HSC Chemistry Question Thread

[bsdfjnlkasn]

Hey Rui,

Thanks for distinguishing between CO₂ acting to increase pressure on it's own (in the provided example) and how any other gases present would be of similar effect in a different scenario.

I still don't understand the connection between pressure and concentration in your line:

I'm just not sure how you related the two for this example (especially since you said not to do this at the very end)

When we talk about increasing the "pressure" in a broad context, we're talking about increasing the concentration of EVERYTHING. We increase the concentration of CO₂, but ALSO the concentration of H₂O and H₂CO₃.

Finally, when you say the "ultimate effect" do you mean what happened after LCP was applied? Because the increase in pressure which comes with more CO₂ will mean that the side with fewer gas moles would be favoured. By having this excess CO₂ being made into products we can then say that CO₂ decreased. Is that right? What I think you're saying is that I can't immediately anticipate that higher pressures will result in lower concentrations with because more substances  could be involved. But in this case, I can say that the higher pressure results in lowered concentrations.

For this example, because CO₂ was the only gas, the ultimate effect was that the concentration of CO₂ DID go down due to an increase in pressure.

Sorry if there wasn't a clear question, I just wanted to make sure that I had consolidated my understanding. If I have said something wrong, could you please let me know? Thank you!

[RuiAce]

Hey Rui,

Thanks for distinguishing between CO₂ acting to increase pressure on it's own (in the provided example) and how any other gases present would be of similar effect in a different scenario.

I still don't understand the connection between pressure and concentration in your line:

I'm just not sure how you related the two for this example (especially since you said not to do this at the very end)

Finally, when you say the "ultimate effect" do you mean what happened after LCP was applied? Because the increase in pressure which comes with more CO₂ will mean that the side with fewer gas moles would be favoured. By having this excess CO₂ being made into products we can then say that CO₂ decreased. Is that right? What I think you're saying is that I can't immediately anticipate that higher pressures will result in lower concentrations with because more substances  could be involved. But in this case, I can say that the higher pressure results in lowered concentrations.


Sorry if there wasn't a clear question, I just wanted to make sure that I had consolidated my understanding. If I have said something wrong, could you please let me know? Thank you!

My bad if I muddled some terms up.

Pressure in an arbitrary context is related to the concentrations of every gas possible. Whilst yes, if we only increase the concentration we are increasing "pressure", simply because we're putting more gas in, it's not what we're talking about.
- Increasing the pressure by ONLY adding CO₂ means only adding CO₂
- Increasing the pressure and NO more info means adding all gases in.
_______________________________

Think of that as a flow chart

[ CO2 is only gas ] ====> *Increase pressure* ====> Only increase the concentration of CO2 ====> [Concentration of CO2 increased] ====> LCP acts to reduce concentration of CO2.

You don't go straight from "increase pressure" to the last line. You go through it step by step so that the final result is that the concentration of CO2 goes down (so yes after LCP applied)

[bsdfjnlkasn]

Is Acid strength determined by the concentration of H+ ions and the degree of ionisation?

What is it about strong acids that makes them have a higher degree of ionisation?

[RuiAce]

Is Acid strength determined by the concentration of H+ ions and the degree of ionisation?

What is it about strong acids that makes them have a higher degree of ionisation?

Yes
___________________________

That's just how they're defined. Strong acids are acids that ionise completely (degree of ionisation = 100%). Weak acids do not ionise completely. (70% ionisation = weak.)

As for WHY HCl and etc. ionise completely, that is out of the course.

[bsdfjnlkasn]

Hi, I posted this a while back on the Chem Megathread (everyone go check it out ) but the time has come for me to answer it myself. So, could someone please check that I have addressed this 4-marker correctly? Thank you!


Analyse the relationship between the position of elements in the Periodic Table, and the acid-base behaviour of their oxides


Non-metals can be found on the right hand side of the periodic table. As their oxides are acidic in solution and donate protons. We can hence deduce that non-metal oxides are formed on the right hand side of the periodic table. Acidity follows the same trends as electronegativity as it increases across periods and up groups (i.e. towards F). Likewise, metals can be found on the right hand side of the periodic table. As their oxides are basic and accept protons when in solution, we can deduce that metal oxides are formed on the left hand side of the periodic table. Alkalinity/basicity is the reverse of trends in electronegativity as it increases across periods from right to left and increases down groups. Oxides which can act as a base or acid in solution are known as amphoteric oxides and we usually find them in the middle of the periodic table, some notable ones include Zn, Pb and Al.

