HSC Chemistry Question Thread

[jakesilove]

Hi!
What is the purpose of using standard solutions in atomic absorption spectroscopy???
Thanks

Hey! In essence, you need something to compare your finding to!

Say that you spray a solution containing 1ppm of some metal into the AAS. It tells you the absorbance is some value, let's call it X. So what? How can you tell from that absorbance what the ppm value of the metal is?

You have to compare it to a standard curve. So, before doing our experiment, we spray a liquid with a known concentration of the metal. We spray 0ppm, 0.5ppm, 1ppm and 1.5ppm (for instance). By sketching a standard curve function, we can compare our unknown solution to the known solution!

[seventeenboi]

Hey! In essence, you need something to compare your finding to!

Say that you spray a solution containing 1ppm of some metal into the AAS. It tells you the absorbance is some value, let's call it X. So what? How can you tell from that absorbance what the ppm value of the metal is?

You have to compare it to a standard curve. So, before doing our experiment, we spray a liquid with a known concentration of the metal. We spray 0ppm, 0.5ppm, 1ppm and 1.5ppm (for instance). By sketching a standard curve function, we can compare our unknown solution to the known solution!

oooohhhh thanks so much!

[scienceislife]

a) 0.1362 of oxalic acid was dissolved in water and the solution made up to 250.0mL. Calculate the concentration of the solution.
b) This acid solution was used to standardise an approximately 0.02mol/L NaOH solution. 20.00mL of NaOH required 20.61mL of the oxalic acid solution to neutralise it. Calculate the exact concentration of the NaOH solution.
c) The standard NaOH solution was then used to standardise an approximately 0.02mol/L HCl solution. 20.00mL of the NaOH solution was placed in a conical flask. The HCl solution was placed in the burette and the initial and final readings were 1.83mL and 20.26mL respectively. Calculate the concentration of the HCl solution.

[jakesilove]

a) 0.1362 of oxalic acid was dissolved in water and the solution made up to 250.0mL. Calculate the concentration of the solution.
b) This acid solution was used to standardise an approximately 0.02mol/L NaOH solution. 20.00mL of NaOH required 20.61mL of the oxalic acid solution to neutralise it. Calculate the exact concentration of the NaOH solution.
c) The standard NaOH solution was then used to standardise an approximately 0.02mol/L HCl solution. 20.00mL of the NaOH solution was placed in a conical flask. The HCl solution was placed in the burette and the initial and final readings were 1.83mL and 20.26mL respectively. Calculate the concentration of the HCl solution.

Hey! I assume you left the 'grams' off the first number

To answer part a), we just need the formula

where 'n' is moles and 'V' is volume in Litres. First, we calculate moles by
Oxalic acid is

and has a molar mass of 90.03g. Thus,


Now, we convert from mL to litres. 250mL is 0.25L. So



Now, onto part b). We know that the neutralisation reaction is


Thus, two moles of Sodium hydroxide requires one mole of Oxalic acid to react! We use 20.61mL Oxalic acid solution. We can quickly calculate how many moles are present here.

There will be twice as many moles of NaOH, so we have 0.00025 moles of NaOH. Putting this into our concentration equation;

You can use a similar methodology to attack part c)! Let me know if you have any trouble

[itssona]

Heey, so it says for chemistry: The Preliminary course contains content that is considered assumed knowledge for the HSC course.
So basically besides the HSC syllabus, we need to know the prelim stuff well???

like do we need to know the history and society impact stuff that's in prelim chem?

[MisterNeo]

Heey, so it says for chemistry: The Preliminary course contains content that is considered assumed knowledge for the HSC course.
So basically besides the HSC syllabus, we need to know the prelim stuff well???

like do we need to know the history and society impact stuff that's in prelim chem?

Hi, preliminary chem in Year 11 builds onto HSC chem because they do assume you know the previous content. This means that you should remember the important things from the preliminary syllabus.

This would include:
-Calculating things with moles. (Maybe the history of moles with Avogardo's number+Gay Lussac and its impact?)
-Memorising valences and ion charges
-Water has 4.18J/g specific heat capacity, what specific heat capacity is, AND why water is so important in relation to its high resistance to temperature change.
-Reactivity series and first ionisation energy trends.
And much more. I have seen HSC/Trial papers where there was a bunch of questions on first ionisation energy, which is Year 11 content. Hence, it would be ideal to know the preliminary course.

As for the history and impacts in the preliminary course, you most likely will have to know the history of the periodic table and how Mendeleev arranged the tables, etc. Maybe Gay Lussac+Avogadro's contributions to chemistry. Social impacts will likely focus on the environmental impacts of certain properties of water like its high heat capacity AND its less-dense solid state. You would need to explain how these are essential in sustaining aquatic and human life, etc.

Ultimately, yes I would know both syllabi inside and out for the HSC.
Hope this helps

[RuiAce]

As for the history and impacts in the preliminary course, you most likely will have to know the history of the periodic table and how Mendeleev arranged the tables, etc. Maybe Gay Lussac+Avogadro's contributions to chemistry. Social impacts will likely focus on the environmental impacts of certain properties of water like its high heat capacity AND its less-dense solid state. You would need to explain how these are essential in sustaining aquatic and human life, etc.

