HSC Chemistry Question Thread

[bsdfjnlkasn]

Yep, add whatever acid you'd like. No need to go into details re why you're adding the particular acid

Awesome thank you very much!

I was just wondering if you could please clarify whether the following (final) step in titration is necessary:

8. To ensure permanent colour change, the nozzle was sprayed from a wash bottle with water and the analyte observed for consistent colour

Why is this allowed? I've seen this in a couple places online and on videos but am convinced that this dilutes the primary standard meaning more volume is required to hit the endpoint. Doesn't this final step then make the whole procedure inaccurate as it suggests more volume of the titrant is needed to neutralise the primary standard?

Would love some clarity on this issue, here's a link that detailed this final step (https://www.youtube.com/watch?v=sFpFCPTDv2w from 5:55).

Thank you!!

[RuiAce]

Can someone please explain how to figure this equilibrium question, I'm getting different answers to the back of the book.

I've found that drawing arrows may help.

[jakesilove]

Awesome thank you very much!

I was just wondering if you could please clarify whether the following (final) step in titration is necessary:

8. To ensure permanent colour change, the nozzle was sprayed from a wash bottle with water and the analyte observed for consistent colour

Why is this allowed? I've seen this in a couple places online and on videos but am convinced that this dilutes the primary standard meaning more volume is required to hit the endpoint. Doesn't this final step then make the whole procedure inaccurate as it suggests more volume of the titrant is needed to neutralise the primary standard?

Would love some clarity on this issue, here's a link that detailed this final step (https://www.youtube.com/watch?v=sFpFCPTDv2w from 5:55).

Thank you!!

Diluting whatever is in the conical flask makes absolutely no difference to the experiment. Remember, once we've put stuff into the flask, we no longer care about concentration. Rather, we care about the number of moles/atoms in that conical flask. You can add as much water as you'd like, and it won't affect the overall calculation. Essentially, we only use concentration so that we can put a set number of moles into the flask. Once we've done this, concentration no longer becomes important!

Still, I don't rate the first half of that last step. No need to wash the nozzle with a wash bottle. However, you should wait 10 seconds to ensure the colour change is permanent (sometimes, the colour change reverts after a few seconds, and you have to add another drop!). As long as your method is consistent (eg. 10 seconds, 20, 30, 5) then you've got yourself a good experiment

[jakesilove]

(Image removed from quote.)I've found that drawing arrows may help.

Yeah, in my working out I totally ignored the original 0.1M of water! So, my answer is 'NEARLY' correct, you just need to add 0.1 to the concentration of water in the final solution

[RuiAce]

Yeah, in my working out I totally ignored the original 0.1M of water! So, my answer is 'NEARLY' correct, you just need to add 0.1 to the concentration of water in the final solution

What's the bet that the answer changes drastically with that 0.1 forgotten?

[jakesilove]

What's the bet that the answer changes drastically with that 0.1 forgotten?

Oh, only by like a factor of 10

[RuiAce]

Oh, only by like a factor of 10

Yeah lol. But that's still funny in an unfortunate way

[kiwiberry]

ok sweet i thought so - what would u suggest?
its core but it's a prac, i was just wondering if we make the solutions in ratio to the overall reaction equation - because will that effect the potential difference if it isnt? - or will one jsut run out faster(just realised this is probably the case)?

Just as a note, when you're using phenolphthalein, it's better to put the base in the burette and acid in the conical flask because it's easier to see the colour change from colourless --> light pink than dark pink --> light pink

Can someone please explain how to figure this equilibrium question, I'm getting different answers to the back of the book.

I like using an ice table for these type of questions

[bsdfjnlkasn]

Diluting whatever is in the conical flask makes absolutely no difference to the experiment. Remember, once we've put stuff into the flask, we no longer care about concentration. Rather, we care about the number of moles/atoms in that conical flask. You can add as much water as you'd like, and it won't affect the overall calculation. Essentially, we only use concentration so that we can put a set number of moles into the flask. Once we've done this, concentration no longer becomes important!

Still, I don't rate the first half of that last step. No need to wash the nozzle with a wash bottle. However, you should wait 10 seconds to ensure the colour change is permanent (sometimes, the colour change reverts after a few seconds, and you have to add another drop!). As long as your method is consistent (eg. 10 seconds, 20, 30, 5) then you've got yourself a good experiment

Hey thanks again Jake!

Why wouldn't diluting the solution in the conical flask change the number of moles? Doesn't n = cv?
As you can probably tell I've got an assessment tomorrow and want to clarify lots of last minute things.
One of the outcomes we're being assessed is on our ability to evaluate the validity of our conclusions and I was wondering if you could give a general explanation of what this means as well as some points on validity for a titration experiment.

Thanks so much


EDIT: This is a reply I received regarding including the origin in AAS calibration curves:
"The line of best fit should only go through the origin if it is appropriate to do so.
Hope that helps."

So should I take that as confirmation only to include (0,0) if it is a specific data point given?

