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### Topic: Finding w/w% with external standard method  (Read 6170 times)

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#### e_spirit

• Guest
##### Finding w/w% with external standard method
« 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?

#### Borek

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##### Re:Finding w/w% with external standard method
« Reply #1 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!
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### vbk

• Guest
##### Re:Finding w/w% with external standard method
« Reply #2 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.