VCE Chemistry Question Thread

[daniel.h]

VCAA 2012 Exam 1 Q1bi): The examiner's report says The presence of an extra C in the structure of the triglyceride meant that there was a valid argument for only two CH3(CH2)16COOH, which was also accepted
What extra C are they talking about??

[-_-zzz]

"A standard solution of sodium carbonate is prepared by dissolving 7.95 grams in water and making the solution up to 250ml in a volumetric flask. A nitric acid solution is standardised by titrating 25.00mL aliquots of the acid with the standard sodium carbonate solution. An average titre of 18.42mL is obtained."

Calculate the concentration of the nitric acid.

Equation: 2HNO₃ +Na₂CO₃ --> 2NaNO₃ +H₂O +CO₂

What are the steps to answering this?

Hey there,

The first step is to find the concentration of the standard solution. So you know that you have 7.95 g of sodium carbonate. You can divide this figure by the molar mass of sodium carbonate to get the moles of sodium carbonate. Once you have the number of moles, you can divide it by 0.250L to get the molar concentration of the sodium carbonate solution. After the concentration has been determined it means that you can find the moles of sodium carbonate in the average titre. In this case you would convert 18.42 mL to 0.01842 L and multiply this figure by the concentration of your standard solution in order to determine the amount of moles of sodium carbonate that was required to reach the end point of the titration. In the equation provided, we know that every 1 mole of sodium carbonate will react with 2 moles of nitric acid. Therefore the number of moles of nitric acid present within the 25.00 mL aliquot will be equal to the number of moles of sodium carbonate multiplied by 2. You now have the moles of nitric acid and the volume of the aliquot. Convert the volume (25.00 mL) to litres and hence determine the concentration of the nitric acid.

Hope this was helpful!

[sweetcheeks]

VCAA 2012 Exam 1 Q1bi): The examiner's report says The presence of an extra C in the structure of the triglyceride meant that there was a valid argument for only two CH3(CH2)16COOH, which was also accepted
What extra C are they talking about??

By the sound of it, they possibly made a mistake on the original exam paper and one of the three fatty acid's was (CH2)17 rather than (CH2)16.

[hums_student]

Hey guys, are we expected to memorise which amino acids are essential? This came up on the 2018 NHT exam (Question 4 of short answer).
Also, are all the fatty acids in the data booklet essential?

Cheers.

[daniel.h]

By the sound of it, they possibly made a mistake on the original exam paper and one of the three fatty acid's was (CH2)17 rather than (CH2)16.

If there was an extra C in the triglyceride, wouldn't we need more than 3 CH3(CH2)16COOH?

With the normal triglyceride, we already need 3 CH3(CH2)16COOH

[daniel.h]

2013 VCAA Exam 2 Q25 MCQ: no reaction is observed and that's because of the slow reaction rate

2011 VCAA Exam 2 Q4a) short answer: reaction does not occur to any significant extent and this is because the strongest reductant and oxidant need to react

but doesn't "reaction does not occur to any significant extent" (mean that the reaction still happens but not to a significant extent, so slow rate would be the reason?

And how are those 2 questions different

[daniel.h]

for 2009 chem exam 2 q3ciii short answer, shouldnt the answer be to 3 sig figs

[Bell9565]

for 2009 chem exam 2 q3ciii short answer, shouldnt the answer be to 3 sig figs

I would think so...

[VanillaRice]

for 2009 chem exam 2 q3ciii short answer, shouldnt the answer be to 3 sig figs

When we perform addition or subtraction, we round to the lowest number of decimal places. Hope that helps

[daniel.h]

thank you! it'd still be to 3 sig figs though since there are 3 decimal places in 0.340?

do we look at sig figs or decimal places for × and ÷?

[Bell9565]

thank you! it'd still be to 3 sig figs though since there are 3 decimal places in 0.340?

do we look at sig figs or decimal places for × and ÷?

If you're multiplying, you answer to sig figs, adding or subtracting to decimal places.
Yet I was still under the impression you answer to the lowest number of significant figures in pieces of data given - if not someone please correct me as the exam is fast approaching.
Also, (I'm not being patronising or anything I'm now also genuinely confused and intrigued) why is n(CH2OH) to 4 sig figs (1.630mol) when it was derived from 2x0.815mol which you're multiplying therefore sig figs. Similarly, I know you use what you have in your calc but to get 0.815, you use data which is only sig figs. I may just be really really confused and over complicated it all but yah ... someone please help me too now 

[VanillaRice]

If you're multiplying, you answer to sig figs, adding or subtracting to decimal places.
Yet I was still under the impression you answer to the lowest number of significant figures in pieces of data given - if not someone please correct me as the exam is fast approaching.
Also, (I'm not being patronising or anything I'm now also genuinely confused and intrigued) why is n(CH2OH) to 4 sig figs (1.630mol) when it was derived from 2x0.815mol which you're multiplying therefore sig figs. Similarly, I know you use what you have in your calc but to get 0.815, you use data which is only sig figs. I may just be really really confused and over complicated it all but yah ... someone please help me too now 

We generally don't round numbers in intermediate steps, so I don't think that 1.630 is actually a "rounded answer". That is, we work to as many significant figures as are necessary until we are required to round, which is when we reach the final answer. Since the final operation is an addition, we round to the least number of decimal places (which is 3).
I could be wrong, so feel free to correct me

[hums_student]

Jumping on board the VCE Sig-figs Question Thread, if we were to use figures from the data book provided that has a lower number of sig figs than the sig figs in the question given, do we stick to the question or use the sig figs in the data book?

For example, in the question the lowest number of sig figs is 3, however the calculations require the molar mass of hydrogen which is given in the data book as 1.0 -- 2 sig figs. Would we leave the answer in 2 or 3 sig figs?

I've asked teachers and have gotten some very inconsistent responses.

[Bri MT]

Jumping on board the VCE Sig-figs Question Thread, if we were to use figures from the data book provided that has a lower number of sig figs than the sig figs in the question given, do we stick to the question or use the sig figs in the data book?

For example, in the question the lowest number of sig figs is 3, however the calculations require the molar mass of hydrogen which is given in the data book as 1.0 -- 2 sig figs. Would we leave the answer in 2 or 3 sig figs?

I've asked teachers and have gotten some very inconsistent responses.

I'm fairly sure in the past they've accepted both due to the ambiguity

[Lear]

According to my teacher we are to assume data booklet values are accurate to an infinite decimal places. I.e 1.0 literally means 1.0 and hence using this does not reduce the accuracy of our answer unlike using measured figures to a certain number of significant figures.