determined the mass of the carbon dioxide gas produced.

after Alka Seltzer was dropped into the test tube

Data Table: 

Calculations: 

Analysis Questions

Discuss an area in this lab where experimental error may have occurred. 

When we were transferring the ground Alka Seltzer tablets from the plastic tray to the balloon, we inevitably lost some of it and could not remeasure how many grams of the alka seltzer tablet remained. Another source of experimental error was our inability to measure the circumference of the balloon perfectly; the string kept sliding down the side of the balloon as we tried to attain its circumference. 

Choose one error from above and discuss if it would make "n" the number of moles of carbon dioxide too big or too small. 

The lost of Alka Seltzer when transferring from the plastic tray to the balloon contributes to a smaller number of moles, "n", of carbon dioxide. This is because when Alka Seltzer powder is lost, the substances reacting with water decrease as well, which ultimately results in a smaller yield of carbon dioxide gas. 

Filling the balloon with water may be one place where error could have occurred. Using the value for the circumference of the balloon in cm, calculate the volume of the balloon mathematically. 

We know the formula for circumference is c=2pi*r, so we can use our circumference measured with the string to find the radius:

With this radius, we can then plug it in to the equation for the volume of a sphere, V=(4/3)(r^3)pi and get the following:

Compare your answer to #3 to the volume attained by filling the balloon with water. Is it close? Which do you feel is more accurate and why? 

The volume we obtained by filling the balloon with water is significantly larger than that of our calculated one. Thus, the values are within about 50 mL of each other, so they are not close at all. However, between the two, I feel as though the calculated answer in #3 is more accurate due to the high levels of variability when measuring the circumference of the balloon with a string. Additionally, when we were filling the balloon up with water in the lab, we put in an excess amount of water but didn't have time to achieve the exact measure of the circumference perviously attained.

The ideal gas law technically applies to ideal gases. Give two differences between a real gas and an ideal gas. 

Essentially, ideal gases are purely theoretical and cannot be found in nature, but gases behave in this "ideal" manner at certain temperatures and pressures. Also, ideal gases are said to not have intermolecular forces, whereas real gas molecules, although often negligible, have size, volume, and intermolecular forces.

Would the carbon dioxide you collected in this lab be considered ideal? Why or why not?

No, the carbon dioxide gas would most definitely not be considered ideal because an ideal gas can only exist in theoretical conditions with perfect pressure, temperature, and no volume lost. 

Advanced Questions

Using the information provided on the label, and stoichiometry calculate the mass of carbon dioxide that should be collected per tablet.

The net ionic equation of this reaction used was:

.

Thus, given that there is 1916 mg of sodium bicarbonate in each tablet, the mass of carbon dioxide collected per tablet is:

What percent is the percent yield for the carbon dioxide collected in your sample?

Since we only calculated the mass of carbon dioxide collected for one tablet in the previous question but used two in our lab, we must double the theoretical yield of 1.382 grams. Thus, we have

.

Carbon dioxide is water soluble. The solubility around room temperature is around 90mL/100mL of water. What effect does this have on your calculated "n" value?

As a water soluble molecule, carbon dioxide is dissolved in the water as it is simultaneously being released as gas in to the balloon. Hence, the mass of carbon dioxide we calculated in the balloon is quite different from that of the theoretical amount; we were only able to account for the carbon dioxide gas released into the balloon and not what was dissolved in the water as well. Therefore, the circumference of the balloon was smaller than what it should have been (if all carbon dioxide was in the balloon), so the calculated "n" value was also much lower than what it would have been theoretically.