HSC Chemistry Question Thread

[WLalex]

This is from 2009 HSC Chem paper. I thought AAS could only be used for metal ions that can be dissolved in solution? Not like...phosphates (which is phosphorous and oxygen anyway, using AAS wouldn't make sense) or are they just playin wit us

also - for qualitative/quantitative tests regarding eutrophication, in my notes i have the fricken randomest things called the 'Kjeldahl method' and 'colorimetry' (just AAS with colours, as far as i understand) that is kinda complex and ceebs remembering. What do other people have for eutrophication tests.... :|

for eutrophication i only have testing phosphate using AAS or photometry because phosphate is the growth-limiting so its better to test than nitrate..if that helps at all haha

[jakesilove]

This is from 2009 HSC Chem paper. I thought AAS could only be used for metal ions that can be dissolved in solution? Not like...phosphates (which is phosphorous and oxygen anyway, using AAS wouldn't make sense) or are they just playin wit us

also - for qualitative/quantitative tests regarding eutrophication, in my notes i have the fricken randomest things called the 'Kjeldahl method' and 'colorimetry' (just AAS with colours, as far as i understand) that is kinda complex and ceebs remembering. What do other people have for eutrophication tests.... :|

Don't worry about that, just accept that the question gives you AAS with phosphates! As for your second question, that's definitely way more information than you need, so don't worry about that either

[jakesilove]

Hey, just wanted some clarity regarding this questions.
On my notes I have written that TDS = dissolved solids + suspended solids

SO for the questions I added the mass difference from both evaporating and filtering but on the answers they one did the mass of the dissolved solids (1.14g instead of 1.38 as I did). Is this right?

Mass of dissolved solids is different to total dissolved solids. The question only asks for dissolved solids, so only do the dissolved solids for this question!

[WLalex]

Mass of dissolved solids is different to total dissolved solids. The question only asks for dissolved solids, so only do the dissolved solids for this question!

but it says percentage of the total dissolved solids...? haha sorry I'm confused

[jakesilove]

but it says percentage of the total dissolved solids...? haha sorry I'm confused

Ah sorry I see! Basically, TDS doesn't include suspended solids, only dissolved solids, so I think you've just written that down wrong in your notes I didn't really know that either, but according to Google dissolved solids shouldn't be able to be removed by filtration through a filter funnel. Basically, if you get exactly the same question in your exam, follow the method the answers gave!

[wesadora]

Ah sorry I see! Basically, TDS doesn't include suspended solids, only dissolved solids, so I think you've just written that down wrong in your notes I didn't really know that either, but according to Google dissolved solids shouldn't be able to be removed by filtration through a filter funnel. Basically, if you get exactly the same question in your exam, follow the method the answers gave!

yah, Jake's right. Suspended solids are more for like turbidity - the .03g of solid left in the filter paper is the suspended solids. The DISSOLVED solids are what couldn't be trapped by filter paper, and thus can only be obtained after evaporating to dryness in an evap basin. (so TDS is 0.26g and go calc from there)

also.... soz continuing with the eutrophication questions: why do algal blooms consume dissolved oxygen anyway? Aren't they photosynthetic o.O (Creating oxygen), but I understand they block sunlight from getting to other photosynthetic organisms and thus lowering DO.

thanks.

[jakesilove]

yah, Jake's right. Suspended solids are more for like turbidity - the .03g of solid left in the filter paper is the suspended solids. The DISSOLVED solids are what couldn't be trapped by filter paper, and thus can only be obtained after evaporating to dryness in an evap basin. (so TDS is 0.26g and go calc from there)

also.... soz continuing with the eutrophication questions: why do algal blooms consume dissolved oxygen anyway? Aren't they photosynthetic o.O (Creating oxygen), but I understand they block sunlight from getting to other photosynthetic organisms and thus lowering DO.

thanks.

It's actually the DECAYING of plant matter, grown in eutrophication, that consumes oxygen. Why? No clue, it just does.

[jarin_]

hey guys I don't understand the sample answer for this so could you please explain it:

Why does the neutralisation of any strong acid in an aqueous solution by any strong base always result in a heat of reaction of approx. -57 kJ/mol ?

[jakesilove]

hey guys I don't understand the sample answer for this so please could explain it:

Why does the neutralisation of any strong acid in an aqueous solution by any strong base always result in a heat of reaction of approx. -57 kJ/mol ?

So, you just need to think about the actual reaction. Every strong acid and strong base will be literally the same reaction! It's just H+ and OH-. Every reaction has a specific heat that is absorbed/released, which is -57 kJ/mol when it comes to neutralisation. So, identify that every strong acid/strong base reaction is the same, regardless of the actual acid or base. Then, discuss the above! Does that make sense!

[IkeaandOfficeworks]

Hi guys, can you explain to me why the answer is B ? Thank you.   

[jarin_]

So, you just need to think about the actual reaction. Every strong acid and strong base will be literally the same reaction! It's just H+ and OH-. Every reaction has a specific heat that is absorbed/released, which is -57 kJ/mol when it comes to neutralisation. So, identify that every strong acid/strong base reaction is the same, regardless of the actual acid or base. Then, discuss the above! Does that make sense!

ohh ok thanks that makes sense
but are we supposed to know the specific heat for all types of reactions?

[g98]

Hi,
What kind of info do we need to know about our chosen scientist?
Thanks

[WLalex]

Hey thanks for the answers to that previous question. Much appreciated and makes sense now.

However, I just came across this question from 2005 and it is virtually an exact repeat however the working to does not make sense to me.

In the success one books they misused the mass of the filtered solid (0.03g) from the sample (500) so it was 499.97 and divided the mass from evaporating (0.26) but that. Why did they do that in this question and not the other? and why would they even minus the suspended solids from the mass of the water in the first place..

thanks

[jakesilove]

Hey thanks for the answers to that previous question. Much appreciated and makes sense now.

However, I just came across this question from 2005 and it is virtually an exact repeat however the working to does not make sense to me.

In the success one books they misused the mass of the filtered solid (0.03g) from the sample (500) so it was 499.97 and divided the mass from evaporating (0.26) but that. Why did they do that in this question and not the other? and why would they even minus the suspended solids from the mass of the water in the first place..

thanks

Hey! I completely disagree with this answer; what the answers are suggesting is that not all 500mL of solution was water (some of it was dissolved solids), so when they divided by volume, they subtracted the weight of the suspended solids. That is wrong. Firstly, 500mL is a volume measurement, and 0.3 is a mass measurement. Whilst 500mL=500g, we don't actually care about the mass of the water, we care about the volume. Which brings me to the second point; concentration includes the entire volume. So, using 500mL would be correct. I'd say that the answer is just wrong.

[youknowwho]

Hey,
would someone please be able to help me solve this question:

what is the pH of a mixture of 20.0 mL of 0.102 mol L^-1 barium hydroxide solution and 40.0 mL of 0.150 mol L^-1 hydrochloric acid diluted to a final volume of 100 mL