VCE Chemistry Question Thread

[dylan.kumar21]

Whats the difference between the validity and the limitations of an experiment?

[keltingmeith]

Hi all,

I'm a bit confused with Significant figure marks. Is it true you only lose one mark for a sig fig mistake on the exam? Also, if a question, for example, gave data with sig figs of 2 but then a subsequent question like 1b or 1c uses sig figs of 3 in that question, should I be using Sig figs of 2 or 3 in my answer?

This has been answered loads of times, so please forgive me for not going into full detail - we don't know that it's only one mark per paper, but it's highly likely that it is. VCAA themselves don't say it publicly, but most high school teachers do, and teachers do have private channels that we can't access.

As for the other question - you always use the smallest number of sig figs in your calculation. Example, question 1 is - "you have 1.0 M hydrochloric acid and neutralise 2.0L of it with 5L of sodium hydroxide"

1.a) How much HCl, in mol, is there?

This would be calculated as n=1.0*2.0=2.0. Even though the smallest number of sig figs in the question is 1 (the 5L), the smallest number of sig figs in THIS calculation is 2 (the 1.0 and the 2.0), and so you round the answer to 2.0.

1. b) What's the concentration of the NaOH in mol/L?

This time, you'd calculate as C=2.0/5=0.4 M. In this case, the smallest number of sig figs you used is 1, so you leave it at one sig fig.

Another example - question 2 is, "50.0kg of soil is treated with acid to remove metal contaminants. After treatment with excess acid and allowing the soil to dry, the soil now weighs 7 kg"

2.a) How much of the soil, in kg, was metal contaminants?

Okay, so to calculate this, the metal contaminants is whatever was gone - so that would 50.0-7=43 kg. Remember that addition/subtraction follow slightly different sig fig rules, and so we do it to the smallest number of decimal places.

2.b) After the acid is treated, it is found that all of the metal waste is copper. If all of the mass of the waste found in 2.a) is converted into solid copper, how much copper, in mol, do we have?

This one is calculated as (43*1000)/63.546=6.8*10^2 mol. In this case, we have two data points - one is two sig figs (the 43) and the other is 5 sig figs (the 63.546). So, we round to two sig figs. 43 is not in the initial set of data - and that's fine. You don't use the 50.0 or the 7 in this calculation, so they should not affect the sig figs in rounding. In fact, if we did, we'd have to round to a different number, because they have 3 and 1 significant figures (respectively). But since the 43 has two, and the 43 is what we used in the calculation, that's the one we pay attention to


Hopefully that all makes sense and clears up any confusion - ignore the sig figs being used in the question, and just focus on the amount of sig figs in the calculation - THOSE are the ones you use

Whats the difference between the validity and the limitations of an experiment?

Validity = is experiment telling you what you want to know. An example of an invalid experiment is eating grass while blindfolded to find out what colour the sky is. An example of a valid experiment is looking at the sky to find out what colour the sky is

Limitations = issues with your experiment that might mean you're not getting the full picture. A limitation of the previous experiment is what if you have vision problems, or what if the colour of the sky changes with time. To fix these limitations, you might ask other people what colour they'd call the sky instead of just observing it yourself, and you might ask at various times in the day/night

[55232Jeremy]

This has been answered loads of times, so please forgive me for not going into full detail - we don't know that it's only one mark per paper, but it's highly likely that it is. VCAA themselves don't say it publicly, but most high school teachers do, and teachers do have private channels that we can't access.

As for the other question - you always use the smallest number of sig figs in your calculation. Example, question 1 is - "you have 1.0 M hydrochloric acid and neutralise 2.0L of it with 5L of sodium hydroxide"

1.a) How much HCl, in mol, is there?

This would be calculated as n=1.0*2.0=2.0. Even though the smallest number of sig figs in the question is 1 (the 5L), the smallest number of sig figs in THIS calculation is 2 (the 1.0 and the 2.0), and so you round the answer to 2.0.

