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March 29, 2024, 03:07:46 am

Author Topic: HSC Chemistry Question Thread  (Read 1040588 times)  Share 

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J.B

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Re: Chemistry Question Thread
« Reply #1695 on: February 20, 2017, 02:56:15 pm »
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Hello,
Today in class, my teacher showed us how that if you have 0.1 molar concentration of Hydrochloric acid solution, the pH is 1. Due to the pH=-log10[H+].
Then we learn't that 0.1 molar concentration of Sulfuric acid solution which produces 2 hydronium ions thus, times the 0.1 by two and the pH is 0.7.
Then he said that 0.1 molar concentration of Acetic acid has pH 3. But I don't understand how you would work this out?
I was just wondering if you would be able to explain this?
Thank you.

RuiAce

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Re: Chemistry Question Thread
« Reply #1696 on: February 20, 2017, 02:57:59 pm »
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Hello,
Today in class, my teacher showed us how that if you have 0.1 molar concentration of Hydrochloric acid solution, the pH is 1. Due to the pH=-log10[H+].
Then we learn't that 0.1 molar concentration of Sulfuric acid solution which produces 2 hydronium ions thus, times the 0.1 by two and the pH is 0.7.
Then he said that 0.1 molar concentration of Acetic acid has pH 3. But I don't understand how you would work this out?
I was just wondering if you would be able to explain this?
Thank you.
Between HCl and H2SO4, which are both strong acids, it is quite clear to work out as you can look at the formula and count the number of H's you have there.

Acetic acid is a weak acid. It does not ionise to completion. This means that its DEGREE of ionisation is not 100%.

You are not expected to memorise the degree of ionisation of CH3COOH

kylesara

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Re: Chemistry Question Thread
« Reply #1697 on: February 20, 2017, 03:21:26 pm »
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Hi I was just wondering if ill have to remember how to draw a Vanadium redox battery during any test this year. whenever i draw it its really bad. thanks.

jakesilove

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Re: Chemistry Question Thread
« Reply #1698 on: February 20, 2017, 03:41:01 pm »
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Hi I was just wondering if ill have to remember how to draw a Vanadium redox battery during any test this year. whenever i draw it its really bad. thanks.

Hey! If one of your chosen batteries is the Vanadium redox battery, then it's probably a good idea to know how to draw it! This is really just an 'icing on the cake' type situation, so if you're not great at it now, don't worry! You'll get used to it over time, so that when the HSC comes you can smash any battery question out of the park.

Again, you don't HAVE to know how to draw it, but I personally think it's helpful!
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J.B

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Re: Chemistry Question Thread
« Reply #1699 on: February 20, 2017, 05:35:19 pm »
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Between HCl and H2SO4, which are both strong acids, it is quite clear to work out as you can look at the formula and count the number of H's you have there.

Acetic acid is a weak acid. It does not ionise to completion. This means that its DEGREE of ionisation is not 100%.

You are not expected to memorise the degree of ionisation of CH3COOH

Oh right, thank you so much!!!

Aaron12038488

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Re: Chemistry Question Thread
« Reply #1700 on: February 20, 2017, 05:41:28 pm »
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I was wondering in future if I could post like my prac report, will I need 15 posts similarly to the Essay Policy?
Thanks.

jakesilove

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Re: Chemistry Question Thread
« Reply #1701 on: February 20, 2017, 10:37:54 pm »
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I was wondering in future if I could post like my prac report, will I need 15 posts similarly to the Essay Policy?
Thanks.

Hey!

Don't worry about the post count, but I don't think I'll be able to mark an assessment in super-detail. I will be able to give you pointers, though; where to focus your attention, where you can improve, that sort of thing!

Jake
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parthie

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Re: Chemistry Question Thread
« Reply #1702 on: February 21, 2017, 08:19:18 pm »
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Hey! So the general things you should be including in your prac reports are:
- Aim
- Hypothesis
- Diagram of set-up - labelled!
- Method - not sure if it's just my school but this should be written in past tense
Eg. Change "Measure 100mL of water and place it into an aluminium can" to "100mL of water was measured and placed into an aluminium can"
- Risk assessment - state the risk, and ways to minimise this risk
- Results - could include observations, tables/graphs, calculations
For graphs, always remember to include a title, label your axes with units, and make sure your scale is appropriate (increments of 1, 2, 5, 10 etc.)
For tables, also remember to write a title, and make sure you draw in the borders - I know this might sound stupid but the teachers at my school are really picky and they took marks off people who didn't draw the outside borders on their tables :/
- Discussion - here you have to talk about accuracy, reliability and validity, as well as any improvements you could make to your experiment.
Accuracy refers to your measuring instruments - How sensitive were the instruments that you used? Could you have used something more accurate (scales with more decimal places or volumetric flask vs. measuring cylinder)? For things like the heat of combustion prac, how close were your results to the actual value?
Reliability refers to consistency of results - How many times did you repeat it? Did you obtain similar results everytime? Important to note that your results aren't reliable if you repeat 100 times but obtain completely different results
Validity refers to whether your method actually tests your aim - Did you control all your variables? Did you pick the most appropriate equipment?
- and finally, Conclusion - just a sentence or two about your results with reference to your aim/hypothesis

