VCE Chemistry Question Thread

[Sine]

This may be a bit late to ask but better now than never. Can someone tell me everything I need to know about significant figures and how to know how many sig figs we need when answering a particular question thank you

In your working out try to keep to the number of sig figs required in the answer.

When multiplying and dividing your answer should have the same amount of sig figs as your least accurate piece of data used.

When adding and subtracting your answer should be do the same amount of decimal places as the data value used with the least number of decimal places.

When taking powers i.e 10^x your answer should be to the amount of significant figures as the value x has decimal places.

When taking logs of numbers i.e log(a) = b (base 10 log) the result b is to the same decimal places as to which a has significant figures.

On the exam only 1 mark is allocated to sig figs and it's pretty obvious which question it is

[sweetcheeks]

Erm,,, Are you guys sure? I did the 2011 Exam 2 this morning and: HCOOH was a liquid. 

(Future2k16: Do you know the answer? Is it supposed to be a l or aq?!)

On all exams of the current study design, the equations show them as aqueous not a liquid. It makes sense for them to be aqueous, due to the fact that pure liquids are meant to be counted as one, rather than have a concentration.

[Adequace]

http://m.imgur.com/a/fLhCC
Q3b) what's wrong with my working? The answer in the book is 63.8g
Q1) I guessed C which was correct, but why can't it be A or B as well?
Q2a) why is this false?
Thanks

[sweetcheeks]

http://m.imgur.com/a/fLhCC
Q3b) what's wrong with my working? The answer in the book is 63.8g
Q1) I guessed C which was correct, but why can't it be A or B as well?
Q2a) why is this false?
Thanks

You forgot to ratio the Na to the Cl2 when calculating limiting reagent. The Na will be limiting, as it only requires 0.543 mol of Cl2.

As for your second question. How would you be able to determine the amount of NaNO3 in a solution through gravimetric analysis. You can only determine salts that have an ion that can be precipitated. There is also the issue of more than one salt containing the same ion (e.g. NaCl and NaNO3)

[larissaaa_]

When multiplying and dividing your answer should have the same amount of sig figs as your least accurate piece of data used.

What do you mean by 'least accurate piece of data used'?

On the exam only 1 mark is allocated to sig figs and it's pretty obvious which question it is

So say in my working out I use completely wrong sig figs for all the questions, the most I can lose from the whole exam regarding to sig figs is 1 mark? They won't tell a mark off like every question for using the wrong decimals places?

[Elizawei]

What do you mean by 'least accurate piece of data used'?

So say in my working out I use completely wrong sig figs for all the questions, the most I can lose from the whole exam regarding to sig figs is 1 mark? They won't tell a mark off like every question for using the wrong decimals places?

Say for example the question goes like: 2.0g of HCl is reacted with 1.76g of blah and 3.065L of H2 gas is produced. In this case the least accurate piece of data is the 2.0g, as it has less sig figs You always answer the question with the smallest sig fig from the data used

Yeah, they only check one question for sig figs, and they don't take more than one mark off for sig figs Still it's a mark you want to have, but don't stress too much in the grand scheme of things.



also guys, if a question asks to list redox pairs in decreasing order of their reductive activity, is that listening them from top of the electrochemical series to the bottom? Does reductive activity refer to their ability to readily undergo reduction and thus their strength as an oxidant? 

[larissaaa_]

Say for example the question goes like: 2.0g of HCl is reacted with 1.76g of blah and 3.065L of H2 gas is produced. In this case the least accurate piece of data is the 2.0g, as it has less sig figs You always answer the question with the smallest sig fig from the data used

Yeah, they only check one question for sig figs, and they don't take more than one mark off for sig figs Still it's a mark you want to have, but don't stress too much in the grand scheme of things.

Okay I swear my teacher told me we answer according to the largest sig figs provided in the question ... Or maybe I was just half asleep  Thankyou heaps!! That makes me feel a lot better I suck with sigs figs as stupid as that sounds.

