In the following question, the water/wine mixing problem was recast in chemical terms. However, partial molar volumes can make this a much more challenging problem than it appears on the surface.


For example, consider a container of $\pu{10 L}$ of ethanol and another container of $\pu{10 L}$ of water. To make a point, let's transfer $\pu{5 L}$ of ethanol to the water container and then transfer $\pu{5 L}$ of the water/ethanol solution back to the ethanol container. What is the purity in each container at the end?

We need to consider that mixtures of water and ethanol have less volume than the two do separately:

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For example, after the first transfer, the water container contains $\pu{10.000 kg}$ of water and $\pu{3.945 kg}$ of ethanol for a total mass of $\pu{13.945 kg}$. However, we need to compute mole fraction to determine the volume of the solution:

$$\begin{array}{c|c|c} \ & \text{water} & \text{ethanol}\\ \hline \text{mass in kg} & 10.000 & 3.945\\ \hline \text{moles} & 555 & 85.6\\ \hline \text{mole fraction} & 0.866 & 0.134 \end{array}$$

From our graph, it looks like we have an excess volume of $\pu{-0.75 mL/mol}$. Since we have total moles $=640.6$, we have an excess volume of $\pu{-0.75 mL/mol}\times \pu{640.6 mol}\times \pu{1 \times 10^{-3} L/mL}= \pu{-0.480 L}$. Thus the actual volume is not $\pu{15 L}$ but more like $\pu{14.5 L}$.

If we transfer $\pu{5 L}$ back, we are left with less than $\pu{10 L}$ in the water container. We will also have less than $\pu{10 L}$ in the ethanol container!

What volume $V_1$, as a fraction of the original volume $V_0$, needs to be transferred in order for both containers to have the same final purity?

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    $\begingroup$ Interesting question. Do you want the purity as mole fraction or $\%v/v$ or $\%w/w$? $\endgroup$ – Mathew Mahindaratne Feb 13 '20 at 17:33
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    $\begingroup$ Well, mol faction can't be 0.5 since original containers have $\pu{555 mol}$ water and $\pu{171.2 mol}$ ethanol. Thus it should be either $\%(v/v)$ or $\%(w/w)$. Would you please clarify which way you want? $\endgroup$ – Mathew Mahindaratne Feb 13 '20 at 18:18
  • $\begingroup$ @MathewMahindaratne I was really hoping you would ‘take your shot’ on this one! Please post something so I can award the bounty to you and not just have it vanish away, softly and suddenly, like the hunter who spies a snark! (Apologies to Lewis Carroll.) $\endgroup$ – Ed V Feb 13 '20 at 21:59
  • $\begingroup$ @MathewMahindaratne Unless Ben Norris says otherwise, just do as you think is most sensible! $\endgroup$ – Ed V Feb 13 '20 at 22:02
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    $\begingroup$ @Ed V: I'll try my best to solve it. :-) $\endgroup$ – Mathew Mahindaratne Feb 13 '20 at 22:39

$$\begin{array}{c|c|c} \ & \text{water} & \text{ethanol}\\ \hline \text{Volume in concern, in L} & 10.00 & 5.00 \\ \hline \text{Density in kg/L} & 1.000 & 0.789 \\ \hline \text{Molar mass in g/mol} & 18.015 & 46.069\\ \hline \text{mass in kg} & 10.000 & 3.945\\ \hline \text{moles} & 555 & 85.63\\ \hline \text{mole fraction} & \frac{555}{555+85.63} = 0.866 & \frac{85.6}{555+85.63} = 0.134 \end{array}$$

From the given graph, it looks like we have an excess volume of $\pu{-0.75 mL/mol}$ (at $\chi_\text{EtOH}=0.134$). Since we have total amount of water and ethanol, $555+85.63=\pu{640.63 mol}$, we can calculate the excess volume of the container:

$$\pu{-0.75 mL/mol} \times \pu{640.63 mol} \times \pu{1 \times 10^{-3} L/mL}= \pu{-0.480 L}$$ Thus the actual volume is not $\pu{15 L}$, but $\pu{(10.00+5.00-0.480) L}=\pu{14.52 L}$

Suppose, we transfer $V_1$ of water/ethanol mixture back to the container-2 containing the remaining of $\pu{5.00 L}$ of ethanol. That $V_1$ contains: $$\frac{\pu{555 mol}\text{ water}}{\pu{14.52 L}}\times V_1 \ \pu{L} = 38.22V_1 \ \pu{mol}\text{ water}$$ and, $$\frac{\pu{85.63 mol}\text{ EtOH}}{\pu{14.52 L}}\times V_1 \ \pu{L} = 5.90V_1 \ \pu{mol}\text{ EtOH}$$

Thus container-2 has $38.22V_1 \ \pu{mol}$ of water and $\left(5.90V_1 + \pu{85.63 mol}\right)$ of ethanol after the addition of $V_1 \ \pu{L}$ of water/ethanol mixture from container-1.

