# Calculating the Activity Coefficient parameters form Henry's Law

Here is the question, I am stuck on part B:

Basically, this is my thought process so far, as $x_a \rightarrow 0$, the Henry's law line becomes tangential to the actual partial pressure curve.. Henry's law states that:
$$p_{\alpha} = Hx_{\alpha}$$

Similarly, using the activity coefficient model: $$p_{\alpha} = \gamma_{\alpha}x_{\alpha}p^*_{\alpha}$$

Where H is Henry's constant and $p^*_{\alpha}$ is the vapour pressure of the pure component. So at small $x_{\alpha}$ values one can assume: $$H = \gamma_{\alpha}p^*_{\alpha}$$

From this I can get values for the $\gamma$ (I've been given values for pure component pressure). Then I can get values for $\ln \gamma_1, \ln \gamma_2$. My next step would be to substitute in values of $x_2$ and $x_1$ into the given correlations. So, for $\gamma_1$ to be correct, $x_1$ must be very small, assume it is zero therefore $x_2 \approx 1$, therefore:

$$\ln \gamma_1 = \frac{A \times 1^2}{\bigg( (1-1)^2\frac{A}{B} + 1 \bigg)^2} \implies \ln \gamma_1 = A$$

Clearly, this is wrong because if I run through the same process for $\ln \gamma_2$, I get a different A value. Can someone please advise, thanks.

• bump, is there an error in the question?? – MathsIsHard Mar 30 '17 at 16:47
• I think there must be an error, the van Laar activity model uses B as a coefficient on for $\ln \gamma_2$ – MathsIsHard Mar 30 '17 at 17:02