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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Twickel on March 17, 2012, 05:19:43 AM

Title: Absorbance solution, dilution problem.
Post by: Twickel on March 17, 2012, 05:19:43 AM

Hi
A practice problem I am having a lot of trouble with. If any one can help it will be much appreciated.
Questionf th
A 0.3gm of powder is digested and nuetralised in 500mL, of which 10mL is diluted to 100mL. To 25mL of this solution 5mL of 0.1 M ammonium molybate, followed by 2mL of 0.012M hydrazine sulfate solution. The sample is heated for 10min cooled and analysed using AAS. What is the number of moles oh phosphorus in the original sample?

Ok I have figured out that the concentration of P = 2.8x10^-4 M. In the final solution ( the one that has a volume of 32ml). I cant figure out how to work backwords for this. I am geting confused as to when the number ofmoles changes/when it stays the same and when the concentration changes.

Please help.

Thank you,

Title: Re: Absorbance solution, dilution problem.
Post by: Borek on March 17, 2012, 06:45:10 AM

When you mix solutions final solution contains all introduced material (mass conservation). When you take part of the solution, it contains only a fraction of the original amount of the substances.