VCE Chemistry Question Thread

[steenasonson21]

We know the number of mols by n = cv & we want to make sure we have accurate info on these so we can do our calculations with the right numbers.

Rinsing with distilled water means we don't add any extra solution but we might dilute the solution going in that glassware i.e. it could mean the real concentration is lower than we think it is.

Rinsing with the solution that will go in the glassware means we won't dilute it but we might add in some extra solution into the glassware.


In the burette you know what the volume you're giving is from reading the difference between your markings so it doesn't matter if there's a bit of extra solution in there - so long as you read the marks on the burette properly and stopped at the equivalence point you'd get the right volume. This means that if we rinse with the solution we're going to run through the burette that's not going to mess us our volume. We also know that rinsing with the solution won't mess up the concentration. Therefore, we should rinse the burette with the solution.

Ok, how about if we rinsed with water? We wouldn't be adding in any extra volume of our solution but we would be diluting it. This would mean that when we do the calculation n = cv, mols in a titre = average titre * concentration calculation we would overestimate the number of mols delivered in a titre compared to what it actually was. This is why we don't want to rinse the burette with water.


Ok, how about in the conical flask then? We already know the volume we'll use for our calculation from measuring the solution going into the flask.  What we really want to avoid is adding in any more mols. We don't care as much if we add in more water since we already have our volume measurement and the water won't add any more mols of our solution. This makes water a good choice for rinsing with. Again, we don't want to rinse with our solution here because we don't want to accidentally add any more mols of it. If we did that, we would need a larger titre from the burette than we should to reach the equivalence point.

Does this make sense so far?

Yes, it makes so much more sense...thankyou so much ! Any tips for these kind of questions, it's so hard for me to know what each of the components are without having a hands on experience this year 

[steenasonson21]

I had a couple of random questions that just popped up during my revision:
- What is a primary standard and how is it used in titrations. Apparently NaOH is not a suitable primary standard due to several reasons, but in many questions, I see NaOH titrated against HCL...is this talking about something different?
- When the temperature is increased, does the Kc value increase as well?
- What are amino acid residues, are they just the part of the amino acid in the peptide chain?
- Just to double check - a glycerol and 3 fatty acid molecules join via condensation reaction to form a triglyceride, which then reacts with methanol ( or is it ethanol? ) to produce glycerol again and 3 methyl esters which are the 3 biodiesel molecules (fatty acid methyl esters?)
- Do we need to know a lot of detail about equilibrium in terms of haemoglobin ie. the reactions?
- Do anions migrate to the anode in electrolysis, despite the anode having a positive charge? In galvanic cells, the anions migrate to the anode right?
- What does the concentration- time graph look like when pressure is increased/ decreased but both sides have equal amount of moles?

[Coolgalbornin03Lo]

I had a couple of random questions that just popped up during my revision:
- What is a primary standard and how is it used in titrations. Apparently NaOH is not a suitable primary standard due to several reasons, but in many questions, I see NaOH titrated against HCL...is this talking about something different?
- When the temperature is increased, does the Kc value increase as well?
- What are amino acid residues, are they just the part of the amino acid in the peptide chain?
- Just to double check - a glycerol and 3 fatty acid molecules join via condensation reaction to form a triglyceride, which then reacts with methanol ( or is it ethanol? ) to produce glycerol again and 3 methyl esters which are the 3 biodiesel molecules (fatty acid methyl esters?)
- Do we need to know a lot of detail about equilibrium in terms of haemoglobin ie. the reactions?
- Do anions migrate to the anode in electrolysis, despite the anode having a positive charge? In galvanic cells, the anions migrate to the anode right?
- What does the concentration- time graph look like when pressure is increased/ decreased but both sides have equal amount of moles?

1. A primary standard is something with a known molar mass, and is soluble which is diluted to make up your known solution in titration. I think NaOH is used in acid-bass titrations so maybe it’s different there? I’m not sure someone please feel free to jump in for this question!!!

