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March 29, 2024, 09:06:26 am

Author Topic: HSC Chemistry Question Thread  (Read 1040663 times)  Share 

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jakesilove

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Re: Chemistry Question Thread
« Reply #1890 on: March 23, 2017, 08:14:22 pm »
+1


I'll leave Jake for that general question. I didn't really study that dot point in depth; I just knew that it was true and went with it.

You're absolutely right here; there's nothing really else you can know. Maybe identify a specific fraction of petroleum? It's really not that important.
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Annie657

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Re: Chemistry Question Thread
« Reply #1891 on: March 23, 2017, 10:01:00 pm »
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Hi,
Is anyone out there doing chem of art? I'm doing it next term and everyone says it's pretty hard, my teacher says it's not popular, but is it really that bad?
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jakesilove

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Re: Chemistry Question Thread
« Reply #1892 on: March 24, 2017, 09:12:07 am »
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Hi,
Is anyone out there doing chem of art? I'm doing it next term and everyone says it's pretty hard, my teacher says it's not popular, but is it really that bad?

Hey! It's definitely not a popular course, however I think I've done a lot of the content at University! So, hopefully, I can help you out as you move through the option :)
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Annie657

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Re: Chemistry Question Thread
« Reply #1893 on: March 24, 2017, 03:46:46 pm »
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Ok great thankyou!
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Annie657

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Re: Chemistry Question Thread
« Reply #1894 on: March 24, 2017, 07:06:28 pm »
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Ok so I have another question,
In the alkenes and alkanes experiment with bromine water, does it matter if you start with bromine water first in your test tube or should you start with the hydrocarbon and add it? Thankyou  :)
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lzxnl

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Re: Chemistry Question Thread
« Reply #1895 on: March 25, 2017, 12:28:47 am »
+1
Ok so I have another question,
In the alkenes and alkanes experiment with bromine water, does it matter if you start with bromine water first in your test tube or should you start with the hydrocarbon and add it? Thankyou  :)

You should start off with your coloured bromine, as the discolouring will be much easier to see if you start with that.
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Ellie__

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Re: Chemistry Question Thread
« Reply #1896 on: March 25, 2017, 08:44:21 am »
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Hey guys,

I'm really unsure on picking out what a titration calculation question is actually asking for, and what I have to do to get there, do you have any suggestions on like a possible general formula type thing that I can use in all titration calculations


Thankyou!
Sorry if this doesn't make sense!!

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Re: Chemistry Question Thread
« Reply #1897 on: March 25, 2017, 11:57:43 am »
+1
Hey guys,

I'm really unsure on picking out what a titration calculation question is actually asking for, and what I have to do to get there, do you have any suggestions on like a possible general formula type thing that I can use in all titration calculations


Thankyou!
Sorry if this doesn't make sense!!

Hey,
I was having the same problem initially and I don't know if you already do this but what helped me was clearly listing out all the information I was already given example. n(C2H6O)= 0.01mol etc.... and then list the formulas that I might need in titration like c=n/V. It's a bit time consuming at first but once you get used to answering different types of questions you won't need it as much. Hope it helps.

Ellie__

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Re: Chemistry Question Thread
« Reply #1898 on: March 25, 2017, 07:17:13 pm »
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Hey,
I was having the same problem initially and I don't know if you already do this but what helped me was clearly listing out all the information I was already given example. n(C2H6O)= 0.01mol etc.... and then list the formulas that I might need in titration like c=n/V. It's a bit time consuming at first but once you get used to answering different types of questions you won't need it as much. Hope it helps.

Ah ok that makes sense!!  Thankyou

bsdfjnlkasn

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Re: Chemistry Question Thread
« Reply #1899 on: March 26, 2017, 09:26:48 am »
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Could someone please help me out with the following:

A student measured out 100 mL of 0.01 HCl and labels it A
25mL of A was poured into a beaker. 75mL of distilled water was added until 100mL was made up. They called this solution B
Compare the pH of the 2 solutions.

