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March 28, 2024, 09:14:51 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040527 times)  Share 

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wesadora

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Re: Chemistry Question Thread
« Reply #570 on: August 04, 2016, 02:31:09 pm »
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For part b), i keep getting 18ppm as my answer, but apparently the answer is 4.5 x 10-3 ppm.
plz explain :'(

my working was just 0.45mg/25mL * 40/40 (to get it into micrograms/L) but I feel there's more to it xD
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jakesilove

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Re: Chemistry Question Thread
« Reply #571 on: August 04, 2016, 02:53:14 pm »
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For part b), i keep getting 18ppm as my answer, but apparently the answer is 4.5 x 10-3 ppm.
plz explain :'(

my working was just 0.45mg/25mL * 40/40 (to get it into micrograms/L) but I feel there's more to it xD

Hey!

I hope the below answer makes enough sense. Let me know if you need more details!



Jake
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wesadora

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Re: Chemistry Question Thread
« Reply #572 on: August 04, 2016, 03:01:18 pm »
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Hey!

I hope the below answer makes enough sense. Let me know if you need more details!

Jake

Legend. :)
Also- in a chemistry exam, when do you have to/don't have to define LCP? My logic says do it the very first time it's required to state it, and then i can just leave it and say 'by LCP' for the rest of the exam later in the paper whenever i need to...but have chem trial tomorrow and wanna make sure HAHA!
Cheers:)
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wesadora

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Re: Chemistry Question Thread
« Reply #573 on: August 04, 2016, 03:04:14 pm »
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hold on....why is the treated water sample "0.450 x 10-3"mg/25mL in the first line of your working? I thought that frmo the previous question before and just reading off the graph it's .450 mg/25mL (450 x -3)
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jakesilove

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Re: Chemistry Question Thread
« Reply #574 on: August 04, 2016, 03:06:43 pm »
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Legend. :)
Also- in a chemistry exam, when do you have to/don't have to define LCP? My logic says do it the very first time it's required to state it, and then i can just leave it and say 'by LCP' for the rest of the exam later in the paper whenever i need to...but have chem trial tomorrow and wanna make sure HAHA!
Cheers:)

You need to define it in every single question to do with LCP. It's not enough to prove to the marker once that you do actually know it; it is vital to any question about equilibrium! Just because you show the marker you can do concentration calculations in one question, doesn't mean you can skip the next calculation question :P For the same reason, ALWAYS define LCP (as painful as that may be)
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Loki98

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Re: Chemistry Question Thread
« Reply #575 on: August 04, 2016, 04:36:45 pm »
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Could someone please help me out with this dot point, "Identify a range of salts which form acidic, basic or neutral solutions and explain their acidic, neutral or basic nature." How to i determine which salts dissolved in water, form acidic, basic or neutral solutions?

RuiAce

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Re: Chemistry Question Thread
« Reply #576 on: August 04, 2016, 04:41:33 pm »
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Could someone please help me out with this dot point, "Identify a range of salts which form acidic, basic or neutral solutions and explain their acidic, neutral or basic nature." How to i determine which salts dissolved in water, form acidic, basic or neutral solutions?
By comparing what ions built up to give that salt.

Ions such as Na+, K+, Cl- and NO3- are all neutral cations. Their conjugate acid/base is so strong that they are only a tiny bit basic/acidic and don't impact.

As opposed to something like NH4+, which is slightly acidic because NH3 is just decently basic.
Or something like CH3COO- which is slightly basic because CH3COOH is just somewhat acidic.
hold on....why is the treated water sample "0.450 x 10-3"mg/25mL in the first line of your working? I thought that frmo the previous question before and just reading off the graph it's .450 mg/25mL (450 x -3)
Milli does not mean micro

1000 micrograms in a milligram

conic curve

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Re: Chemistry Question Thread
« Reply #577 on: August 04, 2016, 05:20:33 pm »
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NiCl2(aq)+NaOH(aq)-->Ni(OH)2(s)+NaCl(aq)

I don't understand the complete ionic equation and net ionic equation behind this?

AgNO3(aq)+cl-(aq)-->AgCl(s)

I don't get the complete ionic equation in this

RuiAce

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Re: Chemistry Question Thread
« Reply #578 on: August 04, 2016, 05:54:46 pm »
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NiCl2(aq)+NaOH(aq)-->Ni(OH)2(s)+NaCl(aq)

I don't understand the complete ionic equation and net ionic equation behind this?