2. It depends on whether the reaction is endothermic or exothermic. If it is endothermic Kc increases however if it is exothermic Kc decreases. The reason for this is because Kc is a ratio of products to reactants and if temperature is increased in an endothermic reaction the net forward reaction is favoured so the products increase and therefore you get a bigger fraction. The opposite is true for exothermic reactions when temperature is increased as endothermic is favoured.

3. Yes, they are the individual amino acids in the polypeptide chain.

4. Correct except it can be any small molecule so both ethanol and methanol are correct. The formation of biodiesel is tranesterification but I’m not sure whether trygliceride formation can be called esterfiication? Someone feel freee to correct me

5. No just no that the equillibrium constant for the harmoglobin equation is quite large which forces the forward reaction hence why small amounts of CO cause poisoning. I can do this more in detail if you like and actually explain. But it’s I the textbook

6. Ooh I’m actually not sure about this one someone please answer! I was about to say anions migrate to the cathode in electrolysis but I think I’m wrong....

7. If the pressure is double for example all species concentration will half (as pressure is inversely proportional to volume). And if both sides have the same amount of particles the reaction will remain At equillibrium. The system will not try to partially oppose the change.

[Bri MT]

Yes, it makes so much more sense...thankyou so much ! Any tips for these kind of questions, it's so hard for me to know what each of the components are without having a hands on experience this year 

No worries!

For titration questions it's really important to set out clear working so you don't get confused about what you are doing and you're showing the examiner each step. If lost about what to do, see if you can find the number of mols of something. For experimental design questions it's important to be able to understand the individual meaning of terms like accuracy, reliability etc. If you're unsure about these check out the advice for teachers link from VCAA's chem page.

I had a couple of random questions that just popped up during my revision:
- What is a primary standard and how is it used in titrations. Apparently NaOH is not a suitable primary standard due to several reasons, but in many questions, I see NaOH titrated against HCL...is this talking about something different?

In a titration you want to have something you know the concentration of which you can titrate against the thing you don't know the concentration of. The standard is what gives you the known concentration. There are particular properties it should have so you can use it in that role (e.g. if it reacts with water in the air then you won't be able to get a known concentration from it since some of the mass you measured out would be from the water)

Edit: and the key thing here is you wouldn't know how much of the mass was from water

[zoharreznik]

Hi guys,
Does anyone have any tips for answering those open-ended questions at the end of the exams? I have to force myself to finish early so I'll have time for them- I really need some help on how to do them!

[steenasonson21]

1. A primary standard is something with a known molar mass, and is soluble which is diluted to make up your known solution in titration. I think NaOH is used in acid-bass titrations so maybe it’s different there? I’m not sure someone please feel free to jump in for this question!!!

2. It depends on whether the reaction is endothermic or exothermic. If it is endothermic Kc increases however if it is exothermic Kc decreases. The reason for this is because Kc is a ratio of products to reactants and if temperature is increased in an endothermic reaction the net forward reaction is favoured so the products increase and therefore you get a bigger fraction. The opposite is true for exothermic reactions when temperature is increased as endothermic is favoured.

3. Yes, they are the individual amino acids in the polypeptide chain.

4. Correct except it can be any small molecule so both ethanol and methanol are correct. The formation of biodiesel is tranesterification but I’m not sure whether trygliceride formation can be called esterfiication? Someone feel freee to correct me

5. No just no that the equillibrium constant for the harmoglobin equation is quite large which forces the forward reaction hence why small amounts of CO cause poisoning. I can do this more in detail if you like and actually explain. But it’s I the textbook

6. Ooh I’m actually not sure about this one someone please answer! I was about to say anions migrate to the cathode in electrolysis but I think I’m wrong....

7. If the pressure is double for example all species concentration will half (as pressure is inversely proportional to volume). And if both sides have the same amount of particles the reaction will remain At equilibrium. The system will not try to partially oppose the change.

Thankyou so so much for this! Super helpful. Don't worry about point 5, ill reread that section on my textbook. All the best for Monday!

[steenasonson21]

Help please !

[Bri MT]

Help please !

