Reef Chemistry Question of the Day #239 Evaporation of DIY NOPOX

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #239

Suppose that a DIY version of NOPOX contains a mixture of the following chemicals:

water (85%)
ethanol (10%)
acetic acid (3%)
methanol (1%)
isopropanol (1%)

If you allow half of it to evaporate away into dry air, the relative concentration of each of these chemicals in the remaining mixture, compared to the starting mixture, has done what?

A. Water and isopropanol increase, the others decrease
B. Water and acetic acid increase, the others decrease
C. Water and ethanol increase, the others decrease
D. Acetic acid and isopropanol increase, the others decrease
E. Acetic acid and ethanol increase, the others decrease
F. Acetic acid increases, the remainder decrease
G. They all increase
H. They all decrease

As a hint, the boiling points for the

water (100 deg C)
ethanol (78 deg C)
acetic acid (118 deg C)
methanol (65 deg C)
isopropanol (83 deg C)

Good luck!

























.

 

[SashimiTurtle]

B?

 

[drawman]

B!

 

[OREGONIC]

F?!

 

[rkpetersen]

B. There's math to solve this numerically but I'm ashamed to admit that I haven't cracked an organic chemistry text in almost 40 years. Just eyeballing it, acetic acid concentration will rise but only to a maximum of 6%, methanol will vanish, isopropanol isn't really a quantitative factor, and ethanol evaporates faster than water.

 

[aquaman30k]

B

 

[Randy Holmes-Farley]

B. There's math to solve this numerically but I'm ashamed to admit that I haven't cracked an organic chemistry text in almost 40 years. Just eyeballing it, acetic acid concentration will rise but only to a maximum of 6%, methanol will vanish, isopropanol isn't really a quantitative factor, and ethanol evaporates faster than water.

I'm assuming people will use a rationale something like yours (I did), rather than an equation.

 

[MnFish1]

The liquids with the lower boiling points will evaporate more quickly - since Acetic acid and water are at the top of the boiling points, their relative concentration will increase 'relative' to the others. (B)

 

[Randy Holmes-Farley]

And the answer is...

Suppose that a DIY version of NOPOX contains a mixture of the following chemicals:

water (85%)
ethanol (10%)
acetic acid (3%)
methanol (1%)
isopropanol (1%)

If you allow half of it to evaporate away into dry air, the relative concentration of each of these chemicals in the remaining mixture, compared to the starting mixture, has done what?

B. Water and acetic acid increase, the others decrease

Critical to this question is the fact that half remains. At other amounts remaining, the answer can be different.

The molecules will evaporate at rates roughly related to their boiling point (a simplification, but close enough here). The lowest boiling solvents evaporate fastest.

Since the three lowest boiling solvents (ethanol, methanol, and isopropanol) only comprise 13% of the total, they will be mostly gone by the time you evaporate down to half of the total remaining. Meaning they are mostly gone before large amounts of the two higher boiling solvents (water and acetic acid) have evaporated away.

The acetic acid is evaporating away more slowly than anything in the mix, so it is increasing in concentration.

However, no matter how much of the initial 3% of acetic acid remains when 50% of the total is gone, its concentration cannot be higher than 6% (which would mean that none of it has evaporated away).

So when 50% of the total remains, at most, 6% of that is acetic acid. Since the others are mostly gone, the remainder must be at least 100-6% = 94% water. So the water has also increased.

Even if substantial amounts of the lower boiling solvents remain, there will still be an increase in the water. They start at 15% of the total. Even if they comprise 5% of the final solution, the water is still 100%-6% (max acetic acid possible) - 5% (the light boiling solvents remaining) = 89% of the final mix, well above the 85% at the start.

Hope this was fun!

Happy Reefing.