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Author Topic: HSC Chemistry Question Thread  (Read 1040546 times)  Share 

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Loki98

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Re: Chemistry Question Thread
« Reply #300 on: May 12, 2016, 05:20:52 pm »
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Hey,
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks

jakesilove

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Re: Chemistry Question Thread
« Reply #301 on: May 12, 2016, 06:12:04 pm »
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Hey,
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks

Hey!

I'm sure that the marking criteria will really only require you to know neutralisation reactions for a Titration prac like this. I wouldn't worry about other areas of Chemistry for that :)

I'll have to think a bit more about your error question, because those sound like all the important errors to me. The dotpoint must mean something else; I feel like the 'area(s) of the lab' part makes it a bit weird and ambiguous, and I don't really know what the suggest. Surely, somehow, it is talking in a more general sense about the actual physical features of the lab. Maybe thinks like the floor being at a slope, causing your reading on the Burrette to be skewed? Air pressure? Humidity? Light causing increased reaction time? Temperature? I really don't know what the suggest, because it feels like you nailed all the important things and we're just pulling at strings.

For validity, you want to talk about whether you actually measured what you were trying to measure. In real terms, that means you want to have controlled all of the variables in your experiment. Things like correctly washing your equipment, keeping the room at a constant temperature, the hydroscopicness of NaOH etc (all of which change the result of your experiment) fall into this category.

For accuracy, you want to talk about your measuring devices. How accurate was your scale, did you account of paralax error, how accurate are your eyes. Essentially, it asks to what extent your measured values are close to the real value.

I'm sorry I couldn't be more helpful for the substance of your question! Maybe ask your teacher to explain the dotpoint; you not understanding it isn't your fault, its theirs.

Jake
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Happy Physics Land

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Re: Chemistry Question Thread
« Reply #302 on: May 12, 2016, 06:43:56 pm »
+1
Hey,
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks

Hey Loki!

Reading your description, I'm assuming that you are conducting a titration involving acetic acid and NaOH? Ok, then let's get started!

When the teacher asks you for equations, you need to know 3 equations: (for convenience sake lm not gonna write in states, but you should write them in an exam)
1. The Balanced chemical equation (CH3COOH + NaOH ----> NaCH3COO + H2O)
2. The net ionic equation (CH3COO- + Na+ ----> NaCH3COO)
3. Hydrolysis equations of salt (CH3COO- + H2O ---> CH3COOH + OH-)

Equation 1 and 2 are pretty straight forwards. Equation 3 is also significant because it shows why the endpoint would be slightly basic. Common exam questions would ask you for whether the endpoint is basic, acid or neutral and would ask you to identify a suitable indicator. If your teacher wants to differentiate between band 5 students from band 6 students, they would ask you why the endpoint would be basic. In this case, referring to equation 3 from above, since hydroxide ion is given off, the endpoint would hence be basic.

With your second question identifying the possible errors in the experiment, I would personally have written a human error or equipment error (it's kinda weird how your teacher is completely rejecting these errors). But look if your teacher says none of these can be included, then we have to find some other flaws in the experiment. One thing you can perhaps talk about is impurities from the atmosphere entering the conical flask or the burette and this can cause slight differences. Another thing maybe carrying out titration at the wrong temperature, which would affect experimental outcome because some indicators are quote sensitive to surrounding temperature. I'm a little confused by the meaning of "area(s) of the lab", because Im not too sure whether your teacher is talking about the experiment or the physical places in the lab that would cause error. Can you please enquire the teacher and come back to us on that? That would be helpful.

With the validity and accuracy part its quite easy, I will list some things for you to consider and you can write it out in a formal manner:

Validity:
- the extent to which the experiment tests the hypothesis
- Is there only one independent variable involved? If yes, then the experiment is valid
- Does your methods answer the aim? If yes, then the experiment is valid
- Is there a very significant difference between your result and your hypothesis? If yes, then the experiment is invalid. (This may sound like a point for accuracy, but accuracy is only concerned with relatively small deviations from your hypothesis. A significant deviation would mean that you are doing something wrong)

Accuracy:
- Did you thoroughly clean your equipments? Did you do it for multiple times? If yes, accurate
- Are your instruments correctly calibrated? If yes, accurate
- Is your weighing instrument presenting a precise quantity? (i.e. the value it displays does not fluctuate) If yes, accurate
- Are you biased in your reading of the scale? If yes, inaccurate
- Is the scale on your pipette and burette correct? If yes, accurate
- Are you reading from the bottom of meniscus (i.e. parallax error)? If yes, accurate
Then talk about how to minimise the impact of these inaccuracies if they exist.

