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Author Topic: HSC Chemistry Question Thread  (Read 1040550 times)  Share 

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nimasha.w

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Re: Chemistry Question Thread
« Reply #345 on: June 17, 2016, 11:25:14 pm »
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hi! i'm stuck on trying to find a ppt test to distinguish between Ca2+ and Ba2+
« Last Edit: June 17, 2016, 11:35:01 pm by nimasha.w »

Jakeybaby

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Re: Chemistry Question Thread
« Reply #346 on: June 17, 2016, 11:51:04 pm »
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hi! i'm stuck on trying to find a ppt test to distinguish between Ca2+ and Ba2+
Sorry, could I ask what a ppt test is?
If you could use a flame test, this would determine the difference, as Ca2+ would produce a 'brick-red' coloured flame whereas Ba2+ would produce a 'yellow-bright green' coloured flame.

Sorry, realised what ppt. means.
You could add NaOH to determine between the two if a precipitate test is the way to go.

Ca2+ will produce a white precipitate, whereas Ba2+ will not produce a precipitate due Ba(OH)2 being soluble.
« Last Edit: June 18, 2016, 12:01:52 am by Jakeybaby67 »
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RuiAce

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Re: Chemistry Question Thread
« Reply #347 on: June 17, 2016, 11:53:17 pm »
+1
hi! i'm stuck on trying to find a ppt test to distinguish between Ca2+ and Ba2+

Add F-

The one that precipitates is the Ca2+

(This is assuming you have to use precipitation. Otherwise a flame test is really easy.)

m_chan98

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Re: Chemistry Question Thread
« Reply #348 on: June 19, 2016, 11:43:03 am »
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Hi! So I was wondering whether someone could provide just a bit of a summary of what solubility rules I need to know? I've consulted a few different sources but each of them are different. Also, I understand that the compounds are soluble, but soluble in what?? Thanks!!!  :)
« Last Edit: June 19, 2016, 11:53:49 am by m_chan98 »
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jakesilove

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Re: Chemistry Question Thread
« Reply #349 on: June 19, 2016, 12:00:48 pm »
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Hi! So I was wondering whether someone could provide just a bit of a summary of what solubility rules I need to know? I've consulted a few different sources but each of them are different. Also, I understand that the compounds are soluble, but soluble in what?? Thanks!!!  :)

That's a seriously good question! I memorised the following solubility rules, which are really easy to remember once you start using the pneumonic device outlined below.



Those are the main solubility rules. I think that there are some others (eg. something to do with Silver Nitrate precipitating a white powder specifically) which discuss the COLOUR of the precipitate, but the actual precipitation is all outlined above. I wouldn't be learning more rules than what I've posted above.

As for your second questions; I assume it's just in water. I've tried doing some research, but everything comes up blank. However, you definitely don't need to think about that!

Let me know if I can be  more specific/helpful regarding any of this,

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amandali

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Re: Chemistry Question Thread
« Reply #350 on: June 19, 2016, 12:56:14 pm »
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can you check my answer ?

CH3COOH(aq) + H2O(l) <---> H3O+(aq) +CH3COO-(aq)
The addition of OH- from NaOH will react with H3O+ which consequently reduces the concentration of H3O+ . This will cause the equilibrium to shift right to increase the concentration of H3O+. Hence, a buffer system is established which minimises pH increase.


The answer given has the equation the other way around so is this one correct as well?

jakesilove

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Re: Chemistry Question Thread
« Reply #351 on: June 19, 2016, 01:00:29 pm »
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can you check my answer ?

CH3COOH(aq) + H2O(l) <---> H3O+(aq) +CH3COO-(aq)
The addition of OH- from NaOH will react with H3O+ which consequently reduces the concentration of H3O+ . This will cause the equilibrium to shift right to increase the concentration of H3O+. Hence, a buffer system is established which minimises pH increase.


The answer given has the equation the other way around so is this one correct as well?

Hey! Your equation is definitely fine; as it's an equilibrium, you can write it whichever way you want. The only thing I would add is a brief explanation of acidity/basidity. Whist it may be obvious to you that an increase in H30+ is an increase in H+, which increased acidity (decreasing pH), perhaps throw in a single sentence saying that. This is just to make sure you give a really comprehensive response!

Great answer though,

Jake
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Re: Chemistry Question Thread
« Reply #352 on: June 19, 2016, 02:41:37 pm »
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can you check my answer ?

CH3COOH(aq) + H2O(l) <---> H3O+(aq) +CH3COO-(aq)
The addition of OH- from NaOH will react with H3O+ which consequently reduces the concentration of H3O+ . This will cause the equilibrium to shift right to increase the concentration of H3O+. Hence, a buffer system is established which minimises pH increase.