That being said, you aren't expected to give a full, comprehensive description in how they work anymore.

In my responses, I always coined relevant prelim content but usually stopped after just a mention, or a brief description.

Also, when I was completing the HSC course I never had to use Gay-Lussac's law or Avodagro's law.

[itssona]

Hi, preliminary chem in Year 11 builds onto HSC chem because they do assume you know the previous content. This means that you should remember the important things from the preliminary syllabus.

This would include:
-Calculating things with moles. (Maybe the history of moles with Avogardo's number+Gay Lussac and its impact?)
-Memorising valences and ion charges
-Water has 4.18J/g specific heat capacity, what specific heat capacity is, AND why water is so important in relation to its high resistance to temperature change.
-Reactivity series and first ionisation energy trends.
And much more. I have seen HSC/Trial papers where there was a bunch of questions on first ionisation energy, which is Year 11 content. Hence, it would be ideal to know the preliminary course.

As for the history and impacts in the preliminary course, you most likely will have to know the history of the periodic table and how Mendeleev arranged the tables, etc. Maybe Gay Lussac+Avogadro's contributions to chemistry. Social impacts will likely focus on the environmental impacts of certain properties of water like its high heat capacity AND its less-dense solid state. You would need to explain how these are essential in sustaining aquatic and human life, etc.

Ultimately, yes I would know both syllabi inside and out for the HSC.
Hope this helps

Omg this helped so much, thank youuuuu!!
 

That being said, you aren't expected to give a full, comprehensive description in how they work anymore.

In my responses, I always coined relevant prelim content but usually stopped after just a mention, or a brief description.

Also, when I was completing the HSC course I never had to use Gay-Lussac's law or Avodagro's law.

Thanks Rui! I'm glad that briefly describing  it will be sufficient enough *phew*

[Jyrgal]

hello! just wondering about partial/total pressure in a gaseous environment. lets say for a simple gas equation of N2+3H2=2NH3, if i was to add an inert gas into the container for example, my understanding is that the partial pressures of the reactants & products doesnt get affected as the inert gas only affects total pressure. my problem is since your adding another gas into the container, wouldnt the container available to the products & reactants decrease in volume, so itd increase the concentrations of them? so why would the equilibrium stay even after adding an inert gas?

[RuiAce]

hello! just wondering about partial/total pressure in a gaseous environment. lets say for a simple gas equation of N2+3H2=2NH3, if i was to add an inert gas into the container for example, my understanding is that the partial pressures of the reactants & products doesnt get affected as the inert gas only affects total pressure. my problem is since your adding another gas into the container, wouldnt the container available to the products & reactants decrease in volume, so itd increase the concentrations of them? so why would the equilibrium stay even after adding an inert gas?

A common misconception among students is what "increasing the pressure means".

Without further description, when the pressure is increased it's assumed that you added only equilibrium mixture. In your example, it would mean you added a bit of N₂, H₂ and NH₃. When a bit of all of that is added in, your analysis is valid.

Suppose instead we add argon. Let's consider what happens.

The presence of argon hinders the forward reaction because there is a greater chance that the reactants end up colliding with the inert argon instead. Hence the forward reaction is slowed down.

Its presence also hinders the products bumping into each other, so the reverse reaction is also slowed down.

Therefore, the overall effect is that both the forward and reverse reactions are affected. As a consequence, no change will occur to the equilibrium position. This change is more like a counter-catalyst; the rate that the equilibrium is achieved is slowed down.

[chelseam]

Hi! I'm really confused about ionic equations in general, including how to get to the net ionic equation and how to determine if there are spectator ions and what they would be. If someone could explain this to me I would appreciate this so so much! Thank you

[MisterNeo]

Hi! I'm really confused about ionic equations in general, including how to get to the net ionic equation and how to determine if there are spectator ions and what they would be. If someone could explain this to me I would appreciate this so so much! Thank you

Hi! These redox reactions have 2 half equations and a net ionic equation. Each half equation is usually an oxidation and a reduction species.
The spectator ion is a species that is not involved in the redox reaction and remains unchanged.

For example, magnesium metal is placed into a solution of copper nitrate.
Normal Equation

Half Equations: Oxidation

Half Equations: Reduction

Net Ionic Equation
(It's basically both half equations combined into one equation but you remove the electron species.)
With spectator ions

(The spectator ion is NO3- because it is not involved in the redox reaction and remains aqueous throughout the reaction. Thus, the NO3-, on both sides, would be crossed out when writing the equation.)

Hope this helps

[chelseam]

Hope this helps

Thank you so so much! Wow, I wish I could upvote this more than once This is so helpful!

[anotherworld2b]

Hi could i have some help with this question please?
I am not sure how you can tell what the product of the reaction would be. All i now is that there will be Mn 2+ on the right hand side

[jakesilove]

Hi could i have some help with this question please?
I am not sure how you can tell what the product of the reaction would be. All i now is that there will be Mn 2+ on the right hand side

Hey! So we have



How did I know that? I googled it. There's no way you need to be able to 'guess' this reaction!