HSC Chemistry Question Thread

[jakesilove]

How do I calculate the percentage of total dissolved solids? Do I have to subtract the mass of suspended solids from the total volume of water?

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/chemistry_05.pdf

Q.25
2005 HSC

No, you shouldn't have to, and again I think those HSC responses are wrong. If I have 400mL of water, which contains 5g of suspended solids, do I really have 395mL of water? No, obviously not, because 1) mL and g are totally different units and 2) we care about the volume of the solvent as a whole, which is 400mL. Later BOSTES exams don't subtract that amount, either.

[wesadora]

Esters vs alkanoic acid boiling points/MP.

I am aware esters have a lower bp/mp than alkanoic acids - alkanoic acids have two sites for hydrogen bonding, dipole-dipole bonding as well as dispersion forces, while esters only have dipole and dispersion forces?
Why do esters not have hydrogen bonding - I mean...they have an OH and double bonded O group as well.

[jakesilove]

Esters vs alkanoic acid boiling points/MP.

I am aware esters have a lower bp/mp than alkanoic acids - alkanoic acids have two sites for hydrogen bonding, dipole-dipole bonding as well as dispersion forces, while esters only have dipole and dispersion forces?
Why do esters not have hydrogen bonding - I mean...they have an OH and double bonded O group as well.

I'm not actually sure why Esters don't form hydrogen bonds! The important thing to know is just that they don't; you don't need to understand why. However, the fact that they don't have hydrogen bonds explains the lower mp/bp!

[Rd2487]

Just a bit confused.
Ozone has a high molecular weight, is that why it has a high BP/MP than molecular oxygen? Or is it something to do with bond energy
Explanation please?

[massive]

how do you do this guys?? the answer is: 1x10^-7 mol/L

[jakesilove]

Just a bit confused.
Ozone has a high molecular weight, is that why it has a high BP/MP than molecular oxygen? Or is it something to do with bond energy
Explanation please?

Yep, molecular weight is the important factor here. Additionally, it's polar, so has strong dipole-dipole intermolecular forces

[jakesilove]

how do you do this guys?? the answer is: 1x10^-7 mol/L

Hey! Remember that, to find the pH of something, we use -log(H+). So, if we plugged in -log(10^-7), we would get 7! What does that mean? As we keep diluting the water, the pH will increase and increase and increase until it gets to neutral (7). But, it wouldn't make sense for the pH to keep rising. Like, we can't just keep adding water, decrease the concentration of H+, and make the solution basic, can we? That wouldn't make any sense. So, the minimum concentration is neutral, as the question states.

[massive]

Hey! Remember that, to find the pH of something, we use -log(H+). So, if we plugged in -log(10^-7), we would get 7! What does that mean? As we keep diluting the water, the pH will increase and increase and increase until it gets to neutral (7). But, it wouldn't make sense for the pH to keep rising. Like, we can't just keep adding water, decrease the concentration of H+, and make the solution basic, can we? That wouldn't make any sense. So, the minimum concentration is neutral, as the question states.

Ohh thanks, so whenever we dilute the pH ALWAYS increases? I thought that was only for weak acids :S

[Cindy2k16]

Hi how come the answer is A? I thought that when a weak acid/base is used, since it sets up an equilibrium, more of a strong acid/base is required to neutralise it?
TIA

[jakesilove]

Ohh thanks, so whenever we dilute the pH ALWAYS increases? I thought that was only for weak acids :S

Yeah, because remember that pH is the -log(concentration of H+). So, if we add more water, the CONCENTRATION will decrease (even if the NUMBER of H+ ions stay the same). Does that make sense?

[lha]

Can you use silver nitrate as the salt bridge for a galvanic cell that has a lead anode and platinum cathode with fe3+/fe2+ solution?

[jakesilove]

Hi how come the answer is A? I thought that when a weak acid/base is used, since it sets up an equilibrium, more of a strong acid/base is required to neutralise it?
TIA

Hey! The simple answer is that it just DOES require the same amount of to neutralise it. Check out Rui's summary here, because I was never very good at this, but really you can just learn that this is a thing that is true and move on with your life.

[jakesilove]

Can you use silver nitrate as the salt bridge for a galvanic cell that has a lead anode and platinum cathode with fe3+/fe2+ solution?

Yep! I would generally recommend using a potassium nitrate salt bridge, but either way works fine

[Cindy2k16]

Hey! The simple answer is that it just DOES require the same amount of to neutralise it. Check out Rui's summary here, because I was never very good at this, but really you can just learn that this is a thing that is true and move on with your life.

ok thanks you!

[JemexR]

Is there a particular reason why you asked the same questions again?

Sorry Rui... I totally missed that