# Could someone explain how to do these questions?

(edited 1 month ago)
Original post by BlackLives

The first question is combustion analysis.
Fact: The vol of a gas is directly proportional to the number of mol (Avogadro's law)
So you treat all of the volumes as if they are mol.

You know the volume of the hydrocarbon, so using the stoichiometry you can predict the volume of carbon dioxide produced.

You are told that water is a liquid so you can simply ignore it in terms of volume.

Do you want to take it from here?
Original post by charco
The first question is combustion analysis.
Fact: The vol of a gas is directly proportional to the number of mol (Avogadro's law)
So you treat all of the volumes as if they are mol.
You know the volume of the hydrocarbon, so using the stoichiometry you can predict the volume of carbon dioxide produced.
You are told that water is a liquid so you can simply ignore it in terms of volume.
Do you want to take it from here?

I have no idea what to do next. I know this equation Vol = mol x 24dm3 but why does that mean that we treat volume as mol??
Original post by BlackLives
I have no idea what to do next. I know this equation Vol = mol x 24dm3 but why does that mean that we treat volume as mol??

Like I said, it's Avogadro's law. It comes from the relationship PV = nRT
At constant P and T,
It follows that 'n' is directly proportional to V.

So treat gas volumes as if they were moles in terms of the stoichiometry of the reaction.
then:
25cm3 of CxHy must make 25x cm3 of carbon dioxide (like I said, you can ignore the water as it's a liquid)
So your problem comes down to basic algebra.
If you let the initial volume of oxygen be V cm3
Then the total initial volume = V + 25
And you are told the final volume of the mixture
If the volume of oxygen reacting = Z
Then excess oxygen = V-Z
So total final volume = volume of CO2 + excess oxygen
The last piece of the puzzle is the NaOH shake. This removes all of the carbon dioxide so the decrease in volume is due to the volume of the carbon dioxide = 25x = 125cm3
Can you take it from here?
Original post by charco
Like I said, it's Avogadro's law. It comes from the relationship PV = nRT
At constant P and T,
It follows that 'n' is directly proportional to V.
So treat gas volumes as if they were moles in terms of the stoichiometry of the reaction.
then:
25cm3 of CxHy must make 25x cm3 of carbon dioxide (like I said, you can ignore the water as it's a liquid)
So your problem comes down to basic algebra.
If you let the initial volume of oxygen be V cm3
Then the total initial volume = V + 25
And you are told the final volume of the mixture
If the volume of oxygen reacting = Z
Then excess oxygen = V-Z
So total final volume = volume of CO2 + excess oxygen
The last piece of the puzzle is the NaOH shake. This removes all of the carbon dioxide so the decrease in volume is due to the volume of the carbon dioxide = 25x = 125cm3
Can you take it from here?

I still don't get it sorry. What is the final volume? And why is the NaOH removing just the carbon dioxide. Shouldn't it react with the remaining oxygen too? (Sorry I'm still really bad at calculations and my a-levels are in June)
(edited 1 month ago)
Original post by BlackLives
I still don't get it sorry. What is the final volume? And why is the NaOH removing just the carbon dioxide. Shouldn't it react with the remaining oxygen too? (Sorry I'm still really bad at calculations and my a-levels are in June)

NaOH is a strong base. It reacts with carbon dioxide which is a weak acid.

2NaOH + CO2 ==> Na2CO3 + H2O

Oxygen is not a weak acid. It is a neutral gas - it cannot react with carbon dioxide.

Let the initial volume of oxygen = a cm3
Let the volume of oxygen reacted = b cm3
The volume of carbon dioxide = the volume loss = 125 cm3
The volume of the hydrocarbon = 25cm3
Hence there are 125/25 = 5 carbon atoms in the hydrocarbon.

To find the oxygen:
Express the final volume in terms of remaining oxygen plus carbon dioxide
Express the excess oxygen as the final volume - the carbon dioxide
(edited 1 month ago)
Original post by charco
NaOH is a strong base. It reacts with carbon dioxide which is a weak acid.
2NaOH + CO2 ==> Na2CO3 + H2O
Oxygen is not a weak acid. It is a neutral gas - it cannot react with carbon dioxide.
Let the initial volume of oxygen = a cm3
Let the volume of oxygen reacted = b cm3
The volume of carbon dioxide = the volume loss = 125 cm3
The volume of the hydrocarbon = 25cm3
Hence there are 125/25 = 5 carbon atoms in the hydrocarbon.
To find the oxygen:
Express the final volume in terms of remaining oxygen plus carbon dioxide
Express the excess oxygen as the final volume - the carbon dioxide

If volume of CO2 is 125 then why did the total gas volume decrease by 75cm3? What does that part mean?
(edited 1 month ago)
Original post by BlackLives
If volume of CO2 is 125 then why did the total gas volume decrease by 75cm3? What does that part mean?

Sorry, my mistake, The volume of CO2 is 75cm3
Original post by charco
Sorry, my mistake, The volume of CO2 is 75cm3

I understand it now. thanks for your help