AgNO3(aq)+cl-(aq)-->AgCl(s)

I don't get the complete ionic equation in this
No tidy typing cause I'm on my phone

With your complete ionic equation all you do is separate the aqueous substances into their components really. It's only for the net ionic equation where you have to throw out your spectator ions.

However, one very important thing to note is that both of your given equations are unbalanced.

First one:
NiCl2(aq) + 2 NaOH(aq) -> Ni(OH)2(s) + 2 NaCl(aq)

In complete ionic:
Ni(2+) + 2 Cl(-) + 2 Na(+) + 2 OH(-) -> Ni(OH)2(s) + 2 Na(+) +2 Cl(-)

Cancel out all the spectators for net ionic
Ni(2+) + 2 OH(-) -> Ni(OH)2(s)

anotherworld2b

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Re: Chemistry Question Thread
« Reply #579 on: August 04, 2016, 10:02:09 pm »
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Hi was wondering if i could get help understanding what this question is asking. Valency?

Skidous

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Re: Chemistry Question Thread
« Reply #580 on: August 04, 2016, 10:10:09 pm »
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It's talking about the number of covalent bonds each element can make, it relates to valency in that it involves the sharing of electrons

So for Cl it has a valency of 7, so it needs an extra electron to become stable (like the noble gases) in which case it forms a covalent bond with another Cl atom, they share that one covalent bond and thus share a valence electron to become stable.

Then you go through and figure out the number of covalent bonds needed to make be valence shells of each element full

For example
O has 6 valence, therefore 2 covalent bonds are needed to fill the valence shell
H has 1 valence and thus forms one covalent bond to fill the valence shell (it only occupies the first shell)

Hope this helps
Skidous
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RuiAce

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Re: Chemistry Question Thread
« Reply #581 on: August 04, 2016, 10:13:05 pm »
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It's talking about the number of covalent bonds each element can make, it relates to valency in that it involves the sharing of electrons

So for Cl it has a valency of 7, so it needs an extra electron to become stable (like the noble gases) in which case it forms a covalent bond with another Cl atom, they share that one covalent bond and thus share a valence electron to become stable.

Then you go through and figure out the number of covalent bonds needed to make be valence shells of each element full

For example
O has 6 valence, therefore 2 covalent bonds are needed to fill the valence shell
H has 1 valence and thus forms one covalent bond to fill the valence shell (it only occupies the first shell)

Hope this helps
Skidous
The question asks for the maximum amount. This makes me question as to whether they mean maximum amount so that the atoms are "happy", or the actual maximum amount.

Why do I say this? Oxygen technically only needs (and ever wants) 2 covalent bonds to be happy. Sulfur, on the other hand, can take 2, 4 or even 6.

I feel that this was a badly worded question due to its lack of clarity.

wesadora

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Re: Chemistry Question Thread
« Reply #582 on: August 04, 2016, 10:13:54 pm »
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is the answer C because citric acid only has a 1% ionisation strength compared to hydrochloric acid's 100%? What's the reasoning/how do you work this out?
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RuiAce

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Re: Chemistry Question Thread
« Reply #583 on: August 04, 2016, 10:16:36 pm »
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is the answer C because citric acid only has a 1% ionisation strength compared to hydrochloric acid's 100%? What's the reasoning/how do you work this out?
pH is 2? Shouldn't the answer then be B because [H3O+] = 10-pH = 10-2 = 0.010
Calling in Jake...

However yes, ionisation strength definitely plays a part.

jakesilove

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Re: Chemistry Question Thread
« Reply #584 on: August 04, 2016, 10:27:18 pm »
+1
pH is 2? Shouldn't the answer then be B because [H3O+] = 10-pH = 10-2 = 0.010
Calling in Jake...

However yes, ionisation strength definitely plays a part.

Yes, the answer is definitely B, not C. Like Rui shows, we know that the concentration of HCl to be 0.010 moles per liter. That equation works directly for all strong acids/bases, so it can't be wrong. This means that the only possible answer is B. We also happen to know that Citric acid is much, much weaker, and so requires a concentration that is much stronger (eg. double) that HCl to produce the same pH. B therefore continues to make sense, and so is the only possible answer.

Jake
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