HSC Chemistry Question Thread

[jakesilove]

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

As I understand it, you actually took measurements, but you didn't take a point where there was a concentration of zero in the apparatus? If that is so then, regardless of other factors, adding that 'zero point' would be fabricating data. This would be easy to justify in a write-up: 'I didn't take that measurement, so I didn't include it'. This is something you can say you would improve for next time, to increase the validity of the experiment. You would argue there is some systematic error in your apparatus to explain why it DIDN'T go through the origin (ie. maybe there was a tiny bit of matter floating around, even when you hadn't put anything in there, left over from past experiments. Maybe the air absorbs some of the light. etc. etc. etc.).

All in all, I would shoot your teacher an email to confirm what I'm saying. Whilst I'm definitely 'right', there is a slim chance that they would prefer what is 100% 'wrong'. That's a decision for you to make, but yeah the above is how I would handle this write up.

[bsdfjnlkasn]

Hey there,

I was wondering if anyone could help me with deducing the reason we do some steps in the gravimetric analysis where we deduce the sulfate content in fertiliser. Why do we add acid? Does it matter which we use? HNO₃ or HCl do they do the same thing? If someone could please provide some chemical equations that would be so helpful! Also, what's the point of adding BaCl₂?

Thank you!!

[jakesilove]

Hey there,

I was wondering if anyone could help me with deducing the reason we do some steps in the gravimetric analysis where we deduce the sulfate content in fertiliser. Why do we add acid? Does it matter which we use? HNO₃ or HCl do they do the same thing? If someone could please provide some chemical equations that would be so helpful! Also, what's the point of adding BaCl₂?

Thank you!!

Hey! First, we grind the fertiliser so it is easily dissolvable. Then, we weigh out a known quantity, and place it in a known volume of water. We add acid (generally, HCl, but really any acid is fine) to try and dissolve the soil as much as possible. Obviously, soil is not soluble in water, so we're just trying to break up all the little particles that it contains using that acid.
Then, we add excess barium chloride. This is to form a barium sulfate precipitate, which we can weigh. The relevant equation is


Then, we filter off the precipitate, weigh it, find the moles of sulfate originally present, and compare that to the mass of soil. Make sense?

[bsdfjnlkasn]

Hey! First, we grind the fertiliser so it is easily dissolvable. Then, we weigh out a known quantity, and place it in a known volume of water. We add acid (generally, HCl, but really any acid is fine) to try and dissolve the soil as much as possible. Obviously, soil is not soluble in water, so we're just trying to break up all the little particles that it contains using that acid.
Then, we add excess barium chloride. This is to form a barium sulfate precipitate, which we can weigh. The relevant equation is


Then, we filter off the precipitate, weigh it, find the moles of sulfate originally present, and compare that to the mass of soil. Make sense?

Yes that was a helpful run down

If we were to add HCl instead of HNO3 what would the differences be? I've got that they neutralise any unwanted carbonates, phosphates and hydroxides which then could precipitate later on when we want to obtain BaSO4. This increases the overall solubility of the solution. I understand NO3 is more general as it is soluble - sorry if this is unnecessarily specific. The main point is that it neutralises and so removes any unneeded ions - right?

Thank you for clarifying!

[jakesilove]

Yes that was a helpful run down

If we were to add HCl instead of HNO3 what would the differences be? I've got that they neutralise any unwanted carbonates, phosphates and hydroxides which then could precipitate later on when we want to obtain BaSO4. This increases the overall solubility of the solution. I understand NO3 is more general as it is soluble - sorry if this is unnecessarily specific. The main point is that it neutralises and so removes any unneeded ions - right?

Thank you for clarifying!

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

[beau77bro]

hey im now very confused. what is the standard procedure, is it the unknown in the burrette or the flask. conquering chem says in the flask. people and teacher say in burrette. IDK WHAT TO DO

but it makes sense to have the burrette with the known, because if say you make a 1 molar solution of the standard - and you have a super weak acid or base as the unknown - it would take forever to do the experiment if you had the unknown in the burrette

but i guess the reverse is true. very unsure about this prac - but very sure it is gonna be the prac we do.

