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delirium happy

Just keep on trying till you run out of cake

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The mass of a fart
A few years ago, I was listening to the radio (in the car with my mum on the way to the hospital to have my wisdom teeth removed, as it happens) and they had one of those inane "Strange but true" features that could only exist on the radio or in email forwards.

One of their alleged facts was that on average a fart weighs 5 grams.

This sounded rather dubious to me, so I did a quick calculation. If we assume so and so as the constituent gases of a fart, then use this, this and this as the relative atomic weights, then take that approximation of the ideal gas equation... then we wind up calculating that a 5 gram fart would have a volume somewhere in the region of 4 litres.

Now, it has to be stressed that that was a fairly rough and ready calculation, and I wouldn't put much stock in its accuracy. But as an order of magnitude approximation, it isn't too bad.

I don't know about you, but I've farted in the bath, where the bubbles make it easy to see the volume, and I'm quite, quite sure that even the biggest farts are nowhere even close to 4 litres.

Something is clearly amiss. Either the statement given by the radio was false, or various bits of science that I have learned were false (or both could be wrong, of course). I have absolutely no empirical evidence either way. I have never measured the mass of a fart. Nor, however, have I ever done a chemical analysis of a fart, nor measured relative atomic masses of carbon, hydrogen, oxygen, nitrogen, etc. The closest I've come to empirical evidence is seeing someone else perform demonstrations of Boyle's Law and Charles' Law, both of which are specific cases of the ideal gas law. The only real empirical evidence I have is the volume of the fart, which doesn't tell me which of the two propositions is right, only that they can't both be.

So, given this situation which of the two opposing theories would you be more inclined to believe, and more to the point why?

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I was inclined to believe you when I first read this this morning. Your numbers sound much more reasonable. However I can't resist a chance to do an experiment (you can see where this is going can't you?).

Aim: To measure the volume and hence the mass of a fart.

Method: One bath with a disposable plastic cup and waterproof pen. Fart and catch bubbles in the inverted water filled cup. Mark gas level on cup with pen. Determine volume of gas by weighing water in same cup filled to gas line.

Results: 115 cm^3 of gas (+- 5) was released. Bath temp was about 29 degrees. At STP this gives 0.0046 mols of gas. Assuming the gas is 100% methane, it isn't, but for this example I'll claim it is, I get a total of 0.074g of methane.

Comment on your results: There isn't a lot to say about this, I have no way of knowing if this was an average fart. The weight determined from this experiment is much less than that given in the radio show. In fact even if I'd been farting pure Radon, the recorded volume of gas would still only weigh just over 1g.

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