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March 29, 2024, 09:59:12 am

Author Topic: HSC Chemistry Question Thread  (Read 1040672 times)  Share 

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jakesilove

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Re: Chemistry Question Thread
« Reply #255 on: April 06, 2016, 10:32:41 am »
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Hey guys!

Got my Chemistry half yearly today and I've got a stupid question D: This is the question:

Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride

What's a good way to tell if you're unsure of the compound? Thanks!

Neutron

Hey!

EDIT: I was mistaken when answering this question: see below posts for the correct answer!

I don't think that's a stupid question at all! I think the most difficult part of the HSC Chemistry curriculum are sections where you just kind of need to memorise content, or rules, to get marks in a question.

Let's approach the multiple choice you've posed. First, I think it is fair to eliminate B straight away, as we know that NaCl is a neutral salt. It kind of has to be: we wouldn't want to be putting acidic/basic substances or our chips.

Next, I would try to recall that anything with an -oate on the end is know as an 'alkanoic acid' (from the Esterifaction part of your curriculum). Knowing that, I think it would be fair to eliminate A as well, since it will be acidic.

As for the last two, I honestly have no idea how to decide which is basic. I hope someone else in the forums can help me out, but I can't think of a way to distinguish between them in terms of any HSC level chemistry. I'm really sorry about that: at least at this stage you have a 50/50 chance of being right!

Again, so sorry about my terrible response.

Jake
« Last Edit: April 06, 2016, 10:05:12 pm by jakesilove »
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Neutron

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Re: Chemistry Question Thread
« Reply #256 on: April 06, 2016, 11:53:45 am »
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Ahh it's okay, but strangely enough the answer was A? :/ No idea, also, what's a good amphoteric substance to use as an example because I originally chose Aluminium Oxide but I don't seem to be able to find what it forms when reacted with NaOH? Thanks :D

Neutron

IntelxD

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Re: Chemistry Question Thread
« Reply #257 on: April 06, 2016, 12:03:45 pm »
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Hey guys!

Got my Chemistry half yearly today and I've got a stupid question D: This is the question:

Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride

What's a good way to tell if you're unsure of the compound? Thanks!

Neutron

Sodium ethanoate will dissociate into sodium ions and ethanoate ions in an aqueous solution. Ethanoate (also known as acetate) is the weak conjugate base of ethanoic acid. Thus, ethanoate will react with the H+ ions, lowering [H+] and slightly increasing the pH of the solution above 7. Ammonium in option c and d is a weak acid and so will have the opposite effect. You should also note that ions such as chloride, sodium and nitrate do in fact have acidic or basic properties. They however are so weak that their effect on the pH of the solution is negligible and usually produces opposing effects (eg, NaCl) leaving the pH unchanged.
2014-2015: VCE
2016: Monash University, Bachelor of Medicine/Bachelor of Surgery (Honours)

helloworld

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Re: Chemistry Question Thread
« Reply #258 on: April 06, 2016, 12:25:24 pm »
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Does anyone have answers to thushans notes for Chem on here?

HopefulLawStudent

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Re: Chemistry Question Thread
« Reply #259 on: April 06, 2016, 12:50:42 pm »
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Hey. This is the HSC Chem thread. This question is specific to VCE Chem (cuz I think Thushan only does notes/stuff for VCE). So if you pop on over to the VCE chem board (Chemistry), you'll probably be more likely to get an answer to your question cuz VCE chem kids are less likely to hang around over in this section of the forum.

helloworld

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Re: Chemistry Question Thread
« Reply #260 on: April 06, 2016, 01:14:22 pm »
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Thanks!

RuiAce

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Re: Chemistry Question Thread
« Reply #261 on: April 06, 2016, 05:43:55 pm »
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Hey guys!

Got my Chemistry half yearly today and I've got a stupid question D: This is the question:

Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride

What's a good way to tell if you're unsure of the compound? Thanks!

Neutron

We're looking for the basic salt. Because ammonium nitrate and ammonium chloride are acidic, they're automatically out. Why did I eliminate them in a matter of 1 second?

NO3- and Cl- are the conjugates of strong acids. That means they're extremely weak; virtually neutral. So they don't impact.

And it just so happens that substances such as Na+ and K+ are neutral cations. Whereas NH4+ is actually weakly acidic.

(Note, if I had ammonium acetate that would be different. That'd be around neutral.)

And then, like mentioned, sodium chloride is neutral. This is trivial, but it's not the first thing I notice due to the difference between sodium ions and ammonium ions.

Now, I think Jake made a mistake. Whilst ethanoic acid (aka acetic acid) is acidic, the ethanoate (acetate) ion is consequently basic. It is the conjugate to acetic acid, and is thus what causes the basic properties. Just like Intel mentioned.

jakesilove

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Re: Chemistry Question Thread
« Reply #262 on: April 06, 2016, 10:06:20 pm »
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Now, I think Jake made a mistake.

You're absolutely right, I'm sorry about that! Your responses all look fantastic, thanks for helping out!

