HSC Chemistry Question Thread

[Natasha.97]

I need help with the steps involved LDPE such as Initiation, Propagation, termination. I'm confused with diagrams to support the description of each step.

Hi!



Hope this helps

[bluecookie]

Okay, I have this scribbled in the side of my note taking book and I have no idea what it means:

[H+][OH-]=1.0*10^-14
Log10[H+][OH-]=log101*10^-14
Log10[H+][OH-]=-14
Log10H+ + log10OH- = -14
PH-log10OH-=14
PH=14+log10OH-

pH=-log10[H+]
POH = -log10[OH]
PH+pOH = 14

I think some elements of it are familiar from the pH/acids and bases module whatever it was called lol, but I'm confused about the conceptual understanding of it. Could someone explain the concept behind the first line? Why does the concentration of hydrogen and hydroxide ions multiply to 1*10^-14

[Somedudelmaoy]

Okay, I have this scribbled in the side of my note taking book and I have no idea what it means:

[H+][OH-]=1.0*10^-14
Log10[H+][OH-]=log101*10^-14
Log10[H+][OH-]=-14
Log10H+ + log10OH- = -14
PH-log10OH-=14
PH=14+log10OH-

pH=-log10[H+]
POH = -log10[OH]
PH+pOH = 14

I think some elements of it are familiar from the pH/acids and bases module whatever it was called lol, but I'm confused about the conceptual understanding of it. Could someone explain the concept behind the first line? Why does the concentration of hydrogen and hydroxide ions multiply to 1*10^-14

At 25 degrees Celsius, the product of the concentration of hydronium and hydroxide ions equals 10^-14. Maybe there is a reason why but I think you just need to know that they do. This means that if given the concentration of hydroxide ions, you can find the concentration of hydronium ions by doing 10^-14/[OH-]. In reality, the only formula you need to know is pH=-log10[H+]. If given the H+ concentration and asked to find the pH, you use the formula. If given the OH- concentration you can do -log10[OH-] to get pOH and then minus it from 14 to get pH. Or you could do 10^-14/[OH-] to get H+ concentration and then do -log10[H+]. It's the same thing but just done in a different order.

[justwannawish]

Hey, guys!

Just wondering how do we do sig figs in the sciences?  Is a number like 2800 considered to be in 2sf or 4?

[MisterNeo]

Hey, guys!

Just wondering how do we do sig figs in the sciences?  Is a number like 2800 considered to be in 2sf or 4?

According to maths, 2800 is 2 sig figs. If it were 2800.0, that would be 5 sig figs. In chem, your answer should be written with the smallest amount of sig figs used in the question.

[justwannawish]

According to maths, 2800 is 2 sig figs. If it were 2800.0, that would be 4 sig figs. In chem, your answer should be written with the smallest amount of sig figs used in the question.

Thank you so much for such a quick answer! I think I get it now

A couple more questions: what's the relationship between the anode/cathode, oxidation/reduction, and activity of metals?
What I currently interpret it as is: the negative anode contains the oxidisation reaction, which is reduction agent, and is generally made out of the more active metals. At the anode, solid metal will be lost as it transforms into a metal ion. And the cathode is pretty much the opposite.
Is this correct? And is there anything else to know regarding anodes and their properties in redox experiments.

But I don't understand entirely the concept of galvanic cells, particularly how the salt bridge helps the electrode reaction. Can anyone please help by explaining the experiment once more? Thank you

[Somedudelmaoy]

According to maths, 2800 is 2 sig figs. If it were 2800.0, that would be 4 sig figs. In chem, your answer should be written with the smallest amount of sig figs used in the question.

would 2800.0 not be 5 sig figs?

[MisterNeo]

would 2800.0 not be 5 sig figs?

Yes, sorry my bad.
2800. is 4, 2800.0 is 5

[f_tan]

Can anyone help explain this question please? Thank you!

[Azurelies]

Hi, can I have some help on this question please?
I have no idea where to even start

Sorry for the blurry photo!

