Need help with Alka-selzer

[blackflame]

hello everyone i hope you can help me out....

 

Today my chemistry teacher gave us this experiment:

 

to find out why the percent yield of the production of CO2, that you get from the reaction when a alka-selzer is put into water, is only 1-6%

 

the chemical reaction is as follows

 

NaHCO3 + H20 ---> CO2 + H2O + NaOH

 

I used 1.78 grams of Alka-selzer (NaHCO3) or 0.02119 moles

 

After doign the math on paper i should be getting 0.02119 moles of CO2, but in reality i can only collect .000447 moles.

 

Could anyone explain as to why it is such a lower percent yield?

 

My teacher also said that the secret lies within the indegeridents, (which are asprin, and citric Acid)

 

(please forgive my spelling i was in a rush to get to work)

 

Thanks in advance,

Greg

[encipher]

You have to remember that Alka-Seltzer is not pure NaHCO3.

 

It consists of Aspirin (acetylsalicylic acid), NaHCO3 and Citric Acid.

 

both aspirin and Citric acid have significantly higher molar masses (180 and 192 respectively) so much of the 1.78 grams of Alka-Seltzer is not NaHCO3.

 

Now by collecting the CO2 gas efficiently, you can get a fairly resonable idea of how much NaHCO3 the 1.78 grams actually contains. Because AFAIK the ratio of the ingredients is probably a 'company secret', or something of that sort. If you can find it online, you can determine the ACTUAL percent yeild from the amount of NaHCO3 it reallly contains.

[blackflame]

well thank you very much

[John Cuthber]

It's not exactly a well kept secret what goes into the tablets.

Ingredients

Active Ingredients: Each Tablet Contains: Aspirin (325 mg), Citric Acid (1000 mg), Heat Treated Sodium Bicarbonate (1916 mg). Alka-Selzer in Water Contains Principally the Antacid Sodium Citrate and the Analgesic Sodium Acetylsalicylate. Inactive Ingredients: None.

BTW, the equation in the first post doesn't reflect what is going on here; in fact that reaction goes the other way.

[YT2095]

it also doesn`t say anywhere that the reaction with the NaHCO3 goes to completion, it may still be Basic when all ingredients are used up

[woelen]

I agree with John Cuthber, that reaction of NaHCO3 never can proceed like this. What is possible though is the following:

 

2NaHCO3 --> H2O + CO2 + Na2CO3

 

It can nicely be observed by dissolving some NaHCO3 in water and then heating. The solution will loose lots of small bubbles of CO2. It is actually quite nice to see this reaction proceed. As if the water is boiling with very fine bubbles.

This reaction also happens when the solid is heated.