1. b) What's the concentration of the NaOH in mol/L?

This time, you'd calculate as C=2.0/5=0.4 M. In this case, the smallest number of sig figs you used is 1, so you leave it at one sig fig.

Another example - question 2 is, "50.0kg of soil is treated with acid to remove metal contaminants. After treatment with excess acid and allowing the soil to dry, the soil now weighs 7 kg"

2.a) How much of the soil, in kg, was metal contaminants?

Okay, so to calculate this, the metal contaminants is whatever was gone - so that would 50.0-7=43 kg. Remember that addition/subtraction follow slightly different sig fig rules, and so we do it to the smallest number of decimal places.

2.b) After the acid is treated, it is found that all of the metal waste is copper. If all of the mass of the waste found in 2.a) is converted into solid copper, how much copper, in mol, do we have?

This one is calculated as (43*1000)/63.546=6.8*10^2 mol. In this case, we have two data points - one is two sig figs (the 43) and the other is 5 sig figs (the 63.546). So, we round to two sig figs. 43 is not in the initial set of data - and that's fine. You don't use the 50.0 or the 7 in this calculation, so they should not affect the sig figs in rounding. In fact, if we did, we'd have to round to a different number, because they have 3 and 1 significant figures (respectively). But since the 43 has two, and the 43 is what we used in the calculation, that's the one we pay attention to


Hopefully that all makes sense and clears up any confusion - ignore the sig figs being used in the question, and just focus on the amount of sig figs in the calculation - THOSE are the ones you use


Validity = is experiment telling you what you want to know. An example of an invalid experiment is eating grass while blindfolded to find out what colour the sky is. An example of a valid experiment is looking at the sky to find out what colour the sky is

Limitations = issues with your experiment that might mean you're not getting the full picture. A limitation of the previous experiment is what if you have vision problems, or what if the colour of the sky changes with time. To fix these limitations, you might ask other people what colour they'd call the sky instead of just observing it yourself, and you might ask at various times in the day/night

Omg! that makes so much more sense!!! Thank you so much. 

[dylan.kumar21]

This has been answered loads of times, so please forgive me for not going into full detail - we don't know that it's only one mark per paper, but it's highly likely that it is. VCAA themselves don't say it publicly, but most high school teachers do, and teachers do have private channels that we can't access.

As for the other question - you always use the smallest number of sig figs in your calculation. Example, question 1 is - "you have 1.0 M hydrochloric acid and neutralise 2.0L of it with 5L of sodium hydroxide"

1.a) How much HCl, in mol, is there?

This would be calculated as n=1.0*2.0=2.0. Even though the smallest number of sig figs in the question is 1 (the 5L), the smallest number of sig figs in THIS calculation is 2 (the 1.0 and the 2.0), and so you round the answer to 2.0.

1. b) What's the concentration of the NaOH in mol/L?

This time, you'd calculate as C=2.0/5=0.4 M. In this case, the smallest number of sig figs you used is 1, so you leave it at one sig fig.

Another example - question 2 is, "50.0kg of soil is treated with acid to remove metal contaminants. After treatment with excess acid and allowing the soil to dry, the soil now weighs 7 kg"

2.a) How much of the soil, in kg, was metal contaminants?

Okay, so to calculate this, the metal contaminants is whatever was gone - so that would 50.0-7=43 kg. Remember that addition/subtraction follow slightly different sig fig rules, and so we do it to the smallest number of decimal places.

2.b) After the acid is treated, it is found that all of the metal waste is copper. If all of the mass of the waste found in 2.a) is converted into solid copper, how much copper, in mol, do we have?