Finally, how should I account for the rough titration in my method for the titration?

[jakesilove]

Hey thanks again Jake!

Why wouldn't diluting the solution in the conical flask change the number of moles? Doesn't n = cv?
As you can probably tell I've got an assessment tomorrow and want to clarify lots of last minute things.
One of the outcomes we're being assessed is on our ability to evaluate the validity of our conclusions and I was wondering if you could give a general explanation of what this means as well as some points on validity for a titration experiment.

Thanks so much


EDIT: This is a reply I received regarding including the origin in AAS calibration curves:
"The line of best fit should only go through the origin if it is appropriate to do so.
Hope that helps."

So should I take that as confirmation only to include (0,0) if it is a specific data point given?

Finally, how should I account for the rough titration in my method for the titration?

Yes, n=CV. However, if you increase the volume of water, you're decreasing the concentration. If I put 10 tennis balls in a beaker (equivalent to moles of a substance), and add loads of water, there will still only be 10 tennis balls in the beaker. So, diluting the conical flask with water does not change the moles, just the concentration.

Validity asks the question: have you tested what you're trying to test? Specific to titration, this comes down to using correct washing techniques, the correct indicator, and the correct standard solution.

Yep, don't use (0,0) unless that was a measurement actually taken.

In your method, just say something like 'Allow _____ to drip from the Burrette in to the Conical flask until a colour change is detected. Record the change in volume, and use this is a 'rough' run-through, from which future 'precise' experiments can be estimated'

[bsdfjnlkasn]

Hey there,

What would happen if we rinsed the burette with the acid (if the base was supposed to go in there). Would a neutralisation reaction begin to occur in the burette increasing it's pH? Does that then reduce the volume required to neutralise the standard solution if it were put in the conical flask?

Is the standard solution often the base?

What difference would it make if we were to put base through a pipette designate to transfer the acid?

[jakesilove]

Hey there,

What would happen if we rinsed the burette with the acid (if the base was supposed to go in there). Would a neutralisation reaction begin to occur in the burette increasing it's pH? Does that then reduce the volume required to neutralise the standard solution if it were put in the conical flask?

Is the standard solution often the base?

What difference would it make if we were to put base through a pipette designate to transfer the acid?

If you rinse the burrette with the acid, and then put the base into it, then some of the base will be neutralised. This means that the concentration of the base would be lower than expected, and would therefore require MORE base to neutralise the acid in the conical flask. Therefore, your final calculations would show a LOWER concentration than the original, 'real' concentration.

Yes, the standard solution is usually NaOH. But it could really be anything.

Again, if you had already washed the pipette in Acid, and then you use it to transfer the Base, the Base will decrease in concentration. Same as above.

[beau77bro]

hey so what is the basic apparatus/ equipment used in titrations - i know generally, but im struggling with what they actually look like and what they are used for.

like what's the difference between a volumetric flask and a conical flask? and how much standard solution should we make up for a titration. expect a posted prac before this night ends as well as possibly a galvanic cell one or a soft drink one

thanks guys

also where do we use pipettes in titrations even?

how do we accurately move the solution to be tested to conical flask under the burrette? do we just rinse the measuring cylinder endlessly with distilled water because it doesnt effect the number of moles

endless questions omg

Mod edit: Merged posts. If you post multiple times, and you haven't received an answer, you can always modify your original post to add questions

[jakesilove]

hey so what is the basic apparatus/ equipment used in titrations - i know generally, but im struggling with what they actually look like and what they are used for.

like what's the difference between a volumetric flask and a conical flask? and how much standard solution should we make up for a titration. expect a posted prac before this night ends as well as possibly a galvanic cell one or a soft drink one

thanks guys

also where do we use pipettes in titrations even?

how do we accurately move the solution to be tested to conical flask under the burrette? do we just rinse the measuring cylinder endlessly with distilled water because it doesnt effect the number of moles

endless questions omg

Mod edit: Merged posts. If you post multiple times, and you haven't received an answer, you can always modify your original post to add questions

Glassware in titration:
Volumetric flask: used to bring your standard solution up to a set volume. ie. 250mL, 500mL etc.
Conical flask: Used to store a set quantity of some solution, below the burrette
Burrette: Used to store a quantity of some solution, and be able to measure the change in solution levels after titration
Pipette: Used to transfer solution from the volumetric flask to the conical flask.

You can make as much standard as you want. You'll generally use around 25mL of standard per titration: generally, you'll make about 250mL of standard solution.

You shouldn't be using a measuring cylinder in a titration. The only thing you can wash with only water (ie. not the solution going into it) is the conical flask.

I think that answers everything!

[beau77bro]

for titrations how should u move the solution being tested in the conical flask? like how do u fill it accurately? like will all conical flasks have measurements all down the side?

also how do we deal with diprotic acids when testing?? do we assume they go to completion, how would we assume they didnt? and what indicator? like i really have no clue what to do with that. so an explanation/ steps would be great (with calculations)