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Author Topic: HSC Chemistry Question Thread  (Read 1040845 times)  Share 

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katherine123

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Re: Chemistry Question Thread
« Reply #375 on: July 06, 2016, 02:27:30 am »
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I did part i but not sure how to part ii)    what are the negative points?
i)
 1.add 150ml of water into measuring cylinder A and 100ml of water into measuring cylinder B
2.transfer 10ml of water using 10ml pipette from measuring cylinder A to B (forward process)
3.Transfer 5ml of water using 5ml pipette from measuring cylinder B to A (backward process)
4.Repeat step 2 and 3 (one transfer cycle) until the volume of water remain constant in both measuring cylinder (equilibrium)

ii)Information collected is highly valid given that it is a qualitative model of an equilibrium reaction. In this procedure, it can be seen that the process occurs 2 directions (water transferred from measuring cylinder A to B or B to A) which is similar to an equilibrium. It is also observed that the volume of water in both measuring cylinder remain constant after transferring, like an equilibrium  with no macroscopic changes. Moreover, no water is removed from the system which is similar to equilibrium which is a closed system

MysteryMarker

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Re: Chemistry Question Thread
« Reply #376 on: July 06, 2016, 11:53:35 am »
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Hey guys, just got a question on eutrophication and the methods used to determine the concentration of phosphates and nitrates/nitrogen.

The method for determining organic nitrogen within waterways is to convert it into ammonia. The nitrogen concentration is then found through acid-base titration. Is this in enough detail for exams or should i know about how they convert the nitrogen into ammonia? (heat in sulphuric acid and stuff.)

katherine123

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Re: Chemistry Question Thread
« Reply #377 on: July 06, 2016, 04:45:36 pm »
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im quite confused about the electrolysis of molten, concentrated and aqueous of NaCl   

what info does the polarity of electrode give?
« Last Edit: July 06, 2016, 04:53:56 pm by katherine123 »

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Re: Chemistry Question Thread
« Reply #378 on: July 06, 2016, 04:51:01 pm »
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Sometimes with examples in chemical equations, I get confused with the states. Like sometimes h2o it is in liquid state and sometimes it is in gaseous state. How do you know when it's in liquid state and when it's in a gaseous state (using h2o as an example)?

Thanks

RuiAce

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Re: Chemistry Question Thread
« Reply #379 on: July 06, 2016, 11:04:58 pm »
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I did part i but not sure how to part ii)    what are the negative points?
i)
 1.add 150ml of water into measuring cylinder A and 100ml of water into measuring cylinder B
2.transfer 10ml of water using 10ml pipette from measuring cylinder A to B (forward process)
3.Transfer 5ml of water using 5ml pipette from measuring cylinder B to A (backward process)
4.Repeat step 2 and 3 (one transfer cycle) until the volume of water remain constant in both measuring cylinder (equilibrium)

ii)Information collected is highly valid given that it is a qualitative model of an equilibrium reaction. In this procedure, it can be seen that the process occurs 2 directions (water transferred from measuring cylinder A to B or B to A) which is similar to an equilibrium. It is also observed that the volume of water in both measuring cylinder remain constant after transferring, like an equilibrium  with no macroscopic changes. Moreover, no water is removed from the system which is similar to equilibrium which is a closed system
Providing the question could help here, thanks. I am not sure where "negative points" comes from just by looking at the answer.
Hey guys, just got a question on eutrophication and the methods used to determine the concentration of phosphates and nitrates/nitrogen.

The method for determining organic nitrogen within waterways is to convert it into ammonia. The nitrogen concentration is then found through acid-base titration. Is this in enough detail for exams or should i know about how they convert the nitrogen into ammonia? (heat in sulphuric acid and stuff.)
Back when I did the course, all I memorised was "Kjeldahl method" and nothing about the specifics at all. It is all that I felt was necessary.
You may find more useful information from another person.
im quite confused about the electrolysis of molten, concentrated and aqueous of NaCl   

what info does the polarity of electrode give?
The polarity of the electrode determines which half-equation is occurring at the electrode. (Always remember that the polarity of the cathode/anode flip for an electrolytic cell; instead of a positive cathode and negative anode, we consider a negative cathode and positive anode.)

Recall the mnemonic RED-CAT AN-OX.
This means that the reduction occurs at the cathode, whereas the oxidation occurs at the anode.

We analyse three cases:

Case 1: Molten NaCl.
The only species present are Na+ and Cl-. This means that sodium ions will be reduced to sodium metal, and chloride ions will be oxidised to produce chloride gas.
+'ve anode: 2 Cl- -> Cl2(g) + 2e-
-'ve cathode: Na+ + e- -> Na(s)

Case 2: Aqueous solution of NaCl.
The two species present are as above, and H2O as well now. We use our data sheet to determine what is more likely to occur.

