HSC Chemistry Question Thread

[jakesilove]

Hello all! Just wondering when you would use NAGSAG? is it genreally in identifying stuff in formulas or is that a syllabus dot point thingy?

I would use NAGSAG whereever it's relevant! Not entirely sure what you mean by the question; if it's something to do with 'will this substance be soluble', or 'how do you separate these substances', NAGSAG is your best friend

[jakesilove]

For those who don't know, check out what NAGSAG is (great mnemonic device) in my comprehensive Chemistry formula sheet!

[RuiAce]

Hello all! Just wondering when you would use NAGSAG? is it genreally in identifying stuff in formulas or is that a syllabus dot point thingy?

Basically as Jake has said, whenever it is relevant. Some situations where it is relevant include precipitation tests and just general writing correct states on chemical equations.

There is no specific time to use it. You use it when they're appropriate.

They are just mnemonics to solubility rules.

[xorichard]

So is NAGSAG a base to understanding which compounds can be broken up into its ions? Thanks!

[RuiAce]

So is NAGSAG a base to understanding which compounds can be broken up into its ions? Thanks!

If by base you meant basis, sure, certainly

Base is a bit misleading in chemistry for... you know... acid/base stuff lol


Technicality note though - make sure you realise that when you say "broken up", it's as a part of dissolution. If you don't dissolve it then it can't be broken up.

[tennis1]

Hey guys,

I just have a couple of random Q's:
(my teacher barely uses email so I thought I'd just post them here)

*Why does a decrease in pH, increase the rate of corrosion


(c) Before performing the experiment, the glassware was washed with the solutions given in the table.  (4 marks)
Washing procedure
The conical flask was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated

The burette was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated

Co(H2O)62+(aq) + 4 Cl–(aq) ⇌ CoCl42–(aq) + 6 H2O(l)

If H20 is added:
 does it favour the side with the H2O (reverse(t,
 OR
 not affect equilibrium at all since it is a liquid & has no concentration,
OR decrease the concentration of all the species and so favour the side with the least moles?  (forward)

and

also if a solid is added does it only not affect equilibrium if it is insoluble
thanks!

[RuiAce]

Hey guys,

I just have a couple of random Q's:
(my teacher barely uses email so I thought I'd just post them here)

*Why does a decrease in pH, increase the rate of corrosion


(c) Before performing the experiment, the glassware was washed with the solutions given in the table.  (4 marks)
Washing procedure
The conical flask was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated

The burette was washed with distilled water.
Effect on the volume of hydrochloric acid used
Effect on the % of aspirin calculated

Co(H2O)62+(aq) + 4 Cl–(aq) ⇌ CoCl42–(aq) + 6 H2O(l)

If H20 is added:
 does it favour the side with the H2O (reverse(t,
 OR
 not affect equilibrium at all since it is a liquid & has no concentration,
OR decrease the concentration of all the species and so favour the side with the least moles?  (forward)

and

also if a solid is added does it only not affect equilibrium if it is insoluble
thanks!

I didn't do shipwrecks but Jake did so he can have those.

H₂O does nothing because like said, it is a liquid and hence no concentration change.
And for a similar reasons - solids have no concentration.
If a solid dissolves, note that it becomes aqueous and thus WILL have a concentration

With that part c), post up the rest of the question please or we cannot be sure of what we're doing

[jakesilove]

Hey guys,


*Why does a decrease in pH, increase the rate of corrosion

Hey! There are a whole bunch of ways to think about this; I think the quantitative method is easiest. Remember, corrosion is essentially when an electrolytic cell is set up between