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March 28, 2024, 07:59:29 pm

Author Topic: VCE Chemistry Question Thread  (Read 2312979 times)  Share 

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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1095 on: July 02, 2014, 02:38:26 pm »
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Also will we be penalised if we name redundantly?

for example, 1,1,1-tricholoro2,2-dimethylpropane, apparently we're supposed to omit the 2,2 since theres no other possible position for it?

Another example is 2-methylpropanoic acid, which is apparently just methylpropanoic acid. This doesnt seem to be an IUPAC convention either
« Last Edit: July 02, 2014, 02:44:23 pm by psyxwar »
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Rod

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Re: VCE Chemistry Question Thread
« Reply #1096 on: July 02, 2014, 02:44:00 pm »
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Is energy from natural gases released via combustion?
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alchemy

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Re: VCE Chemistry Question Thread
« Reply #1097 on: July 02, 2014, 06:36:51 pm »
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Five aspirin tablets, weighing 2.456g in total, are crushed and added to a 250mL beaker. 150mL of 0.252M NaOH is added to the beaker and the contents are heated gently. After heating, 20mL aliquots from the beaker are titrated against 0.15M HCl solution. The average titre is 14.88mL. Calculate the number of mole of aspirin and the % mass of aspirin in each tablet.

Sorry for the easy question, but something in my working out is preventing me from getting the correct answer.


Rishi97

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Re: VCE Chemistry Question Thread
« Reply #1098 on: July 02, 2014, 08:28:52 pm »
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In mass spectrometry,
the relative intensities of the ions depend on:
- the energy of the bombarding electrons
- the stability of the ion fragments formed
- the ease at which ions can lose atoms

What exactly does this mean? What is it referring to when it says 'intensities"?

Thanks :-)
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Rishi97

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Re: VCE Chemistry Question Thread
« Reply #1099 on: July 02, 2014, 08:49:46 pm »
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Five aspirin tablets, weighing 2.456g in total, are crushed and added to a 250mL beaker. 150mL of 0.252M NaOH is added to the beaker and the contents are heated gently. After heating, 20mL aliquots from the beaker are titrated against 0.15M HCl solution. The average titre is 14.88mL. Calculate the number of mole of aspirin and the % mass of aspirin in each tablet.

Sorry for the easy question, but something in my working out is preventing me from getting the correct answer.

I'm not the best at chemistry but I tried and got 81.43% as the percentage mass. I know this is defs wrong because I didn't use dilution factors but no harm in asking  :P
n(aspirin) = 0.002232 mol
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alchemy

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Re: VCE Chemistry Question Thread
« Reply #1100 on: July 02, 2014, 10:08:29 pm »
+1
I'm not the best at chemistry but I tried and got 81.43% as the percentage mass. I know this is defs wrong because I didn't use dilution factors but no harm in asking  :P
n(aspirin) = 0.002232 mol

That's what I got the first time around, so it isn't correct. Btw, I fixed my mistake. Here's my working out:

n(NaOH)=0.252*0.15=0.0378
n(HCl)=0.15*0.0148=0.002232
NaOH(aq)+HCl(aq)-->NaCl(aq)+H20
n(NaOH)=n(HCl)=0.002232 in excess
n(NaOH) in excess in 250mL beaker: n(NaOH)*(150/20)=0.01674 .... this is the part that I initially made a mistake on. It's necessary to multiply the number of moles of NaOH by the number of aliquots that were added to the 250mL beaker.
n(NaOH) reacting with aspirin = 0.0378-0.01674=0.02106
n(Aspirin)=n(NaOH reacting with aspirin)/2 = 0.02106/2=0.0105
m(Aspirin)=0.0105*180=1.89g

Scooby

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Re: VCE Chemistry Question Thread
« Reply #1101 on: July 02, 2014, 10:13:57 pm »
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Also will we be penalised if we name redundantly?

for example, 1,1,1-tricholoro2,2-dimethylpropane, apparently we're supposed to omit the 2,2 since theres no other possible position for it?