Jake
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Johny1234567

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Re: Chemistry Question Thread
« Reply #263 on: April 10, 2016, 08:29:00 am »
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Draw separate concentration-time graphs for the following situations relating to the Haber Reaction.
a) The reactants are placed into the reaction vessel and are allowed to react for a longer period of time.
b) The reaction first reacts to equilibrium, and then the reaction vessel is heated.
c) The formation of ammonia with and without a catalyst, assuming all other conditions are the same. You do not need to show how hydrogen/ nitrogen vary on this graph.

RuiAce

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Re: Chemistry Question Thread
« Reply #264 on: April 10, 2016, 09:01:42 am »
+2
Draw separate concentration-time graphs for the following situations relating to the Haber Reaction.
a) The reactants are placed into the reaction vessel and are allowed to react for a longer period of time.
b) The reaction first reacts to equilibrium, and then the reaction vessel is heated.
c) The formation of ammonia with and without a catalyst, assuming all other conditions are the same. You do not need to show how hydrogen/ nitrogen vary on this graph.
a) As the Haber process does effectively take a HUGE amount of time to reach equilibrium, show this by making the process seemingly unable to obtain equilibrium in graph 1, but then achieving it in graph 2.
b) Recall that the equation 3 H2(g) + N2(g) ⇌ 2 NH3(aq) with ∆H=-92 kJ mol-1
The forward reaction is exothermic. Hence, if we introduce heat into the system, according to Le Chatelier's principle the system will attempt to readjust it's equilibrium to counter the change by shifting it to the left.
From a graph that's already attained equilibrium, suddenly the concentrations of the reactants (hydrogen and nitrogen) must go up a bit, and that of ammonia will go down.
c) Catalyst speeds up the rate of reaction. Hence, you achieve the same equilibrium, but you achieve it faster. This just means that on the graph, say the equilibrium is at 3. You increase up to 3 more quickly, but you end up with the same horizontal line anyway.

I will let someone else post relevant diagrams

P.S. I haven't been helping much these days cause I want to focus on mid sems

Johny1234567

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Re: Chemistry Question Thread
« Reply #265 on: April 10, 2016, 11:05:20 am »
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ty, one more question:
Calculate the yield of the Haber reaction that used 30 kg of Nitrogen gas in excess hydrogen gas to form 1.2 kg of ammonia.

jakesilove

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Re: Chemistry Question Thread
« Reply #266 on: April 10, 2016, 11:22:15 am »
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ty, one more question:
Calculate the yield of the Haber reaction that used 30 kg of Nitrogen gas in excess hydrogen gas to form 1.2 kg of ammonia.

Hey!

Below is my solution. Hope it helps!



Jake
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chloe9756

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Re: Chemistry Question Thread
« Reply #267 on: April 14, 2016, 04:20:07 pm »
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hey, this is a really simply question but i can't manage to get my head around a few things:

The solubilities at 25C of three white barium salts, in grams per 100mL of water are:
Barium Sulfate 0.00025
Barium Nitrate 10.1
Barium Iodide 220

15g of a white powder was prepared by mixing 5.0g of each of these three barium salts. Your task to to obtain a sample of pure barium sulphate and a sample of virtually pure barium nitrate. Write the method for the separation process. Explain at each stage what happens to the mixture.

Firstly, when the question says to obtain PURE barium sulphate and VIRTUALLY PURE barium nitrate, how should that change the process you choose to use. Also, the answers specifically says that you must add more than 55mL of water, why is this so? And it also says after separating barium sulphate through filtration, barium nitrate will stay dissolved but barium iodide will crystallise out. Why is this so, doesn't barium iodide have a higher solubility. Thank you :)

katherine123

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Re: Chemistry Question Thread
« Reply #268 on: April 20, 2016, 03:16:56 am »
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Help with this ques thanks
0.132g of pure carboxylic acid (R-COOH) was dissolved in 25ml of water and titrated with 0.120mol/L NaOH solution
A volume of 14.80ml was required to reach the endpoint of the titration.
The carboxylic acid could be:
A)HCOOH
B)CH3COOH
C)C2H5COOH
D)C3H7COOH
ans:C


question 2: Oxalic acid C2O4H2
A 25.0ml solution of oxalic acid reacts completely with 15.0ml of 2.50mol/L of NaOH
what is the concentration of the oxalic acid solution?
ans: 0.750M
« Last Edit: April 20, 2016, 03:23:16 am by katherine123 »

mmadeleine

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Re: Chemistry Question Thread
« Reply #269 on: April 23, 2016, 06:29:55 pm »
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Hi, just a bit stuck on this titration question - I don't really understand how to do it so any help would be appreciated.  :)

"The electrolyte in car batteries is sulfuric acid. A student decided to determine the concentration of this acid in a well-charged car battery by taking exactly 2 mL by pipette and titrating it with 1.16 mol/L sodium hydroxide solution. 17.1 mL was needed to reach the equivalence point. Calculate the molarity of the sulfuric acid in the battery."

Thanks!