[beau77bro]

for galvanic cell calculations, how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow? - and what does that have to do with the standard potentials

[Alalamc]

Hi guys this question has me baffled?
The ionisation constant of hypobromous acid, HBrO, is 2.06x10-9 at a temperature at which Kw is 1.2x10-14. Determine both the pH and the pOH of a 0.25 M solution of this acid at that temperature.
Also how can we tell if a reaction (at equilibrium) is endothermic or exothermic?
Also does my working out seem correct? (considering the fact that H+ & OH- are the same in water.
What is the value of Kw if the pH of a sample of pure water is 6.77?  Is the temperature of this solution above or below 25oC?

pH = -log[H+]
6.77 = -log[H+]
[H+] = 1 times 10^ (-6.77)
[H+] = 1.6982 times 10^ (-7)
Since at the given pH for the given temperature water is neutral H+ = OH-, [OH-] =1.6982 times 10^ (-7).
Thus Kw= [H+] [OH-]
Kw= 1.6982 × 10^ (-7) times 1.6982 × 10^ (-7)
= 2.88388324 × 10^ (-14)

[jakesilove]

hey im now very confused. what is the standard procedure, is it the unknown in the burrette or the flask. conquering chem says in the flask. people and teacher say in burrette. IDK WHAT TO DO

but it makes sense to have the burrette with the known, because if say you make a 1 molar solution of the standard - and you have a super weak acid or base as the unknown - it would take forever to do the experiment if you had the unknown in the burrette

but i guess the reverse is true. very unsure about this prac - but very sure it is gonna be the prac we do.

Literally will not make any difference; you will find the same value, no matter which way you do it. Most people just always put the base in the burrette, and the acid in the conical flask, regardless of whether they are the standard or the unknown. However, it genuinely doesn't matter, as long as you know which is which, and perform your calculations accordingly.

[jakesilove]

for galvanic cell calculations, how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow? - and what does that have to do with the standard potentials

Is this part of the core course, or your option? If it's part of the core, steer clear of it. Basically, the higher the concentration of ions, the more current will flow. This only matter if they give you data, indicating this trend, and you need to analyse it. You definitely don't need to know it, or anything about it. There are really complicated formulas dealing with this sort of this; your table of standard reduction potentials assume a concentration of 1mol/L, and you should too.

[jakesilove]

Hi guys this question has me baffled?
The ionisation constant of hypobromous acid, HBrO, is 2.06x10-9 at a temperature at which Kw is 1.2x10-14. Determine both the pH and the pOH of a 0.25 M solution of this acid at that temperature.
Also how can we tell if a reaction (at equilibrium) is endothermic or exothermic?
Also does my working out seem correct? (considering the fact that H+ & OH- are the same in water.
What is the value of Kw if the pH of a sample of pure water is 6.77?  Is the temperature of this solution above or below 25oC?

pH = -log[H+]
6.77 = -log[H+]
[H+] = 1 times 10^ (-6.77)
[H+] = 1.6982 times 10^ (-7)
Since at the given pH for the given temperature water is neutral H+ = OH-, [OH-] =1.6982 times 10^ (-7).
Thus Kw= [H+] [OH-]
Kw= 1.6982 × 10^ (-7) times 1.6982 × 10^ (-7)
= 2.88388324 × 10^ (-14)

Unfortunately, I really can't help you out here. I didn't do this option, and so I'm not even clear on the terminology. Hopefully someone else on the forum can help!

[tashlyc]

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

[beau77bro]

Literally will not make any difference; you will find the same value, no matter which way you do it. Most people just always put the base in the burrette, and the acid in the conical flask, regardless of whether they are the standard or the unknown. However, it genuinely doesn't matter, as long as you know which is which, and perform your calculations accordingly.

ok sweet i thought so - what would u suggest?

Is this part of the core course, or your option? If it's part of the core, steer clear of it. Basically, the higher the concentration of ions, the more current will flow. This only matter if they give you data, indicating this trend, and you need to analyse it. You definitely don't need to know it, or anything about it. There are really complicated formulas dealing with this sort of this; your table of standard reduction potentials assume a concentration of 1mol/L, and you should too.

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)?

[jakesilove]

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)?

No suggestion; personally, I always put the base in the burrette.

Yep, it'll just run out faster!

[jakesilove]

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

My understanding is that


Now, we know that the final moles of NO was 0.062 moles. Therefore, 0.038 moles of NO has been reacted. The same number of moles of Hydrogen gas must have been reacted (2:2) leaving 0.051-0.038=0.013 moles. Note that finding the moles is the same as finding the concentration, as there is one litre of substance. Now, the final concentration of water will be 0.038 moles/L (1:1 reaction with NO) and the final concentration of Nitrogen gas will be 0.019 moles/L (2:1 reaction with NO). Therefore