HSC Chemistry Question Thread

[bsdfjnlkasn]

also, for titrations my teacher says to always put the base in the flask - im not sure why though?

is it to do with the base reacting with the glass - wont that still happen in the flask? or maybe its to do with clogging the tap somehow? i really dont know

Hi hopefully this is of some help too

With titrations, you should always be putting the substance you're trying to find the concentration of in the burette as you will be able to obtain an exact volume and then deduce it's concentration (n = cv).

Perhaps you were always doing the same type of titration where you knew the base's concentration so put it in the volumetric flask to detect the colour change and so calculate the unknown acid concentration. But I think you should be approaching titrations a bit more open-mindedly and think about what you're trying to find out and how that affects the set up of the apparatus. It's got nothing to do with the base as a substance, don't worry it won't clog a burette up.  Titration is about using a known concentration of either a base or acid (being the standard solution or analyte when sitting in the flask) to deduce an unknown concentration (titrant being delivered by burette).

Let me know if you have any more questions

[beau77bro]

Hey there!

Here are some reasons for the discrepancies in read voltage and the theoretical value obtained through the standard potentials:

1. Not maintaining SLC is a major reason as we can't exactly ensure that the concentrations are all 1M (except always annotate this in your diagrams)
- Higher concentrations will increase the rate of reaction/current flow
- Lab conditions and concentrations are linked as if we were to increase the temperature of the room, then we make the solutions more concentrated (as water molecules would have evaporated)

2. Current flow is inhibited by resistance in wires (from external circuit)

3. There may be impurities on the electrodes so cleaning them will make it easier for the respective redox reactions to occur

4. Difficult to regulate the dispersion of ions in the salt bridge which also adds resistance
- If the ions haven't moved into their respective beakers, then we don't have a complete circuit preventing charge from flowing. But this doesn't stop the redox reactions from occurring, so the reactions go without powering anything because of the slow dispersion in the salt bridge. By the time there is a full circuit, the voltage would have decreased as there is less metal than what we started off with (as some have already reacted when we couldn't get an accurate reading)
NB: This is a really hazy explanation so would appreciate it if someone could step in and clarify things

Hopefully these helped

ahahha sorry to be a noob but whats SLC? thankyou for all the extra points (esp the temperature one and evaporation-never thought of that). also i think we agree about the salt bridge idea, but i kind of imagine it as the fact that 'you're only as strong as your weakest link' kinda dealio. so if the current in the wire can move through very efficiently (though there will be resistance like you said) then it will, however we have to preserve electrical neutrality throughout the circuit and maintain a closed circuit. so if the ions are moving slowly through, or at a decreased rate (due to small cross sectional area), then current can only move through the wire at a similarly limited rate - it is possible we r saying the same idea and have misunderstood. it is equally likely i am completely wrong ahahahha but hopefully more students can clarify. thanks bsdfjnlkasn

[beau77bro]

Hi hopefully this is of some help too

With titrations, you should always be putting the substance you're trying to find the concentration of in the burette as you will be able to obtain an exact volume and then deduce it's concentration (n = cv).

Perhaps you were always doing the same type of titration where you knew the base's concentration so put it in the volumetric flask to detect the colour change and so calculate the unknown acid concentration. But I think you should be approaching titrations a bit more open-mindedly and think about what you're trying to find out and how that affects the set up of the apparatus. It's got nothing to do with the base as a substance, don't worry it won't clog a burette up.  Titration is about using a known concentration of either a base or acid (being the standard solution or analyte when sitting in the flask) to deduce an unknown concentration (titrant being delivered by burette).

Let me know if you have any more questions

yea i totally agree, i thought the unknown 'titrant' was placed in the burrette always. but my teacher said there was an issue due to bases having some affect on the burrette - something i will have to check with him (im not sure i trust him ahahhhaha because hes not a great chem teacher). but conversely - it really shouldnt matter which substance you place in which - if you know the volume of each part used and the concentration of one you will be able to determine the concentration of the second independant of if it was in the beaker or not. i do recognise that it is the standard procedure so im more questioning my teacher -.-
thanks again bsdfjnlkasn

[bsdfjnlkasn]

ahahha sorry to be a noob but whats SLC? thankyou for all the extra points (esp the temperature one and evaporation-never thought of that). also i think we agree about the salt bridge idea, but i kind of imagine it as the fact that 'you're only as strong as your weakest link' kinda dealio. so if the current in the wire can move through very efficiently (though there will be resistance like you said) then it will, however we have to preserve electrical neutrality throughout the circuit and maintain a closed circuit. so if the ions are moving slowly through, or at a decreased rate (due to small cross sectional area), then current can only move through the wire at a similarly limited rate - it is possible we r saying the same idea and have misunderstood. it is equally likely i am completely wrong ahahahha but hopefully more students can clarify. thanks bsdfjnlkasn

No worries SLC is standard lab conditions. They're detailed at the back of the periodic table and according to which "condition" an experiment is conducted in, you can determine gas volumes. This is the only calculation application I can think of, but it's generally a good thing to mention when discussing the accuracy of particular experiments. Also, I think we are saying the same thing about the salt bridge which is a relief - your way is a much clearer way of explaining it too.

