Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: allym2506 on October 25, 2007, 12:23:13 PM
-
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
-
What solution was in the cuvette? Just a little bit of your 250 mL?
-
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++
-
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?