-'ve cathode: -2.71V is required to reduce sodium ions back to sodium. But -0.83V is required to reduce water into hydrogen gas and base (hydroxide ions). Clearly, it will take considerably less voltage for the latter equation to occur. Hence, provide water is present, that will ALWAYS be the reaction.
2 H2O(l) + 2 e- -> 2 OH- + H2(g)

+'ve cathode: -1.36V is required to oxidise chloride ions to chloride gas. -1.23V is required to oxidise water into acid (hydrogen ions) and oxygen gas. There is a voltage difference, however it's quite small, so we cannot say for sure which reaction will occur. We seperate up our cases:

Subcase 1: Concentrated aqueous solution (generally >2M, although some sources say >1M)
There is a good abundance of chloride ions present, hence they will be oxidised.
2 Cl- -> Cl2(g) + 2e-

Subcase 2: Dilute aqueous solution (generally <0.1M)
Not many chloride ions are present at all. The oxidation of water will be more dominant here.
2 H2O(l) -> 4 H+ + O2(g) + 4 e-

(Note: The electrolysis of dilute NaCl solution is basically the electrolysis of water.)

Sometimes with examples in chemical equations, I get confused with the states. Like sometimes h2o it is in liquid state and sometimes it is in gaseous state. How do you know when it's in liquid state and when it's in a gaseous state (using h2o as an example)?

Thanks

Whilst a bit of this will require your knowledge of chemistry (e.g. aluminium reacts specifically with STEAM), more often than not it should be implied as to what state water is in. You may choose to provide examples to voice your confusion, which I can walk through.

vm1997

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Re: Chemistry Question Thread
« Reply #380 on: July 07, 2016, 12:56:32 pm »
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Few questions on Water quality management

"Describe the tests for any factors that need to be specifically monitored in these areas/with these activities to ensure that we have safe drinking water" I can only name a couple of tests that need to be done for this.
Briefly, what tests can be done for this and what's the chemistry behind them?

"Identify the organisations that monitor the wwaterways in NSW and describe the types of chemistry that is used in the monitoring process"  I know a few suc as SAC, but WHAT TESTS DO THEY DO??

RuiAce

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Re: Chemistry Question Thread
« Reply #381 on: July 07, 2016, 06:35:42 pm »
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Few questions on Water quality management

"Describe the tests for any factors that need to be specifically monitored in these areas/with these activities to ensure that we have safe drinking water" I can only name a couple of tests that need to be done for this.
Briefly, what tests can be done for this and what's the chemistry behind them?

"Identify the organisations that monitor the wwaterways in NSW and describe the types of chemistry that is used in the monitoring process"  I know a few suc as SAC, but WHAT TESTS DO THEY DO??
The HSC expects you to know each of the following tests and how we can use it to determine overall water quality.

- Concentration of common ions: Easy, just use your cation/anion tests and gravimetric analysis.
- Total dissolved solids: This is just an arbitrary way of saying how much solid is there in the water, measured in ppm. We could use gravimetric analysis but it's not that good since the amount of solid present generally isn't much. So we use the fact that the things dissolved are ions and will conduct electricity - thus we use a conductivity meter
- Hardness: This measures the concentration of Mg2+ and Ca2+. We use EDTA titration here.
- Turbidity: The cloudiness of water (on a macroscopic level) is sort of how clear it is. We take measurements of turbidity using a "turbidity tube". We need to know how turbid the water is to assess the ease of aquatic plants in photosynthesizing as an example.
- Acidity: This is obvious. And we can use a pH probe where possible.
- Dissolved oxygen: This is how much oxygen there is in the water. We use Winkler titration to find out this value. Harmful anaerobic bacteria grow when DO is low.
- Biochemical oxygen demand: By definition, "Biochemical Oxygen Demand is the DO necessary completely breakdown organic matter in the water through aerobic bacteria." To measure BOD, we take a sample of water and record the initial DO. We then leave the sample in the dark for 5 days before taking another value. The difference is the BOD.

