HSC Chemistry Question Thread

[bananna]

Hey! This is really beyond the curriculum, and confuses a lot of students. Suffice it to say that if you treat them as exactly the same, you will ALWAYS get the marks.

An amphiprotic substance is anything that can both accept and donate protons (thus, it acts as a B-L acid/base). And amphoteric substance, however, ACTS as an acid and a base without necessarily donating/accepting a proton. For instance, Aluminium trioxide will act as an acid, despite not containing any hydrogen atoms!



Again, you don't need to know this. For the purpose of the HSC, just know that these substances act as an acid and as a base.

 What kind of questions could they ask regarding amphiprotic substances? (I haven't come across very many)

thank you!

[Gabby Mangion]

No worries! So, the initial state is



Which has a net charge of zero (or they would have specified a net charge). As oxygen has a charge of -2, and the charges add up to form the net charge, we can say that



Where x is the charge on Manganese. So, the initial oxidation state of Manganese is going to be 4 (solving for x)

Now, the final state is



Again, we create a formula, noting that the net charge is zero



So, the oxidation number has INCREASED to 6. Thus, the answer is C.

If you figured out the correct answer for Zinc, you can use logic to figure out what's going on with Manganese. You would have found that zinc REDUCES (ie oxidation number decreases). Therefore, the Manganese must OXIDISE). This limits it to A and C. Finally, note that the initial condition of Manganese is not in an elemental form. Thus, it cannot have an initial oxidation state of 0. So, the answer can only be C

Thank you so much!!! It's making a lot more sense now. 

[bananna]

Hi,
why is acetic acid a weaker acid than citric?
And how would I answer a question comparing the degree of ionisation of citric, acetic and HCl acids?

thank you

[jakesilove]

Hi,
why is acetic acid a weaker acid than citric?
And how would I answer a question comparing the degree of ionisation of citric, acetic and HCl acids?

thank you

Hey! Acetic acid is only monoprotic; thus, one molecule of acetic acid will release only one Hydrogen ion. However, citric acid is triprotic; thus, one molecule of citric acid will release three Hydrogen ions. So, citric acid will produce a more acidic solution (assuming equal concentrations) than acetic acid. However, only a small percentage of acetic acid/citric acid molecules actually dissociate in water. Most of them stay as a compound, rather than ionising. This is the definition of a weak acid. On the other hand, HCl is a strong acid, and so ionises completed in solution. This is the definition of a strong acid, and is why a solution of HCl (assuming equal concentration) will be more acidic than a solution of citric acid, which is more acidic that a solution of acetic acid.

[Alalamc]

Hi I was having a bit of trouble with the following 2 questions, I was hoping you could explain them to me \
1. A compound has a percent composition of 38.1% C, 6.3% H and 55.6% Cl and a molar mass of approximately 125 g/mol. What is the empirical and molecular formula?
2. What is the concentration of a sodium carbonate solution prepared by diluting 10.0mL of a 5.0 M Na2CO3 solution to 250mL?
Thanks!

[bsdfjnlkasn]

Hi I was having a bit of trouble with the following 2 questions, I was hoping you could explain them to me \
1. A compound has a percent composition of 38.1% C, 6.3% H and 55.6% Cl and a molar mass of approximately 125 g/mol. What is the empirical and molecular formula?
2. What is the concentration of a sodium carbonate solution prepared by diluting 10.0mL of a 5.0 M Na2CO3 solution to 250mL?
Thanks!

Hiya there,

I managed to solve your second question and hopefully it's right.

2. We will be using the: c₁v₁=c₂v₂ formula to help us solve this question 

First let's write out all the information we've been given:
c₁ = 5M
c₂ = ?
v₁ = 0.01 L (always covert mL to L)
v₂ = 0.25 L

Next we sub in our values to the above formula and get the answer
i.e. 5 x 0.01 = c₂ x 0.25

and you should get 0.2M 

Also, I did some rough working for your first question but was wondering if you could send the solution through so that I can double check that what I got was correct (I won't be surprised if I got it wrong)

Hopefully this was helpful 

[chelseam]

Hi! Can someone please explain the method to solve these questions? I'm especially confused by b) since it's just NaOH! Thank you 

[Alalamc]

Hopefully this was helpful 

Legend! I haven't got the solutions/answers but i did get  oddly big numbers for Q1 :/

[bsdfjnlkasn]

Hi! Can someone please explain the method to solve these questions? I'm especially confused by b) since it's just NaOH! Thank you 

Hey don't be thrown off by NaOH, all that needs to be done differently is we need to calculate pOH before finding the pH

[Kira Leigh]

Hey  there!
I was doing a past mid course my teacher gave me today and came across the term 'amphoteric' I know that in class we covered amphiprotic but not amphoteric. Is there a difference?

[Wren]

Hey  there!
I was doing a past mid course my teacher gave me today and came across the term 'amphoteric' I know that in class we covered amphiprotic but not amphoteric. Is there a difference?

There's a similar question on the previous page thats been answered so you can just check that. But if you can't be bothered it says its basically the same thing for the purpose of HSC.

[katnisschung]

just out of curiosity ( i don't take chem but this is somewhat related to bio test and i don't have a chem background)
what would happen if you heated hydrogen peroxide to like 100°C?

[chelseam]

Thank you so much! 

Hey don't be thrown off by NaOH, all that needs to be done differently is we need to calculate pOH before finding the pH

[jakesilove]

Hey  there!
I was doing a past mid course my teacher gave me today and came across the term 'amphoteric' I know that in class we covered amphiprotic but not amphoteric. Is there a difference?

Yep! Check out my post here

[jakesilove]

just out of curiosity ( i don't take chem but this is somewhat related to bio test and i don't have a chem background)
what would happen if you heated hydrogen peroxide to like 100°C?

Look, when it comes to complex Chemistry, I'm a bit out of my depth aha. However, it has a boiling point around 150 degrees, so my guess is going to be... Nothing?

Also, I just want to say a massive thank you to all the users answering questions. Most of you are current Year 12s yourself, so the fact that you're taking the time to help others is just phenomenal. Plus, it's an insanely good way to study; if you can teach, then you understand. You keep ATAR Notes running