Determination of Caffeine in Moutain Dew

[MitsuShai]

We did an experiment (link provided below), where we were to determine the concentration of caffeine in Mountain Dew. We made 3 standards and measured their absorbance along with the unknown (the mountain dew sample) and we graphed these results to find the concentration of caffeine in Mountain Dew. I wanted to compare this with the amount of caffeine in a 2 Liter Mountain Dew bottle (which was the sample used in the experiment).
I only had to take 4 mL of the same of soda, which was diluted to 100 mL with 10 mL of HCl, so I thought I just had to multiply the concentration that I got by 100 mL to get the mg of caffeine, but I didn't get 310 mg of caffeine (which is the amount of caffeine in Mountain Dew), instead I got a really small number. How do you calculate the mg of caffeine from the concentration (mg/L)?


http://www.mnstate.edu/marasing/CHEM380/Labs/380PLABL/Spectrophotometric%20Analysis%20of%20a%20Mixture%20%20Caffeine%20and%20Benzoic%20Acid.pdf

 

[Borek]

Haven't you calculated amount of caffeine in 4 mL?

 

[MitsuShai]

Haven't you calculated amount of caffeine in 4 mL?

So I can't compare it to the caffiene in a 2 Liter bottle, right?

 

[Borek]

If you know how much caffeine is on 4 mL, you can easily calculate how much is in 2 L.

From the information you posted it is not clear what you did and what you calculated. Procedure calls for measurement of concentration in diluted sample, so I guess the number you have is concentration AFTER dilution. This is not the same as in the original drink.

 

[jackmell]

Hey, I did that experiment too a long time ago. Let me see if I can add something without getting into trouble:

Say you take 2ml of Dew and dilute it to 100ml and you run your sample against all the standards and you come out with a value that says in that 100 ml sample, you have 6 mg/l of caffeine. So wouldn't that mean in the original 2ml sample of Dew that would be 50 (6)=300 mg/l since you diluted 2/100?

 

[MitsuShai]

Hey, I did that experiment too a long time ago. Let me see if I can add something without getting into trouble:

Say you take 2ml of Dew and dilute it to 100ml and you run your sample against all the standards and you come out with a value that says in that 100 ml sample, you have 6 mg/l of caffeine. So wouldn't that mean in the original 2ml sample of Dew that would be 50 (6)=300 mg/l since you diluted 2/100?

Oh duh that's how you do it, I was thinking of multiplying the sample by (2/100) instead of (100/2).
Thank you so much.

 

[chemisttree]

Unit analysis tells you what to do... multiply or divide.