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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: e_spirit on June 18, 2005, 06:58:22 AM

Title: Finding w/w% with external standard method
Post by: e_spirit on June 18, 2005, 06:58:22 AM

Hi again  :)

I would like you to "push" my solution as I am bit stuck there.

Problem:
Standard solution is made and contains the following data:
0.5021 g ethyl propanoate
0.4927 g propyl ethanoate
4.7243 g acetone (solvent)

The standard solution and the sample are run twice each and the following results are obtained:

Component             1 Determ.      2 Determ.    Average of 2  determ.
                              Area             Area

Standard
Ethyl propanoate      525               521             523
Propyl ethanoate      492               487             489.5
Sample
Ethyl propanoate      472               474             473
Propyl ethanoate      380               376             378

Use the external standard method to calculate the content of w/w% ethyl propanoate anw w/w% propyl ethanoate in the sample. Your calculations must be included.

My solution:

1) At first I found average of two determinations

2) I can assume that the volume is 1 L
So C of the standards:
C(ethyl propanoate) = 502.1 mg/L
C(propyl ethanoate) = 492.7 mg/L

3) Calculation of Csample in external standard method is:
C(x) = C(st) * (A(x) / A(st))

so I am calculating concentration of my sample:

C(ethyl propanoate) = 502.1 * (473 / 523) = 454.1 mg/L
C(propyl ethanoate) = 380.5 mg/L

4) w/w% ethyl propanoate anw w/w% propyl ethanoate in the sample.
And here I am stucked. Should I do something with solvent? ???

Thanks in advance!

Title: Re:Finding w/w% with external standard method
Post by: Borek on June 18, 2005, 10:51:58 AM

What is % w/w concentration? Write down the definition and think - what you know, what you can use?

Beware: you are thinking in categories mg/L. That's not w/w!

Title: Re:Finding w/w% with external standard method
Post by: vbk on September 13, 2005, 12:08:24 AM

you can convert from C(mg/L) to %w/w by formulas:

C(mol/L)=C(mg/L)/M; M:mol weight(mg/mol).
and then :
C(%)=C(mol/L)*M/(10*d)
d=density of the sample.
HOPE THIS WILL HELP YOU.