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# Help with calculating the headspace concentration of butyric acid in a 1 mM

## Help with calculating the headspace concentration of butyric acid in a 1 mM

(OP)
Hi everyone,

I´m stucked with a very easy chemistry problem. I have a 10ml crimped vial with 2ml of a 10 mM solution of butyric acid, the headspace volume is 8 ml. Our company is developing a sensor and we need to know the concentration of butyric acid in the headspace of the closed vial.

I´ve tried to make the calculation with the following formula:

n=Pg x Vh/R x T

Where Pg is the partial pressure of butyric acid in the headspace and Vh is the volume of the headspace.

I obtained Pg from the following formula: Pg=Ca/Kh, where Ca is the concentration of butyric acid in aqueous solution and Kh is the henry´s law constant for butyric acid (1900 M/atm).

n= (2.63x10-5)x(0.008L)/0.08205 atm L mol-1 K-1 x 293 K
n=8.75x10-9 mol

To get the concentration in the headspace I divide the mols I obtained above by the headspace volume (0.008L). I get a headspace concentration of 0.00109mM.

This seems extremely low to me, and I have the feeling that there is a mistake in the way I´m making the calculations, but I can´t put my finger on it.

I would really appreciate if somebody out there could give me a hand with this. Thanks so much in advance.

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