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March 29, 2024, 07:13:05 am

Author Topic: HSC Chemistry Question Thread  (Read 1040639 times)  Share 

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RuiAce

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Re: Chemistry Question Thread
« Reply #690 on: August 29, 2016, 06:00:18 pm »
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Guys the answer is D for this question, but couldn't it be A as well???
As far as I can tell, the answer could also be A. The equilibrium should shift away from any increase in concentration. Not sure why the answer is D, and not A, although D is definitely correct
Does the fact that the reactant is a solid play a role?

jyce

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Re: Chemistry Question Thread
« Reply #691 on: August 31, 2016, 12:26:32 am »
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Does the fact that the reactant is a solid play a role?

Yes, I believe it does. Adding more CaCO3(s) does not change its concentration, as it's a solid. Therefore, adding more CaCO3 does not cause a shift in the equilibrium position, leaving D as the correct option.

massive

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Re: Chemistry Question Thread
« Reply #692 on: August 31, 2016, 12:40:41 am »
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Yes, I believe it does. Adding more CaCO3(s) does not change its concentration, as it's a solid. Therefore, adding more CaCO3 does not cause a shift in the equilibrium position, leaving D as the correct option.

Wait what, isn't the concentration still increasing because technically there's more reactants and the system wants to counteract this by decreasing the reactants, thus it shifts right ? :S

Jakeybaby

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Re: Chemistry Question Thread
« Reply #693 on: August 31, 2016, 12:55:23 am »
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Wait what, isn't the concentration still increasing because technically there's more reactants and the system wants to counteract this by decreasing the reactants, thus it shifts right ? :S
The concentration of the solids are constant, only (aq) and (g) are in the equilibrium expressions. You can exclude pure solids and pure liquids from the equilibrium expressions.
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RuiAce

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Re: Chemistry Question Thread
« Reply #694 on: August 31, 2016, 07:38:36 am »
+1
Yes, I believe it does. Adding more CaCO3(s) does not change its concentration, as it's a solid. Therefore, adding more CaCO3 does not cause a shift in the equilibrium position, leaving D as the correct option.
What I thought. I remember being taught this but it was a blur.
Wait what, isn't the concentration still increasing because technically there's more reactants and the system wants to counteract this by decreasing the reactants, thus it shifts right ? :S
When you add more moles of solid, you add the same amount of volume of the solid to counteract a potential increase in concentration. Given C=n/V, if n and V go up in the exact same proportion, the concentration is not affected.

Note that it is the CONCENTRATION of the substance which shifts the equilibrium, not the actual quantity (moles)

massive

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Re: Chemistry Question Thread
« Reply #695 on: September 01, 2016, 02:29:57 am »
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Hey guys, could someone explain how to do hydrolysis of salts to me, i still don't get how to do them :/ (like determining whether the salt is acid or base)
Thanks heaps!

conic curve

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Re: Chemistry Question Thread
« Reply #696 on: September 01, 2016, 10:43:26 am »
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Hey guys, could someone explain how to do hydrolysis of salts to me, i still don't get how to do them :/ (like determining whether the salt is acid or base)
Thanks heaps!

I don't know whether or not this is useful, but maybe you could take a look at this: http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Equilibria/Solubilty/Hydrolysis

Hopes that helps  :D

RuiAce

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Re: Chemistry Question Thread
« Reply #697 on: September 01, 2016, 01:35:29 pm »
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Hey guys, could someone explain how to do hydrolysis of salts to me, i still don't get how to do them :/ (like determining whether the salt is acid or base)
Thanks heaps!
When something is dissolved in water, the ions that form the compound are dissociated (e.g. Na2CO3(s) -> 2 Na+(aq) + CO32-(aq)). To determine the nature of the dissolved substance as acidic or basic, we need to look at the ions that are formed.

Take this example first. CO32- is basic. However Na+ is a neutral cation. Therefore the resulting solution is basic and the original salt was also basic.

Take NH4Cl. NH4+ is acidic, however Cl- is a neutral cation. Therefore we have something acidic.

If we take NaCl, then both Na+ and Cl- are neutral so the result is neutral.


Simply put, analyse the cation and anion separately.
« Last Edit: September 01, 2016, 01:48:57 pm by RuiAce »

anotherworld2b

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Re: Chemistry Question Thread
« Reply #698 on: September 02, 2016, 01:29:33 am »
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How would you solve question b ii and c?

jakesilove

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Re: Chemistry Question Thread
« Reply #699 on: September 02, 2016, 10:36:48 am »
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How would you solve question b ii and c?

Hey! The formula we need for these questions is



For i)




For ii), we WANT a certain volume and concentration out, and we HAVE an initial concentration. The only thing we don't have is the initial volume, which is perfect, because that's exactly how the formula works!




For c), again it's just a simply application of the formula. Remember than C1 is your initial concentration, V1 is your initial volume, C2 is your final concentration, and V2 is your final volume.
The trick to this solution is that the TOTAL final volume is going to be 10ml+55mL=65mL


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anotherworld2b

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Re: Chemistry Question Thread
« Reply #700 on: September 02, 2016, 07:05:09 pm »
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Thank you
Hey! The formula we need for these questions is



For i)




For ii), we WANT a certain volume and concentration out, and we HAVE an initial concentration. The only thing we don't have is the initial volume, which is perfect, because that's exactly how the formula works!




For c), again it's just a simply application of the formula. Remember than C1 is your initial concentration, V1 is your initial volume, C2 is your final concentration, and V2 is your final volume.
The trick to this solution is that the TOTAL final volume is going to be 10ml+55mL=65mL




onepunchboy

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Re: Chemistry Question Thread
« Reply #701 on: September 03, 2016, 07:33:10 pm »
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Yo can anyone help me out on q 16 and 17 ?
Thankss

jakesilove

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Re: Chemistry Question Thread
« Reply #702 on: September 03, 2016, 10:12:39 pm »
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(Image removed from quote.)

Yo can anyone help me out on q 16 and 17 ?
Thankss

Not entirely sure for Q16; is the answer supposed to be A?

For Q17, let's think about what's actually going on. If we had chosen the correct indicator, the endpoint would have been somewhere in the 8-10 range (strong base, weak acid). Therefore, the pH should start on the lower end (ie. pH of 2-3, due to the weak acid) and strong base should be added until the pH is 8-10. However, if the indicator will change colors when the pH reaches 3-4, then much LESS of the base will be required! The greater the pH change, the more base would be required. Since we've used LESS base than expected to neutralise the acid, it will be expected that the acid is WEAKER. Therefore, I believe that the answer is B
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jakesilove

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Re: Chemistry Question Thread
« Reply #703 on: September 03, 2016, 10:15:18 pm »
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(Image removed from quote.)

Yo can anyone help me out on q 16 and 17 ?
Thankss

As far as I can tell, the Cathode should be positive as electrons flow TOWARDS it (remember, Reduction It Gains (RIG)). This won't come 'from' the electrolyte, but rather from the anode. If you consider the anode to be the external circuit, I imagine the answer is D
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massive

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Re: Chemistry Question Thread
« Reply #704 on: September 04, 2016, 02:52:11 pm »
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Hey guys, i was doing a question for my option topic (industrial chem) and the question was to find the equilibrium constant (K). However the question didn't give the concentrations of the reactants and products in mol/L but rather gave it in kPa. When instructed to find K, the answers just substituted these kPa values into the formula. I was just confused, i thought you could only sub the concentration that are in mol/L. :S