[Paul.I]

Hi can someone please help answer this question:
Calculate the pH of the solution produced by:
Mixing 50mL of 0.1 mol/L HCl with 20mL of 0.05 mol/L NaOH

[Shadowxo]

Hi can someone please help answer this question:
Calculate the pH of the solution produced by:
Mixing 50mL of 0.1 mol/L HCl with 20mL of 0.05 mol/L NaOH

pH is -log₁₀[H₃O+] or 14 - pOH

First you find the number of H₃O+ and the number of OH-
n (HCl) = cV = 0.1*0.050 = 0.0050 mol
n (NaOH) = cV = 0.05*0.020 = 0.001 mol
n (H+) = n (HCl) = 0.0050 mol
n (OH-) = n (NaOH) = 0.001 mol
As they react in a 1:1 ratio, H+ is in excess by 0.0050 - 0.001 = 0.004 mol
[H+] = n/V = 0.004/0.070 = 0.057 = 0.06 molL-1 rounded
pH = -log₁₀[H+] = 1.24 = 1.2 (I rounded to 2 sig figs but with the information it should be 1, but with pH questions pH can be a bit weird)

Hope this helps a bit

[Butterflygirl]

Hi guyss

just a quick question for the compound FeSiO3, why is the oxidation number of Si +4 and not -4? because doesn't it need 4 electrons to have a full outershell? or is it because it could lose the 4 electrons too?


Thanks!

[RuiAce]

Hi guyss

just a quick question for the compound FeSiO3, why is the oxidation number of Si +4 and not -4? because doesn't it need 4 electrons to have a full outershell? or is it because it could lose the 4 electrons too?


Thanks!

Basically that.

Technically carbon, silicon, germanium etc. can lose or gain 4, and if they choose to lose 4 then they completely lose their outer shell, which is fine. It just so happens that they have a tendency to choose one over the other, in general.


(However, don't think of it like ionic bonding though. Take CO₂. C has an oxidation state of +4 but it's covalently bonded to the two oxygens though. It's quite complicated and the entire deal isn't explained in the HSC.)

[Paul.I]

pH is -log₁₀[H₃O+] or 14 - pOH

First you find the number of H₃O+ and the number of OH-
n (HCl) = cV = 0.1*0.050 = 0.0050 mol
n (NaOH) = cV = 0.05*0.020 = 0.001 mol
n (H+) = n (HCl) = 0.0050 mol
n (OH-) = n (NaOH) = 0.001 mol
As they react in a 1:1 ratio, H+ is in excess by 0.0050 - 0.001 = 0.004 mol
[H+] = n/V = 0.004/0.070 = 0.057 = 0.06 molL-1 rounded
pH = -log₁₀[H+] = 1.24 = 1.2 (I rounded to 2 sig figs but with the information it should be 1, but with pH questions pH can be a bit weird)

Hope this helps a bit

Thank You Very Much

[anotherworld2b]

Hi Can I please have help with these two questions? I am not sure how to answer them

1. Are atmospheric gases soluble? How does the equilibrium between the atmospheric concentration and ocean concentration of these gases help decrease greenhouse gases in the atmosphere?

I believe atmospheric gases are soluble?

2. Give the sequence of equations that represent the CO2 gas in the atmosphere eventually forming hydrogen ions in the ocean.

[bsdfjnlkasn]

Hi, I posted this a while back on the Chem Megathread (everyone go check it out ) but the time has come for me to answer it myself. So, could someone please check that I have addressed this 4-marker correctly? Thank you!


Analyse the relationship between the position of elements in the Periodic Table, and the acid-base behaviour of their oxides


Non-metals can be found on the right hand side of the periodic table. As their oxides are acidic in solution and donate protons. We can hence deduce that non-metal oxides are formed on the right hand side of the periodic table. Acidity follows the same trends as electronegativity as it increases across periods and up groups (i.e. towards F). Likewise, metals can be found on the right hand side of the periodic table. As their oxides are basic and accept protons when in solution, we can deduce that metal oxides are formed on the left hand side of the periodic table. Alkalinity/basicity is the reverse of trends in electronegativity as it increases across periods from right to left and increases down groups. Oxides which can act as a base or acid in solution are known as amphoteric oxides and we usually find them in the middle of the periodic table, some notable ones include Zn, Pb and Al.

Hey sorry to be annoying but could I please get a little bit of feedback on this question? Thank you so much!!

[shreya_ajoshi]

Hi!
In class we did the 'decarbonating soft drink' experiment. But we used a control along with a can of soft drink to help us. The control was a can of water. I don't quite understand the point of the control. Could someone please explain it to me?
Thanks

[sweetcheeks]

Hi!
In class we did the 'decarbonating soft drink' experiment. But we used a control along with a can of soft drink to help us. The control was a can of water. I don't quite understand the point of the control. Could someone please explain it to me?
Thanks

Could you further elaborate on the experiment? Like how was it performed, what were you measuring? Perhaps the water was used to show CO2 concentration of water in equilibrium with the atmosphere, showing that the CO2 inside the soft drink was being lost due it not being in equilibrium with the environment.

[Vaike]

Hi all,
I was hoping someone could help me with this question. I understand that A is endothermic and thus has +ΔH, and I also figure that C must have +ΔH as it is the reverse of a combustion reaction. However, I am unsure of B and D and how to identify their ΔH value as positive or negative.
Thanks