[allym2506]

Hi there, for my lab report i have to determine the aspirin content.

I have to work out the conc of a salicylate in 250cm^3

I have worked out the concentration of the sample used ofr the absorbance measurement as 3.36 x 10^-4 moldm^3 - using a graph prepared earlier, so that bits ok

BUT how to i work out the conc in 250cm^3 wudnt I have to work out the number of moles in sample then divide that by the volume in dm^3? However I don't have a volume of the curvette(thing i put in the colourimeter)

Please help, i'm in a really big muddle

Thanks

[Borek]

What solution was in the cuvette? Just a little bit of your 250 mL?

[ARGOS++]

Dear Allym2506,

Sorry, it’s very difficult to figure out/interpret what you really did.

If I assume, that your  3.36 * 10^-4 moldm^3 should mean in real 3.36 * 10^-4 mole/dm³   what would be the same as 3.36 * 10^-4 mole/Liter, then an easy solution is possible:
(Maybe you have forgotten to write the “Slash” for the Division.  - I hope so!)

In this case you must know, if you have to determine the concentration in (i)  g/cm³ (= g/ml) or just as  (ii) mole/cm³, or  (iii) the whole amount of the sample (g/250 cm³ = g/Tablet).
In all three cases the cuvette size is not used anymore, because your number is already a concentration.

For (i):
  You have first to get the “Molecular Weight” (MW = g/mole) of your Salicylate and calculate:
    c = 3.36 * 10^-4 mole / 1000 cm³ * MW

For (ii):
  c = 3.36 * 10^-4 mole / 1000 cm³

For (iii):
   c = 3.36 * 10^-4 mole / 1000 cm³ * 250 cm³ * MW

But I could not figure out, what you really have and for what correctly you are asking.

I hope this may bee anyway of some help to you!

Good Luck!
                    ARGOS⁺⁺

[Yggdrasil]

Concentration is independent of volume.  If you have 1L of a 2M solution of NaCl, and you take 10mL of that solution, your 10mL solution will still have a concentration of 2M.

The question is similar to asking, which is more dense, a 1g block of lead or a 10kg block of lead?