I am wondering how is the weight of Co2 being measured, i have just read an article were a Car is emiting 250 gramms of Co2 per Kilometer.
How is it possible for a Gas to be heavier than the Fuel being burnt?
Thanks
The “O2” in CO2 is picked up from the surrounding air during combustion. So much of the molecule isn’t from the gas, but from the atmosphere.
In the carbon dioxide in car exhausts, only 27.3% (by weight) is carbon from the fuel; the remaining 72.7% is oxygen from the atmosphere. The water vapor emitted from combustion of the hydrogen in the gasoline is even more skewed: 11.2% is hydrogen from the fuel, 88.8% is oxygen from the atmosphere.
(For ease of calculation, this assumes gasoline that is 87% 2,2,4-trimethylpentane and 13% n-heptane, no other hydrocarbons present (typical 87-octane gas is chemically similar to but not identical to that mixture), and standard isotopic fractions in the three elements.)
Also remember carbon dioxide is much heavier than the oxygen that went into making it. There have been natural disasters caused by CO2 building up in pockets at the bottom of lakes. Suddenly released via some physical disturbance, the gas rises to the surface where it may suffocate everyone who lives around the lake; google “Lake Nyos” for more information on that. Being heavier it pushes out all the good air.
What? The big worry about CO2 is that it mixes into the atmosphere and goes up high where it blocks some of the thermal infrared radiation heat loss and contributes to global warming. If its heaviness dictated where it went, we wouldn’t have that problem.
No, we’d have the larger problem that we’d all suffocate. 
There is a gradient in the atmosphere, with heavier gases tending to be more concentrated at lower altitudes. (The exosphere, for instance, is composed principally of hydrogen and helium.) Weight is not the only factor in the distribution of a gas in the atmosphere, however. Wind causes mixing, of course, but even in the absence of wind, a high concentration of a particular gas will diffuse into the atmosphere. However, if a large mass of a relatively heavy gas is released somewhere, it can displace the lighter gases already present and will take some time to dissipate.
As a result, you can do things like pour out a cupful of carbon dioxide over a flame to quench it. For heavier gases like sulfur hexafluoride, you can even float lightweight foil boats in a fish-tank full of the stuff.
If these assumptions are needed just for an off-hand back-of-envelope “easy calculation”, maybe we should feel sorry for your students. 
If I were doing the estimate, I’d worry more about Carbon that ends up as soot or CO, but maybe I’m used to poorly running engines. Not to mention that much of the diesel fuel where I live is probably watered down. :smack:
Well, trimethylpentane is just an isomer of octane, and that’s what I’d have used for quick’n’dirty, and not fussed about the 13% n-heptane. But it’s not rocket science.
[QUOTE=There have been natural disasters caused by CO2 building up in pockets at the bottom of lakes.
You mean to say that Co2 as a Gas is heavier than Water? Please excuse my ignorance.
[quote=“Simple_Mind, post:11, topic:537570”]
[QUOTE=There have been natural disasters caused by CO2 building up in pockets at the bottom of lakes.
You mean to say that Co2 as a Gas is heavier than Water? Please excuse my ignorance.[/QUOTE]
I’m going to speculate that the CO2 build up is in solution in water, kind of a natural seltzer.
Exactly. In these cases, the carbon dioxide is dissolved in the water. If it comes rapidly out of solution, which can happen if the water is supersaturated with carbon dioxide, the carbon dioxide can rapidly evolve and suffocate people in the vicinity of the lake.
Read about the Lake Nyos disaster here, which killed over 1,700 people in such an event back in 1986.
Lake Nyos is a very odd place from what I have read.
There is a continuous production of CO2 and on windless days the CO2 concentration builds up within a few inches of the land surface. Grazing animals wander into the meadow, put their head down to graze and fall over and die. Scavangers wander in to see whats for dinner and eventually get low enough into the carcass to collapse as well. And then later that day/week the wind picks up and everything is fine. Except for all the bones in the beautiful well fertilized grass…