Chemistry 3/4 2013 Thread

[teletubbies_95]

I asked my teacher and she said most reactions , except for combustion reactions , don't require states.
And also in our SAC , the question clearly said not to worry about states, even though we did all the reactions and saw the states of each compound, we didnt have to include states.
So confused

[Edward21]

Hey, every so often in checkpoints I come across a question involving the pV=nRT equation, it seems fine. But once they introduce constants...

Example: "2.90g of the ester was vaporised and occupied the same volume as 0.70g of nitrogen gas at the same temperature and pressure" Calculate the molar mass of the ester.

Solutions: pV-nRT and n/V=pRT (how did they re-arrange to get that?) then since p and T are constants n/V=constant

n(N2)/V(N2) = n(compound)/V(compound); hence n(N2)= 0.70/28=0.025mol
Since V(N2) = V(compound), the n(compound) equals 0.025mol
Molar mass = 2.9/0.025=116gmol^-1

OK, SO WHAT JUST HAPPENED. I am so lost, every other gas calculation seemed fine and then I read the solutions for this one that I couldn't get and it's unlike anything they put in the textbooks ! 

[psyxwar]

Hey, every so often in checkpoints I come across a question involving the pV=nRT equation, it seems fine. But once they introduce constants...

Example: "2.90g of the ester was vaporised and occupied the same volume as 0.70g of nitrogen gas at the same temperature and pressure" Calculate the molar mass of the ester.

Solutions: pV-nRT and n/V=pRT (how did they re-arrange to get that?) then since p and T are constants n/V=constant

n(N2)/V(N2) = n(compound)/V(compound); hence n(N2)= 0.70/28=0.025mol
Since V(N2) = V(compound), the n(compound) equals 0.025mol
Molar mass = 2.9/0.025=116gmol^-1

OK, SO WHAT JUST HAPPENED. I am so lost, every other gas calculation seemed fine and then I read the solutions for this one that I couldn't get and it's unlike anything they put in the textbooks ! 

PV=nRT
P = (n/V) RT
(P/RT)V = n

Since pressure, volume, temperature are all constant and R is a constant, then n1 must equal n2.

They found the number of moles of nitrogen gas, which must be equal to the number of moles of ester. Remember that n = m/M and therefore M = mass/ number moles. That's how they got their answer.

(I have no idea how they got n/V=pRT)

[Jeggz]

Just wondering, why is this epic thread not stickied? 

[Mr Keshy]

Just wondering, why is this epic thread not stickied? 

1. No one asked.
2. It's always up the top anyway

[jono88]

2008 VCAA exam 1 Section B q5d), i don't understand it.

[random_person]

2008 VCAA exam 1 Section B q5d), i don't understand it.

The SH bond in two cysteine molecules are oxidised

S-H + H-S
goes to
S - S + H_{2 (g)} + 2e^-

So the answer is S - S

Hope this helps

[Mr Keshy]

Went to GTAC today! Was pretty interesting doing the pracs and using the spectrophotometer (I think! haha). We got to use computers to learn about proteins and their contents too. Anyone going or been to GTAC?

[hardworker]

Went to GTAC today! Was pretty interesting doing the pracs and using the spectrophotometer (I think! haha). We got to use computers to learn about proteins and their contents too. Anyone going or been to GTAC?

Yeah went there on Monday was actually worthwhile.
Btw has anyone done any neap chemistry exams because i'm doing absolutely horrible on them.

[simba]

Went to GTAC today! Was pretty interesting doing the pracs and using the spectrophotometer (I think! haha). We got to use computers to learn about proteins and their contents too. Anyone going or been to GTAC?

We went to VSSEC for our SAC, I don't know if that was similar to what you guys were doing at GTAC. We got to use the gas chromatogram, UV vis spectrometer and AA spectrometer

[Jeggz]

Can someone please explain what the difference is between colorimetry and UV-Visible Spec?

[Homer]

colorimetry is used to analyse coloured substances, UV spec for colourless substances?

[teletubbies_95]

So, both colorimetry and UV vis spec are based on absorption spectroscopy.
The difference is that in UV vis , the light source can be both visible and UV light . On the other hand , in colorimetry light source can be visible.
 Furthermore, in UV vis , there is also a monochromator present , which is a technological advancement compared to the colorimeter , as it selects appropriate wavelengths, thus being more selective as well. On, the other hand, colorimetry  uses a coloured filter , using a complementary colour to the sample.

[Mr Keshy]

So, both colorimetry and UV vis spec are based on absorption spectroscopy.
The difference is that in UV vis , the light source can be both visible and UV light . On the other hand , in colorimetry light source can be visible.
 Furthermore, in UV vis , there is also a monochromator present , which is a technological advancement compared to the colorimeter , as it selects appropriate wavelengths, thus being more selective as well. On, the other hand, colorimetry  uses a coloured filter , using a complementary colour to the sample.

Thank you!

[teany]

I'm having a lot of trouble figuring out the states of compounds, in reactions with alkanes, alkenes, alkanols etc. For example, in the substitution reaction of chloroethane and NaOH, the products are ethanol and NaCl, and they are both aqueous? And when forming an ester with a carboxylic acid and alcohol, both reactants are in the liquid state? :/ Is there some general rule for the states of compounds? Especially for alcohols, chloroalkanes and carb acids. Thanks in advance!

Also, do you have to include states when drawing up pathway reactions?