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Homework Help: Chem Lab Help Please

  1. Feb 10, 2006 #1
    Finding Amount of CO2 Help Please

    I am having trouble with my Alka-Seltzer, Water lab.

    The purpose of the lab is to figure out how much CO2 is produced when an amount of Alka-Seltzer is mixed with an amount of water. The only equation we have is:

    Alka-Seltzer + H2O ---> CO2 + Products

    I have tried every thing I could think of. I measured out 30mL of water, and 1g of Alka-Seltzer. The water was 28°C and the current atm of where I live = .832 atm

    The Experiment: I filled a bucket with water (same temperature), placed a plastic bag (with the Alka-Seltzer in it) over the graduated cylinder full of water. I immediately placed the system (air tight cylinder with water and Alka-Seltzer mixing) under water so that the liquid products stayed about the same height as the water in the bucket. When I looked at the amount of liquid product left, it stayed at 30 mL (still with gas in the container). At this point, I am not sure where to go as I do not have a balanced equation (or the reactants in Alka-Seltzer) that I could solve using Stoichiometry. I have completed a similar task where I filled a plastic bag with approximately 10 mL of water and dropped .5g of Alka-Seltzer in and sealed the bag. I lost 1 mL of water, when poured back into the graduated cylinder, but I assume that was the remaining liquid on the walls of the baggie.

    I would appreciate some advice as to how I can use my current data to finish the lab (Am I forgetting to do something?) or if there are any other experiments I could try / use to figure out the amount of CO2 produced. Thanks for the help. :smile:
    Last edited: Feb 11, 2006
  2. jcsd
  3. Feb 11, 2006 #2


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    what exactly is alka selzer? The chemical compound(s) in question.
  4. Feb 11, 2006 #3
    I'm not sure, I havent found a clear answer on the web either, all of the sites I pull up will tell me that it has aspirin sodium bicarbonate (baking soda) and citric acid. Knowing what the full equation is would really help if anyone knew it. Still, I cannot figure out a way to find the CO2 produced with what I have (I assume it can be done without the full equation), any suggestions?
  5. Feb 11, 2006 #4
    Ok, I believe I have found an equation that shows what is happening. I am not 100% positive that this is the correct equation:

    3HCO3 (-1 charge)(aq) + H3C6H5O7 (aq) ----> 3CO2 (g) + 3H2O + C6H5O7 (-3 charge)(aq)

    Given that the equation is correct, could someone show me where to go with the numbers that I compiled earlier? Thanks for the Help.
  6. Feb 12, 2006 #5


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    bicarbonate itself reacts with an acid solution and then subsequently decomposes to produce carbon dioxide, it's actually your task to find the right acid....I'm not able to make out the details of the experiment you posted originally. I'm thinking that the purpose is to figure out the gas volume due to carbon dioxide emission.
  7. Feb 12, 2006 #6
    Correct. TO make the experiment clearer, here is another instruction set.

    I took one sealable bag and weighed it. I placed exactly 30 mL of water inside the bag. I dropped 1g of Alka-Seltzer in the water after the bag was sealed. I weighed the bag with the gas, products still inside. I determined that there was approximately 1 gram more than with just the bag and water. I opened the bag and got all of the gas out so there was only liquid. There was approximately .5 to .8 grams of liquid than there was water. The temperature decreased about 1 degree C from the original 28 degrees.

    There was about a .2 to .3 gram difference from when the gas and products was in the bag to when the liquid products only was in the bag. Does this mean that the gas CO2 weighs .2 to .3 grams? And if that is correct then I could find the molar mass of CO2 and use:

    .2 / molar mass = mols

    and then use the atm here to find the volume of the gas?

    Is this the correct way to find out how much CO2 is left? Thanks for the help.
  8. Feb 13, 2006 #7


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    Honestly I have never done this experiment before, but from what I'm understanding:

    you weighed the original alka-seltzer tablet correct? Consider the weight of the bag and products after the bag was reopened (all of the gas expelled); find the mass due to the "alka-selzer" still remaining in the liquid.