[beau77bro]

No worries SLC is standard lab conditions. They're detailed at the back of the periodic table and according to which "condition" an experiment is conducted in, you can determine gas volumes. This is the only calculation application I can think of, but it's generally a good thing to mention when discussing the accuracy of particular experiments. Also, I think we are saying the same thing about the salt bridge which is a relief - your way is a much clearer way of explaining it too.

well im glad me helping the great help u gave me helped u, which helped me ahahaha

[bsdfjnlkasn]

yea i totally agree, i thought the unknown 'titrant' was placed in the burrette always. but my teacher said there was an issue due to bases having some affect on the burrette - something i will have to check with him (im not sure i trust him ahahhhaha because hes not a great chem teacher). but conversely - it really shouldnt matter which substance you place in which - if you know the volume of each part used and the concentration of one you will be able to determine the concentration of the second independant of if it was in the beaker or not. i do recognise that it is the standard procedure so im more questioning my teacher -.-
thanks again bsdfjnlkasn

Yeah, you're definitely right about the titrant going in the burette. I personally haven't heard anything about bases affecting glassware, because like you said, we wouldn't be able to hold the base in the beaker/flask either if that were the case... But you seem to have a good understanding of titrations regardless, but just quickly, I do think the standard procedure is the best way to find exact concentrations of unknown substances and I don't think we should really be thinking about it in reverse Just keep it simple and work with the precision of the burette to give you the volume for the unknown you need

[beau77bro]

Yeah, you're definitely right about the titrant going in the burette. I personally haven't heard anything about bases affecting glassware, because like you said, we wouldn't be able to hold the base in the beaker/flask either if that were the case... But you seem to have a good understanding of titrations regardless, but just quickly, I do think the standard procedure is the best way to find exact concentrations of unknown substances and I don't think we should really be thinking about it in reverse Just keep it simple and work with the precision of the burette to give you the volume for the unknown you need

agreed. im sticking by that unless i find out otherwise from my teacher - he will be marking the practical after all. but it is interesting. thanks so much bsdfjnlkasn massive help and enjoyed the discussion honestly

[bsdfjnlkasn]

agreed. im sticking by that unless i find out otherwise from my teacher - he will be marking the practical after all. but it is interesting. thanks so much bsdfjnlkasn massive help and enjoyed the discussion honestly

No worries! It was good revision for me haha. Best of luck with everything, if you have any more questions post them up and hopefully someone will be free to reply

[bsdfjnlkasn]

Hey there,

I was just wondering if the line of best fit for an AAS curve always has to go through the origin? Usually we would base our line off the data given and only from the first data point to the last, but because the line is defined c = ka should we make passing the line through the origin a priority? The particular example I'm working on ends up having all the points above the line if I pass it through the origin. If anyone has any advice that would be super handy - thank you

[beau77bro]

Hey there,

I was just wondering if the line of best fit for an AAS curve always has to go through the origin? Usually we would base our line off the data given and only from the first data point to the last, but because the line is defined c = ka should we make passing the line through the origin a priority? The particular example I'm working on ends up having all the points above the line if I pass it through the origin. If anyone has any advice that would be super handy - thank you

what are the variables of c = ka, is C speed of light or what?

[kiwiberry]

what are the variables of c = ka, is C speed of light or what?

C is concentration and A is absorbance

[jakesilove]

Hey there,

I was just wondering if the line of best fit for an AAS curve always has to go through the origin? Usually we would base our line off the data given and only from the first data point to the last, but because the line is defined c = ka should we make passing the line through the origin a priority? The particular example I'm working on ends up having all the points above the line if I pass it through the origin. If anyone has any advice that would be super handy - thank you

You definitely DO NOT want to create a point that goes through the origin, unless you actually measured that point. Otherwise, you've essentially fabricated results. I agree that the line SHOULD go through the origin, however just ASSUMING that it does is scientific fraud. In fact, a line that does not pass through the origin is almost a good thing. It means you can describe a systematic error; clearly, it should have gone through the origin, but it didn't. Why? What does that mean for the rest of your results? What does that mean for your experimental design?

In summary: do NOT assume anything, unless you've measured it. If you get weird results; GREAT! Explain it, using scientific methodology. Potentially, EXPLAIN what you've done to the marker, and why you've done it

Jake

[chelseam]

Hi! So sorry that this question is going to be a pain but could someone explain the steps to carry out a titration procedure (and why they're important)? I missed both pracs at school and trying to read some of the textbook explanations is killing my brain rn Thank you so so much in advance

[chelseam]

Basically, the titration procedure requires a lot of rinsing to get the most accurate reading of concentration. Even the slightest mess-up can ruin the experiment.
I suggest this really good site!!
 

Thank you so much!! I really appreciate it

[bsdfjnlkasn]

You definitely DO NOT want to create a point that goes through the origin, unless you actually measured that point. Otherwise, you've essentially fabricated results. I agree that the line SHOULD go through the origin, however just ASSUMING that it does is scientific fraud. In fact, a line that does not pass through the origin is almost a good thing. It means you can describe a systematic error; clearly, it should have gone through the origin, but it didn't. Why? What does that mean for the rest of your results? What does that mean for your experimental design?

In summary: do NOT assume anything, unless you've measured it. If you get weird results; GREAT! Explain it, using scientific methodology. Potentially, EXPLAIN what you've done to the marker, and why you've done it

Jake

Hey Jake!
Thanks so much for the reply, I definitely agree with you here because the way graph an AAS calibration curve shouldn't differ from any other graph. BUT all the worked solutions I have seen (both in textbook and online) have considered the graph going through the origin which has me a bit worried. As a part of my assessment we do have to analyse secondary data relating to AAS so if you could possibly suggest a few errors that I would find in secondary experimental data, i'd really appreciate it! Then I could at least argue why I didn't graph the data going through the origin ... I just don't know why resources that HSC students rely on have always shown the graph going through the origin, it's really misleading.


Thanks again so much and hoping to hear back soon