May 29, 2024, 04:44:37 AM
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Topic: Solution Concentration Question (lengthy post for likely short answer)  (Read 2064 times)

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Offline CMR111

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I'll try to keep this short and concise.  We have run a lab twice.  We need to work backwards with the concentration we found from the lab, to find a series of solution concentrations and then a tablet of Fe's iron mass.  I just need help with finding the mass of Fe in the tablet.  Here are the details:
 
  • We boiled a pulverized Fe vitamin tablet in about 20-25 mL HCl, then diluted with DI to 100 mL. This became solution A.
  • We took 1 mL of Solution A and diluted it to 100 mL with DI water. This became Solution B.
  • We took 3 mL of Solution B and diluted it to 25mL with DI water.  This became Solution C.
  • After going through some other stuff to obtain the Fe concentration of C, we need to use that to find the concentration of B
  • From the concentration of B we need to find the concentration of A (we're just using C1V1=C2V2, none of this is hard)
  • Now we're supposed to find the Fe mass of the tablet from the concentration of Solution A.
 
During the first run of this last week, no one was able to get the right number at all...like really off the wall numbers.  I did it differently and actually was pretty close (only about 3.5 mg off...the bottle says it has 65 mg).  The instructions on the lab printout says to use 25 mL HCl for that first step...but I guess in class she said it just had to be enough to fill the bottom of the beaker and powdered tablet.  Either way, when I multiplied Solution A's concentration by 25 mL that's when I ended up with that close number.  Because everyone was way off (if I did it the way we are supposed to, my tablet Fe mass last week would be 245.05 mg and everyone was getting these huge numbers) we had to essentially repeat the lab today.  I showed the lab instructor and she said we multiply the concentration of A by 100 mL since A is 100 mL.  Oddly my concentration doing it their way is still 242.9...so not much off from the time before.  If I use the method I did before, I get 60.73 mg (to be fair we didn't swish the bottle, so some may've settled...I didn't think about that until tonight).  Has anyone done this sort of thing? I'm not sure what to do with this...

Offline Hunter2

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What is the mass of the tablet you dissolved?

You have m(Tbl.)/ 100 ml is 10* m(Tbl.)/l solution A
Take 1 ml and diluted to 100 ml means 1:100, it is 0,1* m(Tbl.)/ l solution B
Then take 3 ml means 0.3* m(Tbl.) /25 ml = 0,012* m(Tbl.)/l solution C

Your iron content in C has to be multiplied by 250 and divided by 3 to get the iron in the tablet.


« Last Edit: April 10, 2024, 02:52:46 AM by Hunter2 »

Offline Borek

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when I multiplied Solution A's concentration by 25 mL that's when I ended up with that close number

That's completely random.

What matters is the final volume of the solution, as it defines the dilution. No matter what volume of the acid you use (assuming tablet gets completely dissolved) you fill it up to 100 mL and the concentration becomes "amount of iron in the tablet/100 mL". It will be the same whether you use 2 mL of acid, 5 mL of acid, or even 50 mL of acid.

Problem is most with likely with

Quote
some other stuff to obtain the Fe concentration
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline CMR111

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UPDATE:  As it turned out, someone in a previous semester mixed the standard Fe solution wrong, so the concentration isn't 10 mg/L we don't know what it is. 

Offline Hunter2

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Then make a new Standard.

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