    Subtract this value from the original 1.000 grams. Conservation of mass tells you that the difference is due to the emission of carbon dioxide.

    From this gram value, convert to moles or whatever else the teacher/professor requested for. You want to make sure that you perform the gravimetric analysis as accurate as possible.
  9. Feb 13, 2006 #8

    Thanks, I got the right answer for the experimental (or atleast I was within my professor's percent error). Now, I have to find a percent yeild, or the percent I was off by the theoretical. I do not understand how to perform the stoichiometry for the equation he gave us today.

    The equation is:

    C3H5O(COOH)3 + 3NaHCO3 --> C3H5O(COONa)3 + 3H2O + 3CO2

    Values I have:

    Water Weight: 10 g or 10mL
    Water Temperature: 29°C
    Alka-Seltzer Weight: .895 g
    ATM where I live: .832 atm

    I cannot figure out where to put my values in the left side of the equation because there is not a reactant for water and one for alka-seltzer. Do you happen to know how to split my values so I can do the stoichiometry? All I can think of is splitting them in half, but that does not seem practical. Thanks for the help.
  10. Feb 13, 2006 #9


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    If alka-seltzer is made of the acid and bicarbonate, C3H5O(COOH)3 + NaHCO3, than all you'll need to do is convert from the moles of CO2 you obtained empirically, to each components independently using the stoichiometric ratios. Than compare this value with the actual weight of the alka-seltzer you weighed out initially.

    you may need to account for the filler, has the professor mentioned that?
  11. Feb 13, 2006 #10

    No he has not mentioned filler, is that something I should research?

    I think what your saying is backward from what I need to find. I am supposed to find the CO2 volume theoretically using just the values of H2O and Alka-Seltzer in grams.

    If I am understanding you correctly though, the alkaseltzer is the

    C3H5O(COOH)3 + 3NaHCO3

    and alone produces all of the products. Therefore I can take my experimental values of CO2 to find the percents of the Bicarbonate and Acid in the Alka-Seltzer. Using those percents, I can then find a theoretical value for the Bicarbonate and Acid and then do stoichiometry to find the theoretical value of CO2.
    Does that sound remotely correct?

    Sorry for all the stupid questions, I missed this part of the lecture and am trying to catch up. Thanks for the help though, I appreciate it.
  12. Feb 14, 2006 #11


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    well you've already found the moles of CO2 right? You've dissolved the alka-seltzer tablet, regardless of the products, everything except the carbon dioxide remains in the liquid content and thus contributes to the weight. When you expel the gas (not that it contributes so much to the weight in the first place, you can't really weigh air), and the carbon dioxide, you can find the resulting mass of the alka-seltzer content (reactants and products) and subtract it from the original weighed alkaseltzer tablet. Are you with me so far?

    If the task is to simply propose an ideal gas volume for CO2, assuming standard conditions you can use the conversion factor, 1 mole/22.4L, to find the volume. That is multiply the CO2 moles you've found by this conversion factor. Something tells me though, that there is more to the experiment than just this. Why are you trying to find the theoretical value of CO2? In order to do that you'll need to know

    -the composition of the reactants in alka-seltzer, as it pertains to the stoichiometry of the reaction equation. You'll need to use these composition values (of which I'm not quite sure....the composition of alka-seltzer that is) to find the theoretical values of CO2, if that is your task (moles,volume,etc.......). You already know the empirical value.

    no, but you can use the empirically (experimentally) determined value for CO2 to suggest the percent composition of the alka-seltzer with respect to the acid and bicarbonate. As I've said, in order to determine the theoretical values of CO2, you'll need to know the actual percent composition (as it may be stated on the actual package).
  13. Feb 14, 2006 #12

    Thanks for all the help, I think I get the stuff now (I got close enough answers anyways). I appreciate it. :biggrin:
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