This one is calculated as (43*1000)/63.546=6.8*10^2 mol. In this case, we have two data points - one is two sig figs (the 43) and the other is 5 sig figs (the 63.546). So, we round to two sig figs. 43 is not in the initial set of data - and that's fine. You don't use the 50.0 or the 7 in this calculation, so they should not affect the sig figs in rounding. In fact, if we did, we'd have to round to a different number, because they have 3 and 1 significant figures (respectively). But since the 43 has two, and the 43 is what we used in the calculation, that's the one we pay attention to


Hopefully that all makes sense and clears up any confusion - ignore the sig figs being used in the question, and just focus on the amount of sig figs in the calculation - THOSE are the ones you use


Validity = is experiment telling you what you want to know. An example of an invalid experiment is eating grass while blindfolded to find out what colour the sky is. An example of a valid experiment is looking at the sky to find out what colour the sky is

Limitations = issues with your experiment that might mean you're not getting the full picture. A limitation of the previous experiment is what if you have vision problems, or what if the colour of the sky changes with time. To fix these limitations, you might ask other people what colour they'd call the sky instead of just observing it yourself, and you might ask at various times in the day/night

What an absurd but effective example. Thank you

[Chocolatepistachio]

For this question
For the following molecules 2-bromo-4-methylhexane, 2,4-dimethylhexane and 2-bromo-4-ethylhexane what is the correct sequence of boiling points from highest to lowest

Wouldn’t it be
2-bromo-4-methylhexane, 2,4-dimethylhexane, 2-bromo-4-ethylhexane
Because more branching means less surface area and weaker dispersion forces, lower boiling point

[Owlbird83]

For this question
For the following molecules 2-bromo-4-methylhexane, 2,4-dimethylhexane and 2-bromo-4-ethylhexane what is the correct sequence of boiling points from highest to lowest

Wouldn’t it be
2-bromo-4-methylhexane, 2,4-dimethylhexane, 2-bromo-4-ethylhexane
Because more branching means less surface area and weaker dispersion forces, lower boiling point

I think you should swap the last two around. Remember that dipole-dipole bonding is a lot stronger than weak dispersion forces, which is why the molecule without bromine would have the weakest intermolecular forces => lowest boiling point.

[jnlfs2010]

I have a question from the 2019 VCAA chemistry exam.

For question MC 13, a question about flashpoint,

Why is flashpoint not higher than the boiling point? I'm currently confused.

[ActivationEnergy]

I have a question from the 2019 VCAA chemistry exam.

For question MC 13, a question about flashpoint,

Why is flashpoint not higher than the boiling point? I'm currently confused.

By definition, the flashpoint is the lowest temperature that a flammable material has enough vapour to ignite, whereas the boiling point is the temperature at which a substance boils and turns to vapour. Therefore, the flashpoint is going to be lower than the boiling point as it is the ability to actually ignite the substance.

[tigerclouds]

Hey guys, can someone please help me with the following questions I have from the 2019 NHT exam, I'd greatly appreciate it! (Sorry they're so many)

- question 6f: I don't understand the solutions from the examiner's report especially with regards to why a narrower tube or meniscus of a pipette increases accuracy, what do they mean by that? And on that note, does anyone know of a way I can improve my understanding of titration errors - like any good videos that explain it because I've tried to look online and I can't find anything.

- question 7a: why do we not include H2O in the Kc equation

- question 7bi: I interpreted it as the addition of H2O will cause the system to favour the reverse reaction so that it is consumed but the answers refer to opposing the dilution by favouring the side with more ions so is my justification incorrect?

- question 7bii: why do the answers talk about the rate of reaction when the question asks for the effect on the position of equilbirum? I spoke about LCP.

- question 8d: How did they use the chemical shift values to determine that there is an ethyl group? It doesn't match up with the values from the data book
 
- question 3cii and 9b: how do we know how reactive a metal is and if it's going to react violently and why is it bad for something to react with oxygen? Also for question 9b, why can't the electrolyte react with Na, isn't that the point?

[interessant]

Just wondering, is question 7b and 9 from the 2016 chemistry exam still relevant?

