VCE Chemistry Question Thread

[HasibA]

just some quick q's for an upcoming sac
1) In AAS, when your sample is being vaporised  , per se, so that it can absorb light specific to the sample/element that's being analysed, would  you say the sample is is an ion e.g Na+ or just the regular atom e.g Na. I know that only 'metal ions' can be analysed in AAS, but the actual sample that has been 'vaporised', would you consider it to be the ion/atomic form? Am i overcomplicating the semantics of it? not too sure..
2) Why can't molecules be analysed in AAS? is it because molecules eg CO2 , will be split into its substituent atoms and thus a molecule cannot be properly tested, only the 'metal ions' ? a resource says this but i am not sure if this is correct

thanks just a brief response will do

[Swagadaktal]

just some quick q's for an upcoming sac
1) In AAS, when your sample is being vaporised  , per se, so that it can absorb light specific to the sample/element that's being analysed, would  you say the sample is is an ion e.g Na+ or just the regular atom e.g Na. I know that only 'metal ions' can be analysed in AAS, but the actual sample that has been 'vaporised', would you consider it to be the ion/atomic form? Am i overcomplicating the semantics of it? not too sure..
2) Why can't molecules be analysed in AAS? is it because molecules eg CO2 , will be split into its substituent atoms and thus a molecule cannot be properly tested, only the 'metal ions' ? a resource says this but i am not sure if this is correct

thanks just a brief response will do

1) So a metal ion becomes "atomised"  (that's the term used) so I would use atom because it aint no ion anymore.
2) Molecules cannot be analysed in AAS coz they don't have any valence electrons. The premise of AAS is that the valence electrons absorb a specific amount of light (or wave length) which tells you how much energy it has absorbed (frmo the amount not absorbed/passed through) Molecules generally have covalent bonds which are SHARED electrons and cannot be excited.
Ya feel?

[Elizawei]

Got another chem question:

When 100mL of a gaseous hydrocarbon was burnt in 740 mL of oxygen, which was more than sufficient, 500mL of the gas was present at the end of the reaction after the products were cooled at STP. The gas mixture was bubbled through concentrated LiOH, which absorbed all of the carbon dioxide present, leaving 140mL of the gas. What was the molecular formula if all gas volumes have been measured at STP?

[Swagadaktal]

Got another chem question:

When 100mL of a gaseous hydrocarbon was burnt in 740 mL of oxygen, which was more than sufficient, 500mL of the gas was present at the end of the reaction after the products were cooled at STP. The gas mixture was bubbled through concentrated LiOH, which absorbed all of the carbon dioxide present, leaving 140mL of the gas. What was the molecular formula if all gas volumes have been measured at STP?

Is it C21H16O14 by any chance? I feel like I have too many oxygens.
If this is remotely correct I'll demonstrate me reasoning

[HasibA]

1) So a metal ion becomes "atomised"  (that's the term used) so I would use atom because it aint no ion anymore.
2) Molecules cannot be analysed in AAS coz they don't have any valence electrons. The premise of AAS is that the valence electrons absorb a specific amount of light (or wave length) which tells you how much energy it has absorbed (frmo the amount not absorbed/passed through) Molecules generally have covalent bonds which are SHARED electrons and cannot be excited.
Ya feel?

thank you!!!!

[Swagadaktal]

thank you!!!!

yo where's my +1 respect fam haha jokes

You're welcome

[HasibA]

yo where's my +1 respect fam haha jokes

You're welcome

dw i gave you it, gotta help out 'fam' xD haaha

just wondering, have you already done a spectroscopy sac? any pointers- got mine soon and i dont know what to expect tbh

[Elizawei]

Is it C21H16O14 by any chance? I feel like I have too many oxygens.
If this is remotely correct I'll demonstrate me reasoning

Unfortunately I don't have the answers   
And it's Hydrocarbons, so I don't think it contains oxygen 
I got C4H3 which seems kinda dodgy

[Swagadaktal]

Unfortunately I don't have the answers   
And it's Hydrocarbons, so I don't think it contains oxygen 
I got C4H3 which seems kinda dodgy

Oh not sure why but i assumed it was an organic molecule hmmmm

why don't my Oxygen add up :O
what's the 500ml gas is it the h2o and co2?

[Elizawei]

Oh not sure why but i assumed it was an organic molecule hmmmm

why don't my Oxygen add up :O
what's the 500ml gas is it the h2o and co2?

Im not sure haha, that was the entire question


Oh also got another quick question:

Identify one method that could be used to verify that the substance produced is pure propyl propanoate.
How do we verify that something is an ester???

_{Have a sac tomorrow pls help} 

Thanks in advance

[Swagadaktal]

Im not sure haha, that was the entire question


Oh also got another quick question:

Identify one method that could be used to verify that the substance produced is pure propyl propanoate.
How do we verify that something is an ester???

_{Have a sac tomorrow pls help} 

Thanks in advance

If my memory serves me right, you can try put some distilled water onto it, the ester should not dissolve in it because it's non polar.

[HopefulLawStudent]

Got another chem question:

When 100mL of a gaseous hydrocarbon was burnt in 740 mL of oxygen, which was more than sufficient, 500mL of the gas was present at the end of the reaction after the products were cooled at STP. The gas mixture was bubbled through concentrated LiOH, which absorbed all of the carbon dioxide present, leaving 140mL of the gas. What was the molecular formula if all gas volumes have been measured at STP?

This question is driving me crazy. Does anyone actually know how to do it? Would love to know.

I get C4H3 too but that can't be right...

The sad thing is I remember a similar question to this which my chem teacher threw onto one of our pop-quiz a month or two ago and apparently everyone got 0 marks for that question...

[Swagadaktal]

This question is driving me crazy. Does anyone actually know how to do it? Would love to know.

I get C4H3 too but that can't be right...

The sad thing is I remember a similar question to this which my chem teacher threw onto one of our pop-quiz a month or two ago and apparently everyone got 0 marks for that question...

Hm, not sure if this is right. But if we get 0.016 mol CO2 right, and 0.0045 mol of the hydro carbon that means there are 7 carbons in the hydrocarbon, and if we do the same for Hydrogen we have 3

So C7H3 --> there's a significant amount of rounding in my answer though

[Elizawei]

If my memory serves me right, you can try put some distilled water onto it, the ester should not dissolve in it because it's non polar.

Thanks!
Hmmm but apparently it's comparing the boiling temp with the known substance and with the sample

@HLS yeah, that question haha..
So I got 0.016 of Carbon, and 0.0125 mol of Hydrogen (from the water), and when I do the ratio thing I get C4H3 haha..

Oh well I'll ask my teacher

[Swagadaktal]

Thanks!
Hmmm but apparently it's comparing the boiling temp with the known substance and with the sample

@HLS yeah, that question haha..
So I got 0.016 of Carbon, and 0.0125 mol of Hydrogen (from the water), and when I do the ratio thing I get C4H3 haha..

Oh well I'll ask my teacher

You gotta factor in the mol of the hydrocarbon!