I 100% rote-learnt the catchment dot point though. You can find information on that (and also more information on the tests) here. https://www.scribd.com/doc/49281194/The-Student-s-Guide-to-HSC-Chemistry

ssarahj

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Re: Chemistry Question Thread
« Reply #382 on: July 08, 2016, 06:35:11 pm »
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You can find information on that (and also more information on the tests) here. https://www.scribd.com/doc/49281194/The-Student-s-Guide-to-HSC-Chemistry

cheers for the link to that guide, it looks super helpful!!!  :)
HSC 2016: SOR 2, Adv. English, Ext. 1 English, Chemistry, 2U Maths, Hospitality
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2017: B Nutrition & Dietetics @ University of Newcastle

katherine123

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Re: Chemistry Question Thread
« Reply #383 on: July 11, 2016, 11:26:57 pm »
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Providing the question could help here, thanks. I am not sure where "negative points" comes from just by looking at the answer.Back when I did the course, all I memorised was "Kjeldahl method" and nothing about the specifics at all. It is all that I felt was necessary.


forgot to add the pic

amandali

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Re: Chemistry Question Thread
« Reply #384 on: July 11, 2016, 11:55:23 pm »
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In this option you studied one natural product that was not a fossil fuel. Describe the issues associated with shrinking world supplies of this natural product, and evaluate progress being made to solve the problems identified. 7 marks
A natural product is one that is used with little or no modification. An example is raw rubber which is a naturally occurring addition polymer formed from the polymerisation of 2-methyl-1,3-butadiene (isoprene) and obtained from sap of rubber trees. Many issues arose when its supply started to shrink thus a replacement was required. In the early 20th century, the demand for rubber outstripped supply as rubber trees can only produce a certain amount of rubber each year. Rubber was needed for tyres for military vehicles thus its limited supply adversely affected countries’ performance at war. In WW2, Japan had control of rubber producing areas in South East Asia which threatened the supply of natural rubber in other countries. Another issue that caused the rising demand of rubber was due to the growing of automobile industry thus car manufacturers needed more and more rubber. In order to solve the limited supply of rubber, a first replacement for rubber was produced which is Styrene Butadiene rubber (SBR) formed from monomers of 1,3-butadiene and styrene. It is vulcanised with short Sulfur chains forming cross-links between polymer chains. It is more favourable than raw rubber as it has the improved properties ie. more durable, more resistant to chemical attack, and stronger. This progress made to increase supply of rubber has been effective as the demand for synthetic rubber as a total % of rubber is around 80%. Hence, allowing demand of rubber to be met while maintaining a low cost for production. However, synthetic rubber made from petrochemicals are non-biodegradable, therefore production from biodegradable polymers is taken into consideration.


since it says  "evaluate progress being made"   does the last sentence suffice the negative point of using synthetic rubber or is it okay to just focus on the positives

RuiAce

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Re: Chemistry Question Thread
« Reply #385 on: July 12, 2016, 08:57:24 am »
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forgot to add the pic
If by negative points you just mean how it might not be invalid, well one thing could be that it doesn't accurately display the rate at which equilibrium is achieved. Two weaker arguments may be that the model also only demonstrates how an equilibrium is achieved, but we don't know if the forward reaction is forcibly going twice as fast (or something) as the reverse reaction. Nor do we know if the final ratio of products to reactants is right.

In this option you studied one natural product that was not a fossil fuel. Describe the issues associated with shrinking world supplies of this natural product, and evaluate progress being made to solve the problems identified. 7 marks
A natural product is one that is used with little or no modification. An example is raw rubber which is a naturally occurring addition polymer formed from the polymerisation of 2-methyl-1,3-butadiene (isoprene) and obtained from sap of rubber trees. Many issues arose when its supply started to shrink thus a replacement was required. In the early 20th century, the demand for rubber outstripped supply as rubber trees can only produce a certain amount of rubber each year. Rubber was needed for tyres for military vehicles thus its limited supply adversely affected countries’ performance at war. In WW2, Japan had control of rubber producing areas in South East Asia which threatened the supply of natural rubber in other countries. Another issue that caused the rising demand of rubber was due to the growing of automobile industry thus car manufacturers needed more and more rubber. In order to solve the limited supply of rubber, a first replacement for rubber was produced which is Styrene Butadiene rubber (SBR) formed from monomers of 1,3-butadiene and styrene. It is vulcanised with short Sulfur chains forming cross-links between polymer chains. It is more favourable than raw rubber as it has the improved properties ie. more durable, more resistant to chemical attack, and stronger. This progress made to increase supply of rubber has been effective as the demand for synthetic rubber as a total % of rubber is around 80%. Hence, allowing demand of rubber to be met while maintaining a low cost for production. However, synthetic rubber made from petrochemicals are non-biodegradable, therefore production from biodegradable polymers is taken into consideration.


since it says  "evaluate progress being made"   does the last sentence suffice the negative point of using synthetic rubber or is it okay to just focus on the positives

Yeah. From memory when I wrote about rubber, with the negatives of SBR I just wrote about how it's still being made out of petrochemicals, and why that's bad (which is something you should know from the production of materials topic).