[Adequace]

You forgot to ratio the Na to the Cl2 when calculating limiting reagent. The Na will be limiting, as it only requires 0.543 mol of Cl2.

As for your second question. How would you be able to determine the amount of NaNO3 in a solution through gravimetric analysis. You can only determine salts that have an ion that can be precipitated. There is also the issue of more than one salt containing the same ion (e.g. NaCl and NaNO3)

Okay thanks. I still don't get how Na has 0.543 moles? Doesn the ratio mean that Na has double the amount of moles of Cl2?

[sweetcheeks]

Okay thanks. I still don't get how Na has 0.543 moles? Doesn the ratio mean that Na has double the amount of moles of Cl2?

What I meant to say is that for the total amount of Sodium you have(1.086 mole), you only need to use 0.543 moles of the chlorine gas. This is due to you requiring twice as much sodium for every chlorine in the reaction.

[HopefulLawStudent]

Does an increase in temperature/change in pH affect the secondary structure of a protein??

My textbook only mentioned tertiary.
So my teacher used to say no.
Then it popped up on a commercial exam
And the exam said both secondary and tertiary were affected.
And then my teacher checked and said that that was a thing.
So now I'm confused...

[sweetcheeks]

Does an increase in temperature/change in pH affect the secondary structure of a protein??

My textbook only mentioned tertiary.
So my teacher used to say no.
Then it popped up on a commercial exam
And the exam said both secondary and tertiary were affected.
And then my teacher checked and said that that was a thing.
So now I'm confused...

The secondary structure is held together through hydrogen bonding. It certainly can be disrupted by temperature. pH is a tricky area. I would say no, as pH changes affect the COO-/COOH and NH3+/NH2 groups on the side chain that form ionic bonds. There are only C=O and N-H bonds on the secondary structure which form hydrogen bonds, not ionic bonding, and these are not affected by pH.

[Elizawei]

The secondary structure is held together through hydrogen bonding. It certainly can be disrupted by temperature. pH is a tricky area. I would say no, as pH changes affect the COO-/COOH and NH3+/NH2 groups on the side chain that form ionic bonds. There are only C=O and N-H bonds on the secondary structure which form hydrogen bonds, not ionic bonding, and these are not affected by pH.

I've also heard that low pH ( the increased H+ ions) may interfere with the hydrogen bonds, by displacing the atoms within the hydrogen bond? (was taught this in bio last year, not sure about chem tho)

[vox nihili]

Does an increase in temperature/change in pH affect the secondary structure of a protein??

My textbook only mentioned tertiary.
So my teacher used to say no.
Then it popped up on a commercial exam
And the exam said both secondary and tertiary were affected.
And then my teacher checked and said that that was a thing.
So now I'm confused...

Both pH and temperature can change the secondary structure, yep. Elizawei's reasoning is right on the money. Secondary structure is stabilised by hydrogen bonds between the backbone carboxyl and amino groups in the protein. If you increase the number of protons in a solution, these bonds are disrupted.

[MandhreeE]

Hey guys!
I was wondering if someone could clarify something for me. When we write an equilibrium expression we only include reactants and products that are either in the gaseous state or aqueous state and NOT liquid or solid. Is this correct?  If so, could someone explain why in the VCAA 2010 exam 2 question 2. why the calculations use liquid lactic acid in the expression?

I hope what I'm saying makes sense.
Thank you!!
Hope everyone's exams are going well

[sweetcheeks]

Hey guys!
I was wondering if someone could clarify something for me. When we write an equilibrium expression we only include reactants and products that are either in the gaseous state or aqueous state and NOT liquid or solid. Is this correct?  If so, could someone explain why in the VCAA 2010 exam 2 question 2. why the calculations use liquid lactic acid in the expression?

I hope what I'm saying makes sense.
Thank you!!
Hope everyone's exams are going well

The lactic acid should be in an aqueous state, due to it being dissolved in water. It seems that in the previous study design, VCAA would state that acids added to water will be liquids. In the current study design exams depict them as aqueous.