Hope my answer helped! Remember to come back to us after you have enquired the teacher about that second point! :D

Best Regards
Happy Physics Land



- Are all other variables controlled? If yes, then the experiment
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Happy Physics Land

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Re: Chemistry Question Thread
« Reply #303 on: May 12, 2016, 06:45:29 pm »
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Hey!

I'm sure that the marking criteria will really only require you to know neutralisation reactions for a Titration prac like this. I wouldn't worry about other areas of Chemistry for that :)

I'll have to think a bit more about your error question, because those sound like all the important errors to me. The dotpoint must mean something else; I feel like the 'area(s) of the lab' part makes it a bit weird and ambiguous, and I don't really know what the suggest. Surely, somehow, it is talking in a more general sense about the actual physical features of the lab. Maybe thinks like the floor being at a slope, causing your reading on the Burrette to be skewed? Air pressure? Humidity? Light causing increased reaction time? Temperature? I really don't know what the suggest, because it feels like you nailed all the important things and we're just pulling at strings.

For validity, you want to talk about whether you actually measured what you were trying to measure. In real terms, that means you want to have controlled all of the variables in your experiment. Things like correctly washing your equipment, keeping the room at a constant temperature, the hydroscopicness of NaOH etc (all of which change the result of your experiment) fall into this category.

For accuracy, you want to talk about your measuring devices. How accurate was your scale, did you account of paralax error, how accurate are your eyes. Essentially, it asks to what extent your measured values are close to the real value.

I'm sorry I couldn't be more helpful for the substance of your question! Maybe ask your teacher to explain the dotpoint; you not understanding it isn't your fault, its theirs.

Jake

Man the one time that I actually get to answer a chem question and you beat me to it
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RuiAce

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Re: Chemistry Question Thread
« Reply #304 on: May 12, 2016, 08:28:01 pm »
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Man the one time that I actually get to answer a chem question and you beat me to it

I'm just too overloaded with work right now or I'd address every chemistry question. For now, I'm focusing on my specialty (even though it's not my top interest).

cajama

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Re: Chemistry Question Thread
« Reply #305 on: May 12, 2016, 09:03:33 pm »
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Hi!
I'm not sure if this has been asked before but I do three sciences; Chemistry, Biology and Physics and I often get my scientists mixed up between the subjects (especially with Chemistry and Biology). What information of the scientist is sufficient for me to know?

jakesilove

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Re: Chemistry Question Thread
« Reply #306 on: May 12, 2016, 09:31:46 pm »
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Hi!
I'm not sure if this has been asked before but I do three sciences; Chemistry, Biology and Physics and I often get my scientists mixed up between the subjects (especially with Chemistry and Biology). What information of the scientist is sufficient for me to know?

Hey!

I can't really help you with Biology, but in terms of learning scientist information in general, you don't need to know very much detail. If it's an important dot-point, tested often (eg. Physicist in the Space section of the curriculum), then you should know at least some history, their contribution to the scientific field, and maybe some extra facts the throw in and sound smart. This applies similarly to the Haber process for Chemistry. When it comes to less important dotpoints, almost never tested (eg. that one weird one about a chemist at the start of Chemical Monitoring and Management), you don't need to worry too much. Know what sort of science they're involved with, maybe some chemical processes they use.

Overall, you don't need to know very much. Mainly, know context and contribution. There aren't many dotpoints requiring you to know about the contribution of specific scientists, so hopefully as you learn each one a bit better you won't get them as mixed up!

This all being said, there are some dotpoints that are person-related and require quite a bit of knowledge (as sometimes you can get a 5-7 marker on them). For instance, Westinghouse vs Edison. Basically just look through some past papers and decide how significant that person is; often, you won't need to know very much.

Jake
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cajama

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Re: Chemistry Question Thread
« Reply #307 on: May 12, 2016, 10:15:25 pm »
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Hey!

I can't really help you with Biology, but in terms of learning scientist information in general, you don't need to know very much detail. If it's an important dot-point, tested often (eg. Physicist in the Space section of the curriculum), then you should know at least some history, their contribution to the scientific field, and maybe some extra facts the throw in and sound smart. This applies similarly to the Haber process for Chemistry. When it comes to less important dotpoints, almost never tested (eg. that one weird one about a chemist at the start of Chemical Monitoring and Management), you don't need to worry too much. Know what sort of science they're involved with, maybe some chemical processes they use.

Overall, you don't need to know very much. Mainly, know context and contribution. There aren't many dotpoints requiring you to know about the contribution of specific scientists, so hopefully as you learn each one a bit better you won't get them as mixed up!