These are not my notes, but basically they were all I memorised for section 1 of industrial chemistry.

(One thing that you will find in these notes at the very end is how progress is being made to develop an alternative to styrene-butadiene rubber, but made out of biomass. This is an example of even further progress to counter the drawbacks of SBR, and may or may not be worth mentioning in your 7-mark response. At this point though, because I'm too out of shape I won't critique your response for now, sorry.)
« Last Edit: July 12, 2016, 09:00:58 am by RuiAce »

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Re: Chemistry Question Thread
« Reply #386 on: July 12, 2016, 10:17:41 am »
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Could someone here please explain to me the important concepts in the metals module (i.e. Moles, spectator ions, half equations, limiting and excess, ionic equations, etc) I struggle to understand the maths behind this and require a lot of help (it'd be great if you could provide examples as well)

CHeers

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Re: Chemistry Question Thread
« Reply #387 on: July 12, 2016, 07:59:02 pm »
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I tried to balance question e but im not sure why i got it wrong

« Last Edit: July 12, 2016, 08:01:08 pm by anotherworld2b »

RuiAce

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Re: Chemistry Question Thread
« Reply #388 on: July 12, 2016, 08:28:50 pm »
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I tried to balance question e but im not sure why i got it wrong
Line 2 is correct.

C3H8(g) + 5 O2(g) -> 3 CO2(g) + 4 H2O(g)

You can use WolframAlpha to show you the answer to balancing out the equations. (Just make sure to use an equal sign.)

Could someone here please explain to me the important concepts in the metals module (i.e. Moles, spectator ions, half equations, limiting and excess, ionic equations, etc) I struggle to understand the maths behind this and require a lot of help (it'd be great if you could provide examples as well)

CHeers
Most of this stuff is quite fundamental to chemistry. Make sure you have read through your textbook carefully enough.

In SI base units, the measurement of the mole is used to measure the quantity of particles that exist. This is opposed to the mass, which measures how much matter there IS in a particle.

1 mol is defined as the number of carbon atoms in exactly 12 grams of carbon solid. The correct value is approximately 6.022 * 1023 which you can find on your data sheet. This number is famously known as Avogadro's number.

NA =  6.022 * 1023 mol-1

This leads to the fundamental relationship:

where n = moles of a substance (measured in mol)
N = the actual number of atoms there are
NA = Avogadro's number

A number's quantity of particles is directly linked to the mass of the substance present. The relationship is given as


where:

n - moles of a substance present (measured in mol)
m - the mass of a substance given (measured in g for the sake of chemistry)
M - molar mass (measured in g mol-1)

By definition, the molar mass is just the mass divided by the quantity present. This is essentially the above formula rearranged (M=n/m). For historical reasons, it has units g mol-1 instead of kg mol-1.

However, every individual element has a different atomic number and atomic mass. The molar mass will differ depending on what species we have.

The number the molar mass (for monatomic elements) is just its atomic mass. (There is a conversion between atomic mass and molar mass but this we will ignore). Hence, the molar mass for, say, carbon, is 12.01 g mol-1. The molar mass of O is 16.00 g mol-1.

The molar mass of compounds is just that of the individual elements added together. E.g. the molar mass of PbSO4 equals
M(Pb) + M(S) + 4 * M(O)
= 207.2 + 32.07 + 4*16
= 303.27 g mol-1

As for the rest? Asking for all that in the one go is THOROUGHLY exhaustive and you will need to provide questions to set a basis. In the future, please also ask one or two questions (especially if they're too related to the same thing) at a time. Just like with the trigonometry questions - all of those at once was quite overloading.
« Last Edit: July 12, 2016, 08:37:48 pm by RuiAce »

Sahar8642

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Re: Chemistry Question Thread
« Reply #389 on: July 12, 2016, 08:46:29 pm »
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Hey,
Need Help for this q.
Its worth 5 marks
A sample of lemon juice is to be analysed in the laboratory. A student took 25.00 mL of the juice
and diluted it to 250.00 mL. Exactly 25.00 mL of the diluted lemon juice is titrated with standard
0.1045 mol L-1
 sodium hydroxide solution using phenolphthalein as the indicator. An average titre
 of 24.05 mL of sodium hydroxide was required
Assuming that the lemon juice contained only citric acid (molar mass = 192.1 g/mol), calculate
the concentration in mol L-1 of citric acid in the undiluted lemon juice.

Thank You!