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March 28, 2024, 11:44:41 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040547 times)  Share 

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jakesilove

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Re: Chemistry Question Thread
« Reply #225 on: March 28, 2016, 01:15:04 pm »
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how do you know that there is change in pressure?  i know that change in concentration is indicated by sharp drop or increase

at 10min can it be increase in temperature as well (endothermic)  although its due to increase in CO concentration as indicated by the sharp increase

Hey!

You know that there is a change in pressure when there is no sharp drop, and the equilibrium clearly shifts to one side. For instance, at four minutes, there is clearly no sharp drop and yet there is an obvious shift to the products! This could either be because of temperature, concentration or pressure.

Because there is no sharp drop/increase, it isn't a change in concentration. As such, you would talk about the fact that it could be a change in temperature, a change in pressure, or both. Because there are more moles of gas on the side of the products, the only reason products would increase in concentration is if you DECREASE the pressure of the system (by Le Chatelier's). As it is endothermic, an increase in temperature will cause a shift to the right, and therefore this could explain the results at 4 minutes as well. There is no way to tell which of these options occurred, so you need to talk about both!

At 10 minutes, the reason for the change is (as you've identified) because of a removal of CO. This will cause a shift to the right, as per Le Chatelier's, giving the results on the graph. There is no clear change in temperature, just a classic change in concentration.
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Maz

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Re: Chemistry Question Thread
« Reply #226 on: March 28, 2016, 06:53:18 pm »
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hey,
can i please get some help with this question? i got an  answer of 120g when the answer is actually 10.5g - big difference haha  :P
the question is
Calculate the mass of quicklime (calcium oxide), which will react completely with 250mL of 1.50mol L-1 hydrochloric acid.

thank you so much :)
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grace_joseph

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Re: Chemistry Question Thread
« Reply #227 on: March 28, 2016, 06:55:34 pm »
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Hi Jake! Would you please be able to explain to me why the answer for 7 is A and why the answer for 11 is C?

Thank you so much!

Grace :)

lazydreamer

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Re: Chemistry Question Thread
« Reply #228 on: March 28, 2016, 07:19:38 pm »
+3
Hi Jake! Would you please be able to explain to me why the answer for 7 is A and why the answer for 11 is C?

Thank you so much!

Grace :)

In Q7, you need to look at how many hydrogen protons are donated by the acid to NaOH. So basically you need to know the protic nature of the acids. The answer is citric acid because it is triprotic, therefore 3mols of acid is needed for 1mol NaOH compared to the other mono/di-protic acids. Degree of ionisation is irrelevant to answering the question, i know, i've seen that Q before

For 11, you'd need to deduce that the compound formed is an alkene (only alkenes discolor bromine water immediately). Then you'd need to be able to link this with the dehydration of an alkanol, by heating with an acid, which produces an alkene. Therefore the answer is C, the alkanol
« Last Edit: March 28, 2016, 08:22:09 pm by lazydreamer »

jakesilove

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Re: Chemistry Question Thread
« Reply #229 on: March 28, 2016, 07:26:00 pm »
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Hi Jake! Would you please be able to explain to me why the answer for 7 is A and why the answer for 11 is C?

Thank you so much!

Grace :)

Totally agree with Lazydreamer's answer! Let me know if you need any further clarification :)

Jake
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jakesilove

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Re: Chemistry Question Thread
« Reply #230 on: March 28, 2016, 07:34:09 pm »
+1
hey,
can i please get some help with this question? i got an  answer of 120g when the answer is actually 10.5g - big difference haha  :P
the question is
Calculate the mass of quicklime (calcium oxide), which will react completely with 250mL of 1.50mol L-1 hydrochloric acid.

thank you so much :)

Hey!

My answer is below, I hope that it helps! Let me know if you need any further clarification. Always just follow the steps that I've outlined: Chemical formula, application of equations, solution!



It should read "the difference between my answer and the 10.5g".
Jake
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lazydreamer

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Re: Chemistry Question Thread
« Reply #231 on: March 28, 2016, 07:38:58 pm »
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Now i got a question

So in the Examples, why does the numbering start where it starts? Like why in 3 does it start closer to the methyl but in 4 it starts at the ethyl instead?
Thanks :D

RuiAce

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Re: Chemistry Question Thread
« Reply #232 on: March 29, 2016, 10:58:51 am »
+1
Now i got a question

So in the Examples, why does the numbering start where it starts? Like why in 3 does it start closer to the methyl but in 4 it starts at the ethyl instead?
Thanks :D

Firstly, the ethyl is written before the methyl because we want to maintain alphabetical order. E lies before M in the English the alphabet. But it seems like you already knew that.