The answer given has the equation the other way around so is this one correct as well?

Also be sure to explicitly say "Le Chatelier's principle" whenever you're referring to equilibrium behaviour.

jakesilove

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Re: Chemistry Question Thread
« Reply #353 on: June 19, 2016, 02:46:34 pm »
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Also be sure to explicitly say "Le Chatelier's principle" whenever you're referring to equilibrium behaviour.

Ah yes of course: every single equilibrium answer should have something like "By Le Chatelier's principle (which says that a system undergoing a change with shift in equilibrium to minimise that change)...." etc.
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Re: Chemistry Question Thread
« Reply #354 on: June 22, 2016, 04:34:39 pm »
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Hey guys!! Why is the answer A and not B?Thanks so much !
« Last Edit: June 22, 2016, 05:01:54 pm by smiley2101 »

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Re: Chemistry Question Thread
« Reply #355 on: June 22, 2016, 05:13:42 pm »
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In the reaction, 2 moles of H2 react with 1 mole of CO.

This means that the moles of hydrogen gas are used up at TWICE the rate the moles of CO are.

Hence the dotted curve represents the concentration of H2 and the blocked curve represents that of CO, because the concentration of H2 should be lower if it goes down more quickly.

Because the dotted curve is the one that spikes up, we know that H2 was what was added. Not CO.

anotherworld2b

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Re: Chemistry Question Thread
« Reply #356 on: June 24, 2016, 08:48:31 pm »
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Hello :)
I was wondering could I ask for help in regards to writing a scientific report (investigation) on molar mass?
I wasn't sure if I could ask this here :/

Design an investigation to determine the composition and molar mass of an unknown white substance
I dissolved the unknown bicarbonate (given info its a bicarbonate) with hydrochloric acid then boiled the substance
using a Bunsen burner to isolate the element. I later used a flame test to identify the substance as sodium bicarbonate

I wasn't sure what to put in the introduction of my report and the depth and what I would need to write to explain the concepts used
I am also confused about how to determine the composition and molar mass of the sodium bicarbonate
I tried to calculate the mass and mole of substances but I'm not sure if I am approaching it properly.
HELP is greatly appreciated TT ^ TT

1. Mass of tube + NaHCO3
 
51.6g
 

2. Mass of tube
 
47.6g
 

3. Mass of NaHCO3
 
4g
 

4. Moles of NaHCO3
 
0.0459 gmol -1
 

5. Mass of tube + NaCl
 
50.7g
 

6. Mass of NaCl
 
3.1g
 

7. Moles of NaCl
 
0.0530 gmol -1
 

Mass of heating basin + water
 
71.5g
 

Mass of water
 
20.8g
 

Mass of carbon dioxide
 
23.9g
 

Ratio: Moles NaCl /Moles NaHCO3
 
0.0530 : 0.0459
 


Mass of NaCo3 = 51.6 - 47.6 = 4g (used in experiment)

Mass of NaHCO3 = 22.98977 + 1.00794 + 12.0107 + (15.9994x3)

                             = 22.98977 + 1.00794 + 12.0107 + 47.9982

                             = 84.00661

Mole of NaHCO3 = 4/ 84.00661 = 0.0459

« Last Edit: June 25, 2016, 01:09:20 pm by anotherworld2b »

jakesilove

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Re: Chemistry Question Thread
« Reply #357 on: June 25, 2016, 02:08:10 pm »
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Hello :)
I was wondering could I ask for help in regards to writing a scientific report (investigation) on molar mass?
I wasn't sure if I could ask this here :/

Design an investigation to determine the composition and molar mass of an unknown white substance
I dissolved the unknown bicarbonate (given info its a bicarbonate) with hydrochloric acid then boiled the substance
using a Bunsen burner to isolate the element. I later used a flame test to identify the substance as sodium bicarbonate

I wasn't sure what to put in the introduction of my report and the depth and what I would need to write to explain the concepts used
I am also confused about how to determine the composition and molar mass of the sodium bicarbonate
I tried to calculate the mass and mole of substances but I'm not sure if I am approaching it properly.
HELP is greatly appreciated TT ^ TT

1. Mass of tube + NaHCO3
 
51.6g
 

2. Mass of tube
 
47.6g
 

3. Mass of NaHCO3
 
4g
 

4. Moles of NaHCO3
 
0.0459 gmol -1
 

5. Mass of tube + NaCl
 
50.7g
 

6. Mass of NaCl
 
3.1g
 

7. Moles of NaCl
 
0.0530 gmol -1
 

Mass of heating basin + water
 
71.5g
 

Mass of water
 
20.8g
 

Mass of carbon dioxide
 
23.9g
 

Ratio: Moles NaCl /Moles NaHCO3
 
0.0530 : 0.0459
 


Mass of NaCo3 = 51.6 - 47.6 = 4g (used in experiment)

Mass of NaHCO3 = 22.98977 + 1.00794 + 12.0107 + (15.9994x3)

                             = 22.98977 + 1.00794 + 12.0107 + 47.9982

                             = 84.00661

Mole of NaHCO3 = 4/ 84.00661 = 0.0459

Hey!

