Chemistry wizzes...

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.

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

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.

depends on the car.

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.

UGA said:

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

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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.

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!

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.

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I believe this is correct. Seem to remember my physics prof mentioning it last year.

be ez said:

I believe this is correct. Seem to remember my physics prof mentioning it last year.

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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.

if i had some weed i would tell you but im sober so thinking gives me a headache

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.

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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.

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?

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.

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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.

conservation of mass....
mass can only be transformed from one state to another, it cant be created or destroyed.

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.

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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.

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.

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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!

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.

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.

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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!

UGA said:

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.

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o rly?

http://www.comcar.co.uk/newcar/companycar/poolresults/co2litre.cfm

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.

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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.