[keltingmeith]

Hey guys, can someone please help me with the following questions I have from the 2019 NHT exam, I'd greatly appreciate it! (Sorry they're so many)

- question 6f: I don't understand the solutions from the examiner's report especially with regards to why a narrower tube or meniscus of a pipette increases accuracy, what do they mean by that? And on that note, does anyone know of a way I can improve my understanding of titration errors - like any good videos that explain it because I've tried to look online and I can't find anything.

- question 7a: why do we not include H2O in the Kc equation

- question 7bi: I interpreted it as the addition of H2O will cause the system to favour the reverse reaction so that it is consumed but the answers refer to opposing the dilution by favouring the side with more ions so is my justification incorrect?

- question 7bii: why do the answers talk about the rate of reaction when the question asks for the effect on the position of equilbirum? I spoke about LCP.

- question 8d: How did they use the chemical shift values to determine that there is an ethyl group? It doesn't match up with the values from the data book
 
- question 3cii and 9b: how do we know how reactive a metal is and if it's going to react violently and why is it bad for something to react with oxygen? Also for question 9b, why can't the electrolyte react with Na, isn't that the point?

Some of these I can answer now, but do you think you can attach pictures of the questions to your post? Just so I have full context

Just wondering, is question 7b and 9 from the 2016 chemistry exam still relevant?

I'm assuming you mean section B? 7b is an artefact from an old study design - at the time, we had to specifically study aspirin especially, you would not be expected to in this study design because the synthesis is a little complicated!

Question 9 section A looks like it shouldn't be relevant, but question 9 in section B looks like something you should still be able to answer. It might look scary because it looks like it's talking about acids and bases, but you actually have all the tools and knowledge to be able to answer it!

[tigerclouds]

Some of these I can answer now, but do you think you can attach pictures of the questions to your post? Just so I have full context

Thank you for that! Because I'm referring to individual parts to questions and their answers, they won't make sense if I only take a picture of that part of the question so I think it'll be easier if I give you the link to the exam:
https://www.vcaa.vic.edu.au/Documents/exams/chemistry/2019/NHT/2019chem-nht-w.pdf

and this is the examiner's report: https://www.vcaa.vic.edu.au/Documents/exams/chemistry/2019/NHT/chemistrynht_examrep19.pdf

[interessant]

I'm assuming you mean section B? 7b is an artefact from an old study design - at the time, we had to specifically study aspirin especially, you would not be expected to in this study design because the synthesis is a little complicated!

Question 9 section A looks like it shouldn't be relevant, but question 9 in section B looks like something you should still be able to answer. It might look scary because it looks like it's talking about acids and bases, but you actually have all the tools and knowledge to be able to answer it!

Oh thank you so much!

[Coolgalbornin03Lo]

Hi guys, I’m back to Chem!

I was just wondering how come in 2017 exam MCQ2

Spoiler

How come the most number of cycles comes from the temperature that’s not too high, not too low and not too average? I understand that if it’s too high or low it damages the battery? Actually I don’t understand at all

And in MCQ 6 I don’t quite get option B I get 14,550? Is this hinting at not 100% efficiency or am I doing something wrong?

[ActivationEnergy]

Hi guys, I’m back to Chem!

I was just wondering how come in 2017 exam MCQ2

Spoiler

How come the most number of cycles comes from the temperature that’s not too high, not too low and not too average? I understand that if it’s too high or low it damages the battery? Actually I don’t understand at all

And in MCQ 6 I don’t quite get option B I get 14,550? Is this hinting at not 100% efficiency or am I doing something wrong?

Hello! For MCQ2, I would look at it as common sense. High temperatures damage the batteries and therefore limits the number of discharges it is able to undergo. A preferable temperature would be at room temperature as batteries are normally used in room temperature, thus option B is looking pretty good! We don't want the battery running at a low temperature because it will ruin its stability and won't be able to provide 100% efficiency. Thus option A is not convincing.

For MCQ7(which i think is what you mean instead of Q6), your calculations are correct, they have just rounded up their answer from 14550 to 14600.