Another example is 2-methylpropanoic acid, which is apparently just methylpropanoic acid. This doesnt seem to be an IUPAC convention either

Yeah, VCAA have penalised students for that in the past
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thushan

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Re: VCE Chemistry Question Thread
« Reply #1102 on: July 02, 2014, 10:32:06 pm »
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Have VCAA actually penalised this in the past? Which year?
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Re: VCE Chemistry Question Thread
« Reply #1103 on: July 02, 2014, 10:54:17 pm »
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Have VCAA actually penalised this in the past? Which year?

Closest I've found is the Exam Assessment Report in the Unit 3 exam 2011, but it only states redundant numbers, not any penalising. (See Short Answer Q7aiv.)
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Scooby

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Re: VCE Chemistry Question Thread
« Reply #1104 on: July 02, 2014, 11:15:57 pm »
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Closest I've found is the Exam Assessment Report in the Unit 3 exam 2011, but it only states redundant numbers, not any penalising. (See Short Answer Q7aiv.)

Yeah, I think that's what I'm remembering (I feel like it came up in some other exam as well but I can't remember where). Since the question asks for the systematic name, and then they've written that "ethan-1-ol is not the systematic name," it's probably safe to assume they didn't accept it. Either way, I'd just be wary of it
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Re: VCE Chemistry Question Thread
« Reply #1105 on: July 04, 2014, 03:04:52 pm »
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What is the stationary & mobile phase for GLC and HPLC? And is there a difference between GC and GLC or are the terms used interchangeably?

Ty :)
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1106 on: July 04, 2014, 03:57:05 pm »
+1
What is the stationary & mobile phase for GLC and HPLC? And is there a difference between GC and GLC or are the terms used interchangeably?

Ty :)
GLC uses a porous solid coated with high boiling point liquid hydrocarbon (so it doesnt vaporise in the chromatography machine) as the stationary phase, and uses an inert carrier gas as the mobile phase, for example N2 or He. Inert doesnt necessarily mean noble gas btw, just one that will not reactive with the species under analysis.

HPLC uses small, high surface area solid particles as the stationary phase, eg. Alumina and uses some solvent as a mobile phase. Whats actually used depends on the setup and what you're trying to separate or identify

I dont know if they can be used interchangably, but I know there is also gas solid chromatography.
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Penguuu

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Re: VCE Chemistry Question Thread
« Reply #1107 on: July 06, 2014, 10:08:28 am »
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Hi guys, quick question about properties of alkanes and alkenes. Which one has a higher boiling point? First I though alkenes because double bonds are harder to break, but apparently its alkanes? As alkenes have irregular shapes so the dispersion forces are weaker. Alkanes have regular structures that fit closely together allowing for stronger dispersion forces to exist between the molecules.


thushan

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Re: VCE Chemistry Question Thread
« Reply #1108 on: July 06, 2014, 10:11:19 am »
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To boil a substance, you need to break bonds BETWEEN molecules, which include:
- hydrogen bonds
- dipole dipole interactions
- dispersion forces (as for an alkene)

You don't need to break the covalent bonds within a molecule. Yes, double bonds are stronger (in an alkene), but boiling a substance does not require that double bonds be broken.

Dispersion forces are stronger when:
- the molecule is bigger
- the molecules are closely packed.
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Re: VCE Chemistry Question Thread
« Reply #1109 on: July 06, 2014, 11:16:29 am »
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To boil a substance, you need to break bonds BETWEEN molecules, which include:
- hydrogen bonds
- dipole dipole interactions
- dispersion forces (as for an alkene)

You don't need to break the covalent bonds within a molecule. Yes, double bonds are stronger (in an alkene), but boiling a substance does not require that double bonds be broken.

Dispersion forces are stronger when:
- the molecule is bigger
- the molecules are closely packed.

Thank you!