We would like to choose, so that we always minimise the sum of the locant numbers in the molecule.

For Q3, if we count from left to right we will have
3-ethyl-5-methyl-hex-3-ene
3 + 5 = 8
If we count right to left we will have
4-ethyl-2-methyl-hex-3-ene
4 + 2 = 6

So we favour the latter.

For Q4, firstly keep in mind that there is an ethyl group on the right, because the double bond is what gives pentadiene it's name.
Now, using the same process

If we count from left to right we will have
4-ethyl-2,3-dimethyl-1,4-pentadiene
If we count from right to left we will have
2-ethyl-3,4-dimethyl-1,4-pentadiene

And here we have a problem. From maths, we know that 4+2+3=2+3+4. Oh dear!

Which is exactly the point I call you out for being a selective school student (or attempt to) and having to do harder questions.

The reason why the second one is favourable is that in the event that your sum of locants are the same (very unlikely to occur in the HSC), we again treat alphabetical order as priority. E lies before M, so the ethyl will get the lower number here.

RuiAce

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Re: Chemistry Question Thread
« Reply #233 on: March 29, 2016, 11:06:42 am »
+1
how do you know that there is change in pressure?  i know that change in concentration is indicated by sharp drop or increase

at 10min can it be increase in temperature as well (endothermic)  although its due to increase in CO concentration as indicated by the sharp increase

For questions like this there exists a rule of thumb

  • No spike, and then things change -> Temperature change. If the nature of the reaction as exothermic or endothermic is given, you can work out if heated or cooled
  • Spike in every substance -> Pressure change. Downward spike always means increase in pressure (or alternatively decrease in volume of the vessel)
  • Spike in only one substance -> Concentration change. One of the things has been tampered with. Could be you introduced more or suddenly extracted a ton depending on spike up or down.

RuiAce

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Re: Chemistry Question Thread
« Reply #234 on: March 29, 2016, 11:10:55 am »
+1
In Q7, you need to look at how many hydrogen protons are donated by the acid to NaOH. So basically you need to know the protic nature of the acids. The answer is citric acid because it is triprotic, therefore 3mols of acid is needed for 1mol NaOH compared to the other mono/di-protic acids. Degree of ionisation is irrelevant to answering the question, i know, i've seen that Q before

For 11, you'd need to deduce that the compound formed is an alkene (only alkenes discolor bromine water immediately). Then you'd need to be able to link this with the dehydration of an alkanol, by heating with an acid, which produces an alkene. Therefore the answer is C, the alkanol
To build on:
It is irrelevant, but you should be aware that the reason why it is irrelevant is due to the titration with a strong base. The strong base being present, regardless of the strength of the acid, forces the reaction to go to completion no matter what.

indiago

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Re: Chemistry Question Thread
« Reply #235 on: March 29, 2016, 06:04:43 pm »
0
Hello !!!

I have come across similar questions to this one but I can't seem to figure out how to do them. Please help !!!

pH 2.00 of 0.01M HCl.

Calculate the pH after 20 mL of 0.01 mol L-1 hydrochloric acid is diluted with 180 mL of water.


Thanks, India

lazydreamer

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Re: Chemistry Question Thread
« Reply #236 on: March 29, 2016, 11:02:01 pm »
0
Firstly, the ethyl is written before the methyl because we want to maintain alphabetical order. E lies before M in the English the alphabet. But it seems like you already knew that.

We would like to choose, so that we always minimise the sum of the locant numbers in the molecule.

For Q3, if we count from left to right we will have
3-ethyl-5-methyl-hex-3-ene
3 + 5 = 8
If we count right to left we will have
4-ethyl-2-methyl-hex-3-ene
4 + 2 = 6

So we favour the latter.

For Q4, firstly keep in mind that there is an ethyl group on the right, because the double bond is what gives pentadiene it's name.
Now, using the same process

If we count from left to right we will have
4-ethyl-2,3-dimethyl-1,4-pentadiene
If we count from right to left we will have
2-ethyl-3,4-dimethyl-1,4-pentadiene

And here we have a problem. From maths, we know that 4+2+3=2+3+4. Oh dear!

Which is exactly the point I call you out for being a selective school student (or attempt to) and having to do harder questions.