Have you been able to do d?
Note that you're provided with c(Na2CO3) and v(Na2CO3)

For e) for each mol of Na2CO3, how many mol of citric acid would there be?

for f) you have n(citric acid) from e, and you have v(citric acid before dilution) from the question


If you're still a bit unsure please reply back with any thinking/working you have and what you're confused on

[steenasonson21]

Have you been able to do d?
Note that you're provided with c(Na2CO3) and v(Na2CO3)

For e) for each mol of Na2CO3, how many mol of citric acid would there be?

for f) you have n(citric acid) from e, and you have v(citric acid before dilution) from the question


If you're still a bit unsure please reply back with any thinking/working you have and what you're confused on

Hi! So basically was able to do up to part (d)
I'm not sure how to do parts (e) and (f) and how to find the moles of citric acid from sodium carbonate. How do you create the formula?
I found the mole of sodium carbonate as 9.4 x 10^-5mol
The molar mass of citric acid was also given as 192.0g/mol in the Q

[markmorstentine]

Hi! So basically was able to do up to part (d)
I'm not sure how to do parts (e) and (f) and how to find the moles of citric acid from sodium carbonate. How do you create the formula?
I found the mole of sodium carbonate as 9.4 x 10^-5mol
The molar mass of citric acid was also given as 192.0g/mol in the Q

I've done this question before, I'm pretty sure you don't need the equation but in the previous page you work out that the acid has 3 carboxyl groups and can thus donate 3H+ per molecule, thus for every one mole of the acid you need 3 moles of the base, so you can simply divide the moles of the base by 3

[tigerclouds]

Hey guys,
Can someone please help me with the following questions from the 2019 exam?
- With question 3, is the graph on the examiners report drawn in correct proportions? I drew the top and bottom curves as changing by two lines rather than 1
- With question 5a, why do we use the 100 mL instead of the 5mL in the last calculations if that's what the question is asking for?
- With question 9g, why is Cl2 more likely to be produced when the concentration of NaCl increases?

[steenasonson21]

I've done this question before, I'm pretty sure you don't need the equation but in the previous page you work out that the acid has 3 carboxyl groups and can thus donate 3H+ per molecule, thus for every one mole of the acid you need 3 moles of the base, so you can simply divide the moles of the base by 3

Wow! That was so simple ... thanks a lot. However, how would you take the dilation into account?

I'm also assuming you've done the QATS 2020 exam then - so for question 2, have you thought about why the answer can't also be butyl methanoate or methyl butanoate?

[steenasonson21]

Can someone give me some advice in regard to efficiency questions? How do you know when to divide or multiply by the efficiency...likewise with dilation calculations as well

[markmorstentine]

Can someone give me some advice in regard to efficiency questions? How do you know when to divide or multiply by the efficiency...likewise with dilation calculations as well

- With efficiency questions, if it says its at e.g. 40% efficiency, if it asks for the mass required to produce x amount of energy, once the mass has been calculated, simply divide by 40 then times by 100 to get the actual mass required, due to the system only being 40% efficient, that means more mass would be required then at 100% efficiency.
- With dilution questions they usually follow the process of
- original sample  (e.g. 10ml) is diluted to (e.g.100ml) , sometimes they say they take a sample of 20 mls from the 100 but this doesn't matter cause the new concentration is still the same. Usually you will use molar concentration calculations to find the concentration of the 100ml sample, this may be from a titration or even graph. Then simply always remember to use the equation C1V1=C2V2, sub the found conc in for C2 and 10ml volume for C1 and 100ml volume for C2, and the concentration of the original undiluted sample can be found

Hope this helps

[markmorstentine]

Wow! That was so simple ... thanks a lot. However, how would you take the dilation into account?

I'm also assuming you've done the QATS 2020 exam then - so for question 2, have you thought about why the answer can't also be butyl methanoate or methyl butanoate?

Also the answer for this one is butyl methanoate, can't be methyl butanoate as it says there there is one singlet in a proton NMR, and methyl butanoate would not result in a singlet