I have to admit, I'm not 100% sure what this investigation is actually asking you to do. Looks like you're having particular trouble with the introduction, depth, and explanation, so I'll just generally talk about that here. Hope it helps!

You don't really need an introduction for a scientific report. You want to have an aim, a hypothesis, equipment, a method, results (potentially calculations) and a discussion. Don't worry too much about having an introduction; just make sure to include the above listed features.

In terms of depth, just make sure your method includes every step that you used, including recording data, washing glassware, repetition etc. You don't really need to explain WHY you do anything until the discussion.

In the discussion, definitely talk about your method. Particularly, outline the solubility rules/flame tests that you knew to be true, and why you used them in this specific example. You basically want to go through accuracy, reliability and validity in every discussion. Talk about the flaws of the experiment, where sources of error lie, that kind of thing. This really comes down to your interpretation of the question, and I don't think I have enough information to properly give you advice for that.

Just make sure to go through the key components of a standard scientific paper. Make sure to include tables, graphs etc. with all the correct units, and make sure that your method is in past tense. Those sorts of things just make everything seem super professional. Your calculations seem right, but that's only a tiny part of an experimental report. Just as an example, check out the report someone else posted here!

Let me know if you have other specific questions; sorry that I have been extremely vague and general with my response, I'm just not sure what you have or haven't done yet.

Jake
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Re: Chemistry Question Thread
« Reply #358 on: June 25, 2016, 09:19:45 pm »
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Hey,
could you please help me?
I have a fuel cell test on Monday and I was just wondering if you had any idea what kind of questions they generally ask. It is on the hydrogen-oxygen and methanol fuel cell specifically :)

Also,
Would you happen to know if there are any variations in electrolytes used (if any) and chemical reasons for these variations?

I'd really appreciate any ideas
Thankyou :)
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Re: Chemistry Question Thread
« Reply #359 on: June 25, 2016, 11:23:18 pm »
+1
Hey,
could you please help me?
I have a fuel cell test on Monday and I was just wondering if you had any idea what kind of questions they generally ask. It is on the hydrogen-oxygen and methanol fuel cell specifically :)

Also,
Would you happen to know if there are any variations in electrolytes used (if any) and chemical reasons for these variations?

I'd really appreciate any ideas
Thankyou :)

Hey mq123! Thanks for asking (it's been a long time since I lasted posted in the forums)

I didnt really do fuel cell for my secondary investigation on batteries, I will try my best to help you here but if you dont seem to benefit from it Im extremely sorry and I hope someone more professional can be here to provide better advices.

In direct methanol fuel cell:
- Fundamental mechanism involves the exchange of protons
- Main fuel being methanol, which offers the advantage of being portable, energy-dense and chemically stable under normal temperature and pressure
- Methanol is reacted to hydrogen through steam reforming
- Common operation temperature 50 - 120 degrees celsius
- Know about how methanol cross-over reduces battery efficiency
- CO2 produces at the anode, negatively impacting the environment
- Anode equation: CH3OH + H2O --> CO2+6H[/sup]+[/sup] + 6e-
- Cathode equation: 6H+ + 6e- --> 3H2
- Overall cell equation: CH3OH + H2O --> CO2 + 3H2
- Know about the structure and purpose of: Oxygen flow field, anode/cathode backing, anode/cathode current collector
- Requires a continuous source of fuel and oxygen to sustain the chemical reaction, hence usually used in an ambient condition

In Hydrogen-Oxygen Cell:
- Anode equation: 2H2 --> 4H+ + 4e-
- Cathode equation: O2 + 4e- + 4H+ --> 2H2O
- Overall cell equation: 2H2 + O2 --> 2H2O
- Energy is released in the form of heat and electricity
- Unused H2 is circulated/reused for maximum efficiency
- Proton exchange reaction
- Know the purpose of proton exchange membrane
- Know the catalyst used at both the cathode and anode

Common exam questions will often involve:
- asking about the structure/composition of the battery
- asking about the chemistry behind the functioning of battery (I.e. need to state anode and cathode equations, overall equation and the working process that leads to redox reaction)
- asking about the impact of the battery on society and environment
- asking about the applications/practicality of the battery, advantages and disadvantages
- asking about cost and compare this cost to the more common dry cell batteries

Hope this helps! :)

Best regards
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