14

I asked my first question last night, Are 19.6 pounds of CO2 produced from burning a gallon of gasoline?, which has gotten quite a response. I'm really grateful to everyone who contributed.

This morning I saw some answers going into the explanation of why the answer to my question is yes. Some were better than others, but the second-highest-voted answer struck the right balance of being accessible yet explanatory. I didn't accept mark it as accepted because I wanted to give some time for others to answer the question, but I really think it was a good answer and want to mark it as correct.

I just took a look at the thread now, and it looks like all except the top answer have been deleted. I'd like to know why this happened and whether the other answers were ineligible for some reason. In the event it was some sort of mistake (or if a mod was trying to help me by deleting answers besides the top one), I'd like to request that answer be reinstated so I can accept it.

Remember, I'm totally new to this SE site so bear with me if there's some cardinal rule I'm totally overlooking. Thanks!

  • 1
    I think one of those answers was tagged with an "original research" disclaimer, even though it didn't contain any original research or complicated analysis -- it was a explanation of where the extra weight came from that any high school chemistry student would understand. With references for its data (exclude for lb->kg, which shouldn't need a reference). It wasn't the best answer, but it was still a valid answer. I don't know if the original author deleted it, or if a moderator did. – Johnny Jan 24 '18 at 18:32
  • answer here: meta.skeptics.stackexchange.com/q/2929 – Sklivvz Jan 25 '18 at 0:45
  • @Sklivvz, oh wow, that totally answers it. TY – user1717828 Jan 25 '18 at 1:47
12

Here's a list of the deletions and their reason (note, I did not delete all of these so most is just guessing, but it's quite obvious why):

  • Four answers calculated the reaction for octane, not gasoline. They did not bother to check if that was a valid approximation. They often further calculated the reaction for ethanol, which is not part of the question. Actual answers for a gallon of octane (which is not gasoline): 8.25kg, 8.83kg, more than 8.3kg, 8.4kg

  • One showed a graph of the result (which incorrectly mixed up air and oxygen) without showing calculations or references. Actual answer: 5.9kg

  • One made an unreferenced point about not being able to equate gasoline and octane, then made a calculation using a model based on energy density. Actual answer: 9.287kg

  • One used a model based on atomic mass. Actual answer: 3 times the mass of fuel, which is not given nor calculated

  • One used an "average carbon atoms per hydrogen" calculation. Actual answer: 8.7kg

All of these answers got quite different results, some of them close to the value in the question, some not, some ...we don't know. No answer states how much uncertainty they have, so we can't really match their values to the question.

This shows that the answers are either displaying the error of false precision or they are altogether wrong. In any case: they are theoretical and this is why they were removed.


A side note on uncertainty, matching values and theoretical answers.

Uncertainty

In science all, all measurable numbers are imprecise. Thus, it is essential to keep track of the uncertainty of values when performing any calculation. This is a quite interesting university level exercise, but it is certainly always done when "serious" science is made.

Scientific notation

In scientific notation one either gives a specific uncertainty value, like so

8.8±0.2kg
meaning: 68% of the times this value is found to be within 8.6kg and 9.0kg

or if omitted, then the last given digit is assumed to have ±1 uncertainty

8.4kg → 8.4±0.1kg
meaning: 68% of the times this value is found to be within 8.3kg and 8.5kg

Therefore we can desume that the article in the question has found a value of 8.89±0.01kg, 3 significant digits.

Comparing values

As a first approximation it is possible to compare two uncertain value by checking whether their ranges overlap (in reality it's a bit more complicated since these ranges are probabilistic and we need to apply statistics).

If we want to compare any of the theoretical values found by our answerers with the claim, we need to know their imprecision (which we don't, because they did not calculate it).

This is fundamental and here's why. Do any of the calculated values confirm or disprove the claim? It depends on how many digits of the calculated values are significant:

  • If we assume 1 digit of precision: 5 answers match the claim, 1 does not
  • With 2 digits of precision: 2 answers match the claim, 4 do not
  • Taking 3 or more digits of precision: no answers match the claim

Which is the correct interpretation? I don't know.

Conclusion

Unless a careful calculation and a careful comparison is made, it's really tricky to prove or disprove a claim with a theoretical answer, no matter how trivial the formula we use.

4

I can't see the deleted answers, but from the descriptions here, it sounds like at least some of them satisfy this test from the canonical Meta answer:

They must also show that these are the right numbers to add, and that adding only those two numbers is the right operation to match reality, and no other factors or complications have been omitted.

In my opinion, these are neither calculations that can only be verified by experts in the field, nor back-of-the-envelope calculations. While I like the accepted answer, some of the others sound like calculations based on common knowledge of the observed reality, that can be checked by anyone who knows middle school chemistry, and should have been left in place for site members to vote on.

  • 7
    To be clear, the highly upvoted one starts off with the line "Lets assume pure ethanol"..... thats textbook why we try to avoid these sort of answers. – Jamiec Jan 25 '18 at 17:24
3

I added a post notice to every answer which was theoretical so that those users could see why their answer had been deleted - mainly to stop them unfairly getting upvotes for answers which are not generally acceptable here.

Unfortunately, as a low rep user you were unable to see those deleted answers. In hindsight I should probably have left a comment on the question for you to see.

  • 1
    Actually, even your rep wouldn't be enough to see deleted answers if you weren't a mod -- ordinary users need 10k before they can see deleted posts. – David Richerby Feb 6 '18 at 21:23
1

I actually think that your question is not a good fit for this stack and would fit better on Chemistry.SE. The problem being that this stack is driven by citations and it really seemed to me like you were looking for more explanation than a citation allows.

As others have noted many answers were deleted for being theoretical in nature. However, I thought they answered your question better than the one cited answer. That's a sign to me that this was the wrong stack for this particular question.

The remaining answer is the kind of answer you should expect on this stack. It cited and quoted a canonical source. However, it could not provide much explanation, as the source didn't have it.

I would think that a good Chemistry.SE answer would explain that oxygen atoms are individually more massive than carbon atoms; that CO2 has twice as many oxygen atoms as carbon; therefore, most of the mass of carbon dioxide comes from the atmospheric oxygen and not the fuel. If that's the kind of answer that you want, then Chemistry.SE is probably a better place to ask your question.

If you really just wanted a citation, then you were correct to ask here.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .