Chemistry wizzes...

Skroatz

Skroatz

Well-Known Member
A mate was asking for some help... so I thought I'd try.. and have no idea.... who can answer this lol

A car weighs 1000 kg. It is filled up with 50 kg of petrol. The car is driven until
all of the petrol is used up. The car now weighs 1000 kg again. What is the
approximate weight of the exhaust gases that have been given off during the
drive?

I googled for a while... just gave me a headache... sometimes I miss those days, sometimes I don't.
 
U

UGA

Active Member
I assume you are using C8H18 as the formula for gasoline.
If so I calculated 143.9 kg of CO2 emitted.
 
U

UGA

Active Member
You probably don't have a degree in physics or science but I was just thinking how relatively simple that problem was.

What would be cool is if the problem gave the engine displacement, weight of the vehicle and a constant velocity RPMs and length time the engine ran then asked for that information.

Then it would be a physics problem then a chemistry problem.
Anyways hope my answer helped but like I said I have to make a few assumptions.
 
U

UGA

Active Member
Umm no. 50 kg of fuel burns the same way in a large truck as it does in a small car. Might be different rates but in the end the result will be the same.

I hate when people try and answer questions they have no idea how to answer.
 
doc111

doc111

Well-Known Member
UGA said:
I assume you are using C8H18 as the formula for gasoline.
If so I calculated 143.9 kg of CO2 emitted.
Click to expand...
How can the mass of the exhaust gasses be greater than the mass of fuel?:-?

I believe the answer would be 50 kg. It's probably a trick question. If 50 kg of fueld were burnt then there should be approximately 50 kg of exhaust gasses. ;-)

Also, it's going to make a difference if he's talking about diesel or gasoline. bongsmilie
 
josae

josae

Active Member
You have to find and balance the formula for burning of gasoline. There are many different chemicals in gasoline so the best way I suppose would be to start with octane which is :
2CH8H18 + 25O2 ----> 16CO2 + 18H2O
Then you must find the molar mass of the first 2 compounds; that is the atomic weight of each element added for the compound weight To do this you must know the mass( or percentage of the compound to get the mass) you are looking for, i.e in your example 50kg(you must convert to grams, but octane is not the complete chemical breakdown of gasoline) but you could say 50Kg of octane in place. You then divide mass/molar mass to get the number of moles. Whenever you are dealing with reaction you must use moles! You then place the values of the product into a Stoichiometric table this will allow you to find the limiting reactant and finally the number of moles for the product which can be used in the same equation n = (mass)/(molarmass).
Hope this helps!
 
be ez

be ez

Well-Known Member
doc111 said:
I believe the answer would be 50 kg. It's probably a trick question. If 50 kg of fueld were burnt then there should be approximately 50 kg of exhaust gasses. ;-)
Click to expand...
I believe this is correct. Seem to remember my physics prof mentioning it last year.
 
josae

josae

Active Member
be ez said:
I believe this is correct. Seem to remember my physics prof mentioning it last year.
Click to expand...
This is true through the law of conservation of matter; as matter cannot be destroyed or created. The question asks the mass of the exhaust GASES that are released.
 
VER D

VER D

Well-Known Member
if i had some weed i would tell you but im sober so thinking gives me a headache
 
doc111

doc111

Well-Known Member
josae said:
This is true through the law of conservation of matter; as matter cannot be destroyed or created. The question asks the mass of the exhaust GASES that are released.
Click to expand...
If combustion was complete or close to it then the mass of the exhaust gasses should be very close to the mass of the fuel. If not then there would be something left over from the reaction. The weight of the vehicle and the fuel being gone from the car doesn't account for this.bongsmilie
 
josae

josae

Active Member
CO2(g) & H2O(aq) are the products of combustion. I am having trouble understanding the question you pose. The weight of the vehicle plays no part in this, if the question was "you have 1000kg car and 50kg of fuel was put in the car, the car was driven and re-weighed and now weighs 1030kg what is the mass of the exhaust gasses", then the weight of the car would come into play.

