-1
$\begingroup$

I'm trying to work backwards to obtain the original concentration of a 1 litre solution. I feel my calculations have gone wrong and need another set of eyes to have a look.

To start, a tablet containing arginine was dissolved in water and made up to 1 litre solution. A 250 μL aliquot was taken from the solution and diluted to 25mL.

Analysis was done and a concentration value for the 25mL sample was 4.25 x10^-5 mol/L.

When I tried to work backwards from that concentration value and determine what the original concentration is in the 1 litre solution, I got 17 mol/L. Which seems excessively high for what was suppose to be a tablet.

I then need to give the value of mass of arginine (172.2 g/mol) in the tablet, but using the 17 mol/L, gives a value ~3kg.

What do I need to do to get the concentration value for the 1 litre solution?

$\endgroup$
2
$\begingroup$

You took an aliquot of $0.25 ml$ with unknown concentration. You diluted 100 times by adding 100(-1) times the volume of your aliquot. The result was $4.25*10^{-5} mol/L$, or, $$4.25*10^{-5} mol/L * 172.2 g/mol = 7.38*10^{-3} g/l$$

You diluted 100 times, so going back it becomes 100 times more concentrated: $ 100*7.38*10^{-3} g/l = 0.738 g/l$.

A good habit, even if you're not stuck, is to write the units corresponding to the values. This way it is very easy to spot when you've for example multiplied by a molar weight you should have divided by.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.