HSC Chemistry Question Thread

[RuiAce]

Thank you RuiAce and ml125 for your help 

I was wondering how would you know the expected observations from a reaction? I know general base and acid reactions

The image didn't appear the first time.

For that question, unless you have the skills to combine a ton of equations together you need to keep working backwards with the calculations. Easy but EXTREMELY tedious.


If you want me to show you how to combine equations here, feel free to ask.
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And yeah. With identifying reactions, you need to know how to deduce what makes sense. Know the type of reactions and thus anticipate the products.

[anotherworld2b]

Yes could you please show me?

The image didn't appear the first time.

For that question, unless you have the skills to combine a ton of equations together you need to keep working backwards with the calculations. Easy but EXTREMELY tedious.


If you want me to show you how to combine equations here, feel free to ask.
___________________________________

And yeah. With identifying reactions, you need to know how to deduce what makes sense. Know the type of reactions and thus anticipate the products.

[RuiAce]

Yes could you please show me?

I'm sorry, I take it back. Combining for that one can be done in too many different ways and I don't know which one is correct.

Yeah, it's just an extremely tedious question. You'll have to work through each individual thing step by step through every single equation.

Using n=m/M you have the moles of HNO3 being 396.71 or something

Then 1 mol of HNO3 is formed from 1 mol of NO2 so moles of NO2 also equals 396.71

Then 1 mol of NO2 is formed from 1 mol of NO so moles of NO is also 396.71 and just keep going.

[anotherworld2b]

Ok okay. Which question are u referring to because I posted two images for two questions

I'm sorry, I take it back. Combining for that one can be done in too many different ways and I don't know which one is correct.

Yeah, it's just an extremely tedious question. You'll have to work through each individual thing step by step through every single equation.

Using n=m/M you have the moles of HNO3 being 396.71 or something

Then 1 mol of HNO3 is formed from 1 mol of NO2 so moles of NO2 also equals 396.71

Then 1 mol of NO2 is formed from 1 mol of NO so moles of NO is also 396.71 and just keep going.

[RuiAce]

Ok okay. Which question are u referring to because I posted two images for two questions

If there was a second one I didn't see it because I only saw one.

[anotherworld2b]

How accurate do the answets have to  be? I got 144ml for this question but the answer is 146ml in the book

[RuiAce]

How accurate do the answets have to  be? I got 144ml for this question but the answer is 146ml in the book

Never round halfway through. Only ever round at the end.

[jakesilove]

How accurate do the answets have to  be? I got 144ml for this question but the answer is 146ml in the book

Make sure to ONLY round to sig figs at the END of the question (ie. when you get the final answer). That being said, many HSC questions will allow for some small variation in answer (generally less than 2mL though!). I used to just always write down at least 5-8 decimal places, if I couldn't keep all the values stored on my calculator, so that I had values to come back to with enough accuracy. No point losing silly marks when you've done all the hard work!

[anotherworld2b]

Oh ok. Would my working out be right Though?

I was also wondering for this question i got the wrong answer but im not sure why

[jakesilove]

Oh ok. Would my working out be right Though?

I was also wondering for this question i got the wrong answer but im not sure why

Yep, if you got an answer that is 2mL off the actual answer, your working would be right!

The problem with your working out in the last question is that you used 5.09g, where you should have used the CHANGE in mass. So, a certain amount has reacted, and THAT is the number of moles that you care about. So, subtract the final mass from the initial mass, and find how many moles of Calcite that is

[anotherworld2b]

Thank you very much for your help

For these two questions I was confused on how to do them as well.

I appreicate all the help I am recieving. It is really helping me practise using stoich which is currently my weakest point in chem.

Yep, if you got an answer that is 2mL off the actual answer, your working would be right!

The problem with your working out in the last question is that you used 5.09g, where you should have used the CHANGE in mass. So, a certain amount has reacted, and THAT is the number of moles that you care about. So, subtract the final mass from the initial mass, and find how many moles of Calcite that is

[jakesilove]

Thank you very much for your help

For these two questions I was confused on how to do them as well.

I appreicate all the help I am recieving. It is really helping me practise using stoich which is currently my weakest point in chem.

That's really no problem! It's clear you're getting heaps better at these questions, so I have no doubt that you'll be confident come exam time.

For the first question (which is definitely a weird one), you need to start by interpreting the whole 'driving for a year' bit correctly. If it releases 3g per kilometer, and it travels for 1.6*10^4 km, then we can find the TOTAL grams emitted by multiplying the two numbers together! Then, we can find the moles of NO released based on the mass just found and the molar mass. Finally, we use the 4:6 molar ratio to find the moles of Ammonia, and convert that into litres using the molar volume formula.

For the second question, you need to oxidise liquid Hydrogen. This means 'inventing' a formula; I'd go with something like this



2kg of Hydrogen was used up; you can figure out the rest!

Jake

[angiezhang9]

Hey guys,

I am stuck on Q18 of the 2012 HSC Paper.

Which of the following changes take place when 50 ml of water is added to 50ml of 0.1 mol/L acetic acid? The answer is pH increases and degree of ionisation increases.

I don't understand why ionisation increases. Wouldn't this then mean that ph will decrease?

Thanks

[jakesilove]

Hey guys,

I am stuck on Q18 of the 2012 HSC Paper.

Which of the following changes take place when 50 ml of water is added to 50ml of 0.1 mol/L acetic acid? The answer is pH increases and degree of ionisation increases.

I don't understand why ionisation increases. Wouldn't this then mean that ph will decrease?

Thanks

Hey! It's a really good question, and quite a tough one too. Firstly, we need to remember that ionisation is the removal of a hydrogen ion. For weak acids, like acetic acid, this will only occur a small percentage of the time. If we add more water, more acid ions/hydrogen ions will dissociate. This means that the ionisation increases, just because it has more water to react with!

So, we know that there are MORE hydrogen ions in the solution after we add water than before. Why doesn't the pH change? Remember that the formula for pH is -log[H], where [H] is the CONCENTRATION of Hydrogen ions. Whilst the AMOUNT of Hydrogen ions have increased, the amount of liquid has doubled too. Ionisation will only barely increase, so the relative concentration of ions will actually decrease. As such, the concentration of Hydrogen ions will decrease, and thus the pH will increase.

Let me know if this explanation makes sense!

Jake

[lha]

Should i only be practicing long response questions rather than short responses?

Alright thank you