This all being said, there are some dotpoints that are person-related and require quite a bit of knowledge (as sometimes you can get a 5-7 marker on them). For instance, Westinghouse vs Edison. Basically just look through some past papers and decide how significant that person is; often, you won't need to know very much.

Jake

Ahh thanks!  ;D

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #308 on: May 12, 2016, 10:40:33 pm »
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Hi!
I'm not sure if this has been asked before but I do three sciences; Chemistry, Biology and Physics and I often get my scientists mixed up between the subjects (especially with Chemistry and Biology). What information of the scientist is sufficient for me to know?

Hey Cajama:

Adding onto Jake's point, in chemistry there is actually a syllabus dotpoint which demands for students to know about the works of different chemists and the importance of their collaboration. To satisfy this dotpoint you would need to be familiarised with the various fields of chemistry such as analytical chemists, industrial chemists, environmental chemists, and radiography chemists. It is also recommended that you look up the techniques each one would use in their individual specialties. For example, analytical chemists use the technique of AAS (Atomic Absorption Spectroscopy) to qualitatively detect the presence of lead in water. It is also recommended that you know the name of a chemist that works for a particular field (This actually came up in a HSC exam a few years ago) and you would be required to talk about the details of the works that relate to the chemist.
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Re: Chemistry Question Thread
« Reply #309 on: May 15, 2016, 12:16:37 pm »
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why is the answer B? thank you!!!!! :)

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Re: Chemistry Question Thread
« Reply #310 on: May 15, 2016, 03:41:05 pm »
+1
why is the answer B? thank you!!!!! :)

Using n=m/M, for every 1 gram of substance x we have y moles of it:

x is CO, y = 1/28.01 mol
x is CH4, y = 1/16.042 mol
x is C2H2, y = 1/26.036 mol
x is C2H6, y = 1/30.068 mol

So since the heat q released of reaction is just enthalpy change * moles:

CO: 1/28.01 * 233 = 8.318 kJ
CH4: 1/16.042 * 890 = 55.479 kJ
C2H2: 1/26.036 * 1300 = 49.93 kJ
C2H2: 1/30.068 * 1560 = 51.88 kJ

So the answer is B

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Re: Chemistry Question Thread
« Reply #311 on: May 16, 2016, 06:00:23 pm »
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(Hydrogen Iodide) there,

So in the acidic env. topic we were told that phosphoric acid is weak, nitric acid is strong etc. Is there a definite list of strong/weak acids/bases we need to know for the HSC course?

jakesilove

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Re: Chemistry Question Thread
« Reply #312 on: May 16, 2016, 06:56:04 pm »
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(Hydrogen Iodide) there,

So in the acidic env. topic we were told that phosphoric acid is weak, nitric acid is strong etc. Is there a definite list of strong/weak acids/bases we need to know for the HSC course?

Hey!

Yep there certainly is! The acids that you need to know are strong acids are Nitric acid, Sulfuric acid and Hydrochloric acid. You can assume that any other acid you are given is weak (there are very few strong acids!).

Similarly, when it comes to strong bases you only really need to know that Sodium Hydroxide is weak.

Hope that helps!
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Re: Chemistry Question Thread
« Reply #313 on: May 16, 2016, 09:01:07 pm »
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Hey!

Yep there certainly is! The acids that you need to know are strong acids are Nitric acid, Sulfuric acid and Hydrochloric acid. You can assume that any other acid you are given is weak (there are very few strong acids!).

Similarly, when it comes to strong bases you only really need to know that Sodium Hydroxide is weak.

Hope that helps!
Whoops.

@ProfLayton2000

Anyway, as Jake has pointed out, the main ones that you need to know are:

Strong acids:
HNO3
HCl
H2SO4

However, it should be quite obvious as to why HBr and HI are also strong. (What may be surprising instead, is that HF is in fact, weak. This occurs due to the fact fluorine is the most electronegative element.)

Strong bases are simple. Group I hydroxides and that is it.

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Re: Chemistry Question Thread
« Reply #314 on: May 16, 2016, 09:14:54 pm »
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Hey guys!

I was learning about coordinate covalent bonds today and I was wondering how you can tell which pairs form a product that does contain a coordinate covalent bond? Sorry if this seems simple but here's an example of a question:

Which of the following pairs form a product containing a coordinate covalent bond?
a) Ca and 2H+
b) H2O and H+
c) Ag+ and Cl-
d) NH4+ and OH-

Thank you! you're all amazing :D

Neutron