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What to do for this one???

Q) For which of the following substances will 0.125 moles of oxygen be enough for complete combustion?

A 0.21 mol of magnesium
B 0.19 mol of aluminium
C 0.15 mol of carbon
D 0.075 mol of methane

Please Help!
D because you are looking to make oxygen to have more moles (hence excess) than other reactant., as you want to use up all of other reactant
Reply 2
Avatar for Yatayyat
Yatayyat
OP
14
Original post by neluxsan
D because you are looking to make oxygen to have more moles (hence excess) than other reactant., as you want to use up all of other reactant


I checked up the answer and it says it's apparently A.

I don't know why, but I wrote up all the combustion equations for each substance it is reacting with first, and this is what I got...



Magnesium:

2Mg + O2 ---> 2MgO

Aluminium:

4Al + 3O2 ---> 2Al2O3

Carbon:

C + O2 ---> CO2

Methane:

CH4 + 2O2 ---> CO2 + 2H2O

I used the molar ratio for the methane combustion and it turned out to give moles of methane as 0.0625 mol. Maybe it can't be D since it has to be at least 0.0625 mol or less (oxygen still in excess or exact amount) in order to fully burn completely

I did the same thing for the rest which left me with only option A and B...
B gives moles of aluminium to be 0.167 moles while A gives moles of magnesium to be 0.250 moles. I think maybe then you have to choose magnesium in option A as this is still just enough. You could choose option B but I think they want you to select the maximum amount of moles which is enough.

I really don't know what else there is to do than this. What do you think?
Yes I think I figured out why it is A

Look at your equations:

2Mg + O2 -> MgO
Here for 0.125 moles of O2, we can burn 0.25 moles
So let's look at option A, they give us 0.21 moles
0.21 moles is less than 0.25 moles (the one we worked out for magnesium), so complete combustion will occur

4Al + 3O2 -> 2Al2O3
Here for 0.125 moles of O2, we can burn 0.167 moles (3 s.f.)
So let's look at option B, they give us 0.19 moles
0.19 moles is more than 0.167 moles (3 s.f.) so complete combustion will not occur as there is more magnesium than the oxygen can burn to form complete combustion products (this reason applies for below so (option c and d))

C + O2 -> CO2
Here for 0.125 moles of O2, we can burn 0.125 moles
So let's look at option C, they gives us 0.15 moles
0.15 moles is more than 0.125 moles so complete combustion will not occur (same concept applies below)

CH4 + 2O2->CO2 + 2H2O
Here for 0.125 moles of O2, we can burn 0.0625 moles
Reply 4
Avatar for Yatayyat
Yatayyat
OP
14
Original post by neluxsan
Yes I think I figured out why it is A

Look at your equations:

2Mg + O2 -> MgO
Here for 0.125 moles of O2, we can burn 0.25 moles
So let's look at option A, they give us 0.21 moles
0.21 moles is less than 0.25 moles (the one we worked out for magnesium), so complete combustion will occur

4Al + 3O2 -> 2Al2O3
Here for 0.125 moles of O2, we can burn 0.167 moles (3 s.f.)
So let's look at option B, they give us 0.19 moles
0.19 moles is more than 0.167 moles (3 s.f.) so complete combustion will not occur as there is more magnesium than the oxygen can burn to form complete combustion products (this reason applies for below so (option c and d))

C + O2 -> CO2
Here for 0.125 moles of O2, we can burn 0.125 moles
So let's look at option C, they gives us 0.15 moles
0.15 moles is more than 0.125 moles so complete combustion will not occur (same concept applies below)

CH4 + 2O2->CO2 + 2H2O
Here for 0.125 moles of O2, we can burn 0.0625 moles


Thanks your explanation makes perfect sense to me. Don't know why I was also considering option B in my last post. Just was getting a bit muddled up with all the numbers.

Thanks again for all the help! :smile:
Original post by Yatayyat
Thanks your explanation makes perfect sense to me. Don't know why I was also considering option B in my last post. Just was getting a bit muddled up with all the numbers.

Thanks again for all the help! :smile:


Np....Im ngl, if you didn't write those equations out clearly, I guess I wouldn't have been able to reach this conclusion...

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