Hope this helps :)

Thankyou so much this really helps!!

anotherworld2b

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Re: Chemistry Question Thread
« Reply #1703 on: February 21, 2017, 08:26:15 pm »
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I am bit confused about ice melting. Why does water have more energy than ice? Does that mean that if you freeze ice the opposite will happen?
Melting ice -> heat absorbed & water has more energy than ice -> endothermic

I was also wondering if i could have some help with these questions. In particular for q3 how do you know what is being consumed? Is it always the first reactant in the equation? If not, how do you tell?
« Last Edit: February 21, 2017, 08:28:32 pm by anotherworld2b »

bananna

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Re: Chemistry Question Thread
« Reply #1704 on: February 21, 2017, 08:50:10 pm »
+1
hi!

can someone pls help with this q the answer's (C)

i sorta get the calculations, but don't know WHY you'd use mol weight

its from the 2003 HSC q 6

thank u!!

Mathew587

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Re: Chemistry Question Thread
« Reply #1705 on: February 21, 2017, 09:20:30 pm »
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I am bit confused about ice melting. Why does water have more energy than ice? Does that mean that if you freeze ice the opposite will happen?
Melting ice -> heat absorbed & water has more energy than ice -> endothermic

I was also wondering if i could have some help with these questions. In particular for q3 how do you know what is being consumed? Is it always the first reactant in the equation? If not, how do you tell?
Water has more energy than ice simply because water flows while ice doesn't. This means water must have kinetic energy which MAKES it flow. Also steam has more energy than water and ice simply as to create steam, you need to ADD heat to water just as how you need to ADD heat to ice to make it water. :D

Idek how you can freeze ice?!?

Yes as far as what you know now, the reactant is whats being consumed (obv...). When solid water i.e. ice is transformed into liquid water i.e. water, the ice i.e. the reactant in this case is the one being consumed :D
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parthie

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Re: Chemistry Question Thread
« Reply #1706 on: February 21, 2017, 09:50:02 pm »
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Can someone please explain validity in experiments to me?? I am so confused

and

For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??

Any help would be great thanks

bsdfjnlkasn

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Re: Chemistry Question Thread
« Reply #1707 on: February 21, 2017, 09:54:03 pm »
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hi!

can someone pls help with this q the answer's (C)

i sorta get the calculations, but don't know WHY you'd use mol weight

its from the 2003 HSC q 6

thank u!!


Hey, I'd also like some help approaching the calculations part of the question if that's ok?

bsdfjnlkasn

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Re: Chemistry Question Thread
« Reply #1708 on: February 21, 2017, 10:05:56 pm »
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Can someone please explain validity in experiments to me?? I am so confused

and

For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??

Any help would be great thanks

Validity is mostly concerned with maintaining all the variables the same to ensure the experiment, as implied by the name, valid. If you changed what you were measuring in the middle of an experiment then your experiment wouldn't have valid results as your results are not specific to the dependent variable anymore as you've introduced another one.

The heat of combustion experiment wouldn't be considered invalid if you kept your controlled variables controlled, measured the change in mass and changed the fuels accordingly. If you've kept these all as they should be, then you have yourself a valid experiment. But, what I think you're hinting at in your question is actually accuracy rather than validity. Accuracy is concerned with how close your values are to the accepted ones and the way in which you recorded your results. Because the latter is far less significant (only really concerned with parallax error in observing thermometer), we don't really need to focus on it as much. This is because our results do not drastically change when small readings on the thermometer are taken incorrectly.

Now on to what really shakes up our results:

Heat loss is by far, the most significant contributor to inaccurate results; i.e. results which deviate from the accepted values for the heat of combustion of your independent fuels.
 
Your heat of combustion experiment depends on the mass changes which is a result of the fuel having evaporated. This is what we are measuring. We obtain these mass changes by heating the fuel. If all the heat from the flame was conserved and directed straight into the alkanol, (i.e. through use of a bomb calorimeter) then we wouldn't see large discrepancies. But, because the experiment wasn't performed in a closed system, heat inevitably escaped - dissipated into the surroundings. By not having the most efficient heat transfer, our results are no longer reliable as such a big portion of what our results depended on was unreliable.

I hope that makes sense. Essentially you need to know that the heat losses to the surroundings significantly decreases the reliability of the experiment. Let me know if you want me to clarify the above further.
« Last Edit: February 21, 2017, 10:11:53 pm by bsdfjn;lkasn »

kiwiberry

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Re: Chemistry Question Thread
« Reply #1709 on: February 21, 2017, 10:06:55 pm »
+1
hi!

can someone pls help with this q the answer's (C)

i sorta get the calculations, but don't know WHY you'd use mol weight

its from the 2003 HSC q 6

thank u!!

So for this question, we're trying to convert heat of combustion from kJ/g to kJ/mol. Molar mass is in g/mol. Looking at the units, if we times heat of combustion in kJ/g by molar mass in mol/g, we'll get kJ/mol, which is what we want!

Does that make sense? So to find heat of combustion in kJ/mol, you would multiply the values in the table by their respective molar masses :)
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