[MisterNeo]

Can anyone help explain this question please? Thank you!
(Image removed from quote.)

The absorption is proportionate to the concentration, so you can just divide 10ppm by 4 then multiply by 5 to get the absorbance for 0.500.
Parts per million is the same as mg/L, and you can find mol/L using copper molar mass. Then divide the mol/L by 10 to get the moles in 100mL, and it’s just stoichiometry from there.

Hi, can I have some help on this question please?
I have no idea where to even start
Sorry for the blurry photo!

This is what I got, not sure if correct so answers would be nice. Since they give you the NaCl data, I’d assume it plays a role in the calculations because Na⁺ initial concentrations would be increased. You would find the initial concentrations, then work backwards with the K values to find the concentration of bicarbonate and carbonate. The given pH can be converted into H⁺ using 10^(-pH).

[anotherworld2b]

I had some question about equilibrium questions. For the first table wouldn't [NoBr] increase when you remove NoBr?
Also when you decrease volume wouldn't
Pressure increase? So every thing except NoBr decreases? (NoBr increases?)

I understand the principles but I get confused when there is a solid involved.
 I got confused doing the second table how do you know there isn't effect for the reactants/products? I understand the reaction rates and I have put what the answers are in the tables

[MisterNeo]

I had some question about equilibrium questions. For the first table wouldn't [NoBr] increase when you remove NoBr?
Also when you decrease volume wouldn't
Pressure increase? So every thing except NoBr decreases? (NoBr increases?)

I understand the principles but I get confused when there is a solid involved.
 I got confused doing the second table how do you know there isn't effect for the reactants/products? I understand the reaction rates and I have put what the answers are in the tables

So for [NOBr], it has decreased its concentration compared to what it was prior to any changes. Whenever you remove some of a species, it will shift to recover it, but not all of it. The equilibrium shifts to minimise the change.
The volume decrease does increase pressure. Gas pressure is like the concentration of gas because you fit more particles into a given space. Same concept with the thing above, the concentration will increase but it won’t go back to its orginal concentration, as the equilibrium will minimise the change.
[H₂O] is a pure liquid and is always equal to 1. 

[anotherworld2b]

I get confused when there is a solid involvements. An example is the second table. Do I treat as just a reactant\product?
I was also wondering what would be the easiest and quickest way to identify the nature of a salt solution?

So for [NOBr], it has decreased its concentration compared to what it was prior to any changes. Whenever you remove some of a species, it will shift to recover it, but not all of it. The equilibrium shifts to minimise the change.
The volume decrease does increase pressure. Gas pressure is like the concentration of gas because you fit more particles into a given space. Same concept with the thing above, the concentration will increase but it won’t go back to its orginal concentration, as the equilibrium will minimise the change.
[H₂O] is a pure liquid and is always equal to 1. 

[MisterNeo]

I get confused when there is a solid involvements. An example is the second table. Do I treat as just a reactant\product?
I was also wondering what would be the easiest and quickest way to identify the nature of a salt solution?

Solids and pure liquids are still treated as reactants and products because they shift like every other chemical species. But, the [X] brackets specifically measure the concentration of the species at equilibrium. In the second table, the water will be used up if the system shifts to the right, but the concentration of that water is unchanged because it remains constant, it's still 100% pure water but just less of it.
Aqueous and gaseous species do change in concentration, however.
When an aqueous species gets used up, the water stays there but the solute gets used up. This reduces its concentration. When a gas species gets used up, the volume remains unchanged but moles of gas are used up. This reduces its concentration too.
Hope this explanation is clear, it used to baffle me too.   

For the salt question, the easiest way to tell if a salt solution is acidic/neutral/basic is to look at the cation and anion of the salt.
Anions are derived from acids (Cl^-, CH₃COO^-, etc), whereas cations are derived from bases (Na⁺, NH₄⁺, etc). Strong-Strong, Weak-Weak make neutral salts, whereas Strong-Weak and Weak-Strong make basic and acidic salts respectively. The only thing you should know is which is a strong acid/base and which is a weak acid/base.