EDIT: Ok think I realize what you are saying. Yes it should be similar, I was looking to much into the question and trying to hint at how to get the masses of the compounds. At what level is the question posed from?
 
doc111

doc111

Well-Known Member
josae said:
CO2(g) & H2O(aq) are the products of combustion. I am having trouble understanding the question you pose. The weight of the vehicle plays no part in this, if the question was "you have 1000kg car and 50kg of fuel was put in the car, the car was driven and re-weighed and now weighs 1030kg what is the mass of the exhaust gasses", then the weight of the car would come into play.
Click to expand...
There are other products of combustion as well. CO anyone? lol! If all the fuel was BURNT then that would mean that there is approximately 50 kg's of exhaust gasses. If the entire 50 kg's of fuel weren't turned into exhaust gasses, what happened to the unburnt portion? H20 as a product of combustion is in the form of water vapor which is a gas.;-)
 
shmow52

shmow52

Well-Known Member
conservation of mass....
mass can only be transformed from one state to another, it cant be created or destroyed.
 
josae

josae

Active Member
doc111 said:
There are other products of combustion as well. CO anyone? lol! If all the fuel was BURNT then that would mean that there is approximately 50 kg's of exhaust gasses. If the entire 50 kg's of fuel weren't turned into exhaust gasses, what happened to the unburnt portion? H20 as a product of combustion is in the form of water vapor which is a gas.;-)
Click to expand...
Yes, I understand. I thought the OP was looking for a more specific question, and I misunderstood what you were trying to say. CO is incomplete combustion as there is not enough oxygen to attached to carbon.
 
doc111

doc111

Well-Known Member
josae said:
Yes, I understand. I thought the OP was looking for a more specific question, and I misunderstood what you were trying to say. CO is incomplete combustion as there is not enough oxygen to attached to carbon.
Click to expand...
I'm aware of this, but it's still a gas. The OP didn't give nearly enough info. It simply gives the mass of the car and fuel burned and asks what the mass of the exhaust gasses is. With only that information the only answer that can be given is approximately 50 kg. If you burn a log that weighs 10 kg and you have say 1000 grams of ashes left the logical conclusion is the exhaust gasses' mass would be approximately 9 kg. The rest of the log had to go somewhere. Where did it go? It was burned so knowing the beginning mass of the log and the mass of what was left over after combustion gives you the total mass of the products of combustion. Same thing applies with anything burned. There's no need for a complex equation or counting moles or anything like that. It's a trick question used to illustrate the law of conservation of mass. I'm guessing this is a high school chemistry course, or at the very least community college chemistry. lol!:lol:
 
josae

josae

Active Member
Yeah I agree w/ you on that it must be a lower level then. I just finished 2nd year chem exams so I am on the thought process that I would need to know how to build and balance the equation, find the moles blahblahblah. I assumed because he had used gases i took it as plural that he was looking for specific masses of compounds. My bad! Ha, now I look stupid.
 
doc111

doc111

Well-Known Member
josae said:
Yeah I agree w/ you on that it must be a lower level then. I just finished 2nd year chem exams so I am on the thought process that I would need to know how to build and balance the equation, find the moles blahblahblah. I assumed because he had used gases i took it as plural that he was looking for specific masses of compounds. My bad! Ha, now I look stupid.
Click to expand...
lol! I have a master's degree in chemistry and I almost fell right into that trap too. He didn't give enough info to be balancing equations though, and it simply asks for the mass of the exhaust gasses, meaning all of the gasses expelled during combustion. We don't even know what kind of fuel he was using. It just says "petrol" which I was under the impression was diesel but I think it's more of a generic term for any auto fuel. I'm not from the U.K. so I really don't know. lol!:lol:
 
toastycookies

toastycookies

Active Member
doc111 said:
How can the mass of the exhaust gasses be greater than the mass of fuel?:-?

I believe the answer would be 50 kg. It's probably a trick question. If 50 kg of fueld were burnt then there should be approximately 50 kg of exhaust gasses. ;-)

Also, it's going to make a difference if he's talking about diesel or gasoline. bongsmilie
Click to expand...

How can the mass of exhaust gasses be greater than the mass of fuel?

easy. simple chemistry my friend.

Because the gasoline is combined with air to create a more volatile substance which is combusted inside the cylinder.
 
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