Suppose we have made both containers have the same purity (by $\%(w/w)$) by this action. Let's calculate the masses of water and ethanol ($m_{w2}$ and $m_{et2}$, respectively) in container-2:

$$m_{w2} = 38.22V_1 \ \pu{mol}\text{ water}\times \frac{\pu{18.015 g}\text{ water}}{\pu{1.0 mol}\text{ water}}= 688.5 V_1 \ \pu{g}\text{ water}$$


$$m_{et2} = \left(5.90V_1 + 85.63\right) \ \pu{mol}\text{ EtOH}\times \frac{\pu{46.069 g}\text{ EtOH}}{\pu{1.0 mol}\text{ EtOH}}= \left(271.8V_1 + 3945\right) \ \pu{g} \text{EtOH}$$

Now, we'll calculate the masses of water and ethanol ($m_{w1}$ and $m_{et1}$, respectively) in container-1. Amounts of water and ethanol in this container is:

$$\frac{\pu{555 mol}\text{ water}}{\pu{14.52 L}}\times (14.52 - V_1) \ \pu{L} = \left(555 - \frac{555V_1}{14.52}\right) \ \pu{mol}\text{ water}$$ and, $$\frac{\pu{85.63 mol}\text{ EtOH}}{\pu{14.52 L}}\times (14.52 - V_1) \ \pu{L} = \left(85.63 - \frac{85.63V_1}{14.52}\right) \ \pu{mol}\text{ EtOH}$$


$$m_{w1} = \left(555 - \frac{555V_1}{14.52}\right) \ \pu{mol}\text{ water}\times \frac{\pu{18.015 g}\text{ water}}{\pu{1.0 mol}\text{ water}}= \left(555 - \frac{555V_1}{14.52}\right)\times \pu{18.015 g}\text{ water}$$


$$m_{et1} = \left(85.63 - \frac{85.63V_1}{14.52}\right) \ \pu{mol}\text{ EtOH}\times \frac{\pu{46.069 g}\text{ EtOH}}{\pu{1.0 mol}\text{ EtOH}}= \left(85.63 - \frac{85.63V_1}{14.52}\right)\times \pu{46.069 g}\text{ EtOH}$$

Theoretically, the condition for same purity by $w/w$ is:

$$\frac{m_{w1}}{m_{w1}+m_{et1}} = \frac{m_{w2}}{m_{w2}+m_{et2}} $$


$$\frac{m_{et1}}{m_{w1}+m_{et1}} = \frac{m_{et2}}{m_{w2}+m_{et2}} $$

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    $\begingroup$ It took you less than two hours to get an answer for a question that sat for more than 4 years (!) without even a comment! Wow! Thank you! $\endgroup$ – Ed V Feb 14 '20 at 1:09
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    $\begingroup$ I will now award the bounty! Boy, this is fun! Thanks again! $\endgroup$ – Ed V Feb 14 '20 at 1:14
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    $\begingroup$ @ Ed V: I apologize to take this long to response. I was rushing to go home last night. Well, I also calculated the first situation (5 L ethanol to water and back 5 L of water/ethanol). For first transfer, answer is ther (14.52 L). When second transfer has been done, new volume of original ethanol container is 9.65 L (water to ethanol mole fraction: 0.624:0.376). After transfer, volume of original water container is 9.52 L. Lost about 0.8 L! $\endgroup$ – Mathew Mahindaratne Feb 14 '20 at 16:21
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    $\begingroup$ No problem at all! Even if it should happen that your answer is not totally correct, and I am not implying that, the important thing is you stepped up and screwed this problem to the wall like so much sheetrock! That is worth 50 points anytime! $\endgroup$ – Ed V Feb 14 '20 at 16:26
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    $\begingroup$ I think we don't have to calculate masses in container-1 using $V_1$. I'd correct it sometimes today. I have a meeting in few min. :-) $\endgroup$ – Mathew Mahindaratne Feb 14 '20 at 16:36

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