I understand we have to calculate the moles of HCl but I don't know what to do with the next step where the dilution occurs. The concentration changes and the moles do too so I don't know which formula to apply or if there is a handy shortcut that works with 75mL being 3/4 of the total 100mL - does that make sense? I want to kind of get to know the shortcuts so I don't waste time applying formulas when I don't need to.

Thank you so much for your help!


EDIT: another question  :)

The heat of combustion of ethanol is 1346 kJ/mol. What mass of ethanol would be required to produce 250 kJ of energy upon its complete combustion?
« Last Edit: March 26, 2017, 09:36:51 am by bsdfjn;lkasn »

ag5946

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Re: Chemistry Question Thread
« Reply #1900 on: March 26, 2017, 11:03:53 am »
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Could someone please help me out with the following:

A student measured out 100 mL of 0.01 HCl and labels it A
25mL of A was poured into a beaker. 75mL of distilled water was added until 100mL was made up. They called this solution B
Compare the pH of the 2 solutions.

I understand we have to calculate the moles of HCl but I don't know what to do with the next step where the dilution occurs. The concentration changes and the moles do too so I don't know which formula to apply or if there is a handy shortcut that works with 75mL being 3/4 of the total 100mL - does that make sense? I want to kind of get to know the shortcuts so I don't waste time applying formulas when I don't need to.

Thank you so much for your help!


EDIT: another question  :)

The heat of combustion of ethanol is 1346 kJ/mol. What mass of ethanol would be required to produce 250 kJ of energy upon its complete combustion?

Dont you just use C1V1=C2V2, calculate the C2 and then calculate pH of the resulting solution and compare it to the original pH of 2

jakesilove

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Re: Chemistry Question Thread
« Reply #1901 on: March 26, 2017, 11:20:13 am »
0
Could someone please help me out with the following:

A student measured out 100 mL of 0.01 HCl and labels it A
25mL of A was poured into a beaker. 75mL of distilled water was added until 100mL was made up. They called this solution B
Compare the pH of the 2 solutions.

I understand we have to calculate the moles of HCl but I don't know what to do with the next step where the dilution occurs. The concentration changes and the moles do too so I don't know which formula to apply or if there is a handy shortcut that works with 75mL being 3/4 of the total 100mL - does that make sense? I want to kind of get to know the shortcuts so I don't waste time applying formulas when I don't need to.

Thank you so much for your help!


EDIT: another question  :)

The heat of combustion of ethanol is 1346 kJ/mol. What mass of ethanol would be required to produce 250 kJ of energy upon its complete combustion?

Yep, ag is exactly right! Could can quickly calculate the initial pH (ie. -log(0.01). Then, we use C1V2=C2V2 to calculate the final concentration, and sub that straight into our pH formula again!

Now, the heat of combustion question is always a tough one. We know that



Now, we want to release 250kJ of energy. Just as a matter of logic, this means we must be burning LESS than one mole of Ethanol (as one mole produces MORE energy). So, we divide the smaller number by the bigger number, which will be equal to the number of moles. This is probably just a technique to memorise.



Then, you can multiply by the molar mass to get the total mass
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bsdfjnlkasn

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Re: Chemistry Question Thread
« Reply #1902 on: March 26, 2017, 11:24:42 am »
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Yep, ag is exactly right! Could can quickly calculate the initial pH (ie. -log(0.01). Then, we use C1V2=C2V2 to calculate the final concentration, and sub that straight into our pH formula again!

Now, the heat of combustion question is always a tough one. We know that



Now, we want to release 250kJ of energy. Just as a matter of logic, this means we must be burning LESS than one mole of Ethanol (as one mole produces MORE energy). So, we divide the smaller number by the bigger number, which will be equal to the number of moles. This is probably just a technique to memorise.



Then, you can multiply by the molar mass to get the total mass

Thank you so much Jake!!