The reason why the second one is favourable is that in the event that your sum of locants are the same (very unlikely to occur in the HSC), we again treat alphabetical order as priority. E lies before M, so the ethyl will get the lower number here.

HAHAHA quite the opposite, i was struggling with naming organic compounds so i went on that website to practice and came across something that made me more confused...but your explanation made it clearer so thanks! :D

RuiAce

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Re: Chemistry Question Thread
« Reply #237 on: March 30, 2016, 12:17:07 am »
+2
Hello !!!

I have come across similar questions to this one but I can't seem to figure out how to do them. Please help !!!

pH 2.00 of 0.01M HCl.

Calculate the pH after 20 mL of 0.01 mol L-1 hydrochloric acid is diluted with 180 mL of water.


Thanks, India

Logical consideration:

The dilution increases the volume from 20mL to 200mL. Note that 200 = 20 * 10
Because it's been diluted to a ten-fold value critically speaking, the pH goes up by 1. pH = 3.
This is because the pH scale is logarithmic (pH = -log10[H3O+])

Formal method:
According to the dilutions formula C1V1=C2V2
0.01 mol L-1 * 20 mL = [HCl]final * 200 mL
0.001 mol L-1 = [HCl]final

As HCl is both monoprotic and a strong acid
[H3O+]=[HCl]=0.001 mol L-1

So pH = -log10[H3O+] = -log10(0.001) = 3

katherine123

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Re: Chemistry Question Thread
« Reply #238 on: March 30, 2016, 04:55:07 pm »
0
During an investigation you are provided with:
- 3 volumetric flasks - each with volume capacity of 100ml, 205ml and 500ml
-2.2g anhydrous NaHCO3 molar mass of 84.01
You are asked to prepare a 0.250mol/L solution of NaHCO3
Only one of volumetric flask is useful for the task
Choose one appropriate flask and justify your choice with calculations


what i did was   i calculated the moles of NaHCO3 and find the volume required to have all the moles in the flask which turned out to be 105ml  so i thought 250 ml is the most suitable since it can contain all the volume of solution

but the answer is 100ml


2nd ques: NaOH absorbs moisture from air. If it was weighed and prepared for use as basic solution in X, what effect will this have?
a) calculated acid concentration will be correct
b) calculated acid concentration will be too low
c) calculated acid concentration will be too high
d) Ph at end point will be much lower than it should be
« Last Edit: March 30, 2016, 05:04:14 pm by katherine123 »

jakesilove

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Re: Chemistry Question Thread
« Reply #239 on: March 30, 2016, 06:09:02 pm »
0
During an investigation you are provided with:
- 3 volumetric flasks - each with volume capacity of 100ml, 205ml and 500ml
-2.2g anhydrous NaHCO3 molar mass of 84.01
You are asked to prepare a 0.250mol/L solution of NaHCO3
Only one of volumetric flask is useful for the task
Choose one appropriate flask and justify your choice with calculations


what i did was   i calculated the moles of NaHCO3 and find the volume required to have all the moles in the flask which turned out to be 105ml  so i thought 250 ml is the most suitable since it can contain all the volume of solution

but the answer is 100ml


2nd ques: NaOH absorbs moisture from air. If it was weighed and prepared for use as basic solution in X, what effect will this have?
a) calculated acid concentration will be correct
b) calculated acid concentration will be too low
c) calculated acid concentration will be too high
d) Ph at end point will be much lower than it should be

Hey Katherine!

I agree that, for the first part, you would require 105mL to get the required concentration. However, remember that a volumetric flask has to be FILLED UP COMPLETELY in order to be accurate and effective. Therefore, your options are to use slightly less water (100mL), and therefore get a concentration slightly higher than required, or use much more water (250mL) and get a concentration much lower then required. Obviously, using the 100mL will get you closer to the desired value, and so we choose the first volumetric flask!

Okay, let's talk about the second question. If the NaOH has been measured to weigh MORE than it actually is (due to water), then in your calculations you will use a basic solution with a HIGHER concentration than it actually is. If you use LESS base than you think you are using to neutralise the SAME amount of acid, then the acid is going to be weaker than you expect (for instance, you might think it neutralises 0.5 moles of base, but it is actually only neutralising 0.45 moles of base. As C=n/V, the concentration will lower if n lowers). Therefore, the answer is c) as the actual concentration is lower than the calculated concentration.

Jake
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