Could I get help with approaching this 5 mark question? I don't know how you can involve equations with polymerisation? I know it might be annoying to ask you to draw it but I was never taught this properly and it just came up in a practice paper and it's obviously worth a lot of marks so if you could maybe refer me to a website that shows it well that would be really awesome!


EDIT: I'm not sure what the difference between an oxidising agent is and something which oxidises. They're not the same but I don't know how to remember a good definition to not get confused. Any advice would be greatly appreciated :)
« Last Edit: March 26, 2017, 02:24:28 pm by bsdfjn;lkasn »

bsdfjnlkasn

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Re: Chemistry Question Thread
« Reply #1903 on: March 26, 2017, 02:37:08 pm »
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Hey there,

I have a few miscellaneous questions that I was hoping to get some help with. I've attached them as screenshots and because I've got mostly pH related questions wrong, it would be really nice to have someone explain the theory behind it (if there is any) to help me clarify.
Thank you!!

For Q2: I answered A because I thought molar heat of combustion was independent of the mole ratio. Sorry if that's really stupid because they  both have the word mole in them.... I'm just confused. Ans - D

For Q10: I'm not sure what the best approach would be to answering this because there are 5 different elements and their spread through multiple reactants/products. Ans - B

Q11: I was almost certain the answer was C but it's A

Q12: I was also pretty sure the answer was D not C


jakesilove

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Re: Chemistry Question Thread
« Reply #1904 on: March 26, 2017, 03:27:55 pm »
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Thank you so much Jake!!

Could I get help with approaching this 5 mark question? I don't know how you can involve equations with polymerisation? I know it might be annoying to ask you to draw it but I was never taught this properly and it just came up in a practice paper and it's obviously worth a lot of marks so if you could maybe refer me to a website that shows it well that would be really awesome!


EDIT: I'm not sure what the difference between an oxidising agent is and something which oxidises. They're not the same but I don't know how to remember a good definition to not get confused. Any advice would be greatly appreciated :)

Hey,

The polymerisation reactions of these two reactions are something you need to know. There is a dotpoint about PVC and Polystyrene. I would recommend going through the HSC dot points, and figuring out if there's anything you've missed out on.

You need to be able to distinguish between addition polymerisation and condensation polymerisation. Addition polymerisation occurs when the monomers 'link' up without any elimination of a small molecule. Condensation polymerisation occurs when the monomers 'link' up and elimination a small molecule. The first one will look like this



You can tell that, since the monomer has a double bond, the bond will just 'split' open and react with other monomers.

The second one doesn't have a double bond, so a small molecule will be eliminated. One of the -OH groups will be eliminated from one side, and one -H from the -OH will be eliminated from the other side. This means that a product of the reaction will be water (H2O).

Additionally, you can talk about the standard initiation, propagation, termination steps required of polymerisation.

An oxidising agent is something that causes something else to oxidise. Thus, it reduces.


Molar heat of combustion has to do with how many bonds are broken. Thus, the more a molecule 'breaks down', the higher the molar heat of combustion. The bigger the molecule, the more it can combust into CO2 and H2O. So, the answer is D

Oxygen rarely changes oxidation state, so let's ignore it for now.
The initial charge of KMnO4 is zero. Thus,




The oxidation state of Manganese, initially, is +7. Looking at the right hand side




The final oxidation state of Manganese is +3. Therefore, it has decreased in oxidation number (gained electrons). The answer is B


Each step on the pH scale is a factor of 10. So, the Hydrogen ions in Milk 10 times more concentrated than in Distilled Water. The Hydrogen ions in Rain water is 10*10=100 times more concentrated than in Distilled water. the concentration in Wine is 10*10*10=1000 times more concentrated that in Distilled water. The answer is A.


The initial pH is going to be



We want the final pH to be five, so we need



Thus, initial concentration is 0.001 and final concentration is 0.00001.





So, we need to ADD 1980mL, as there is already 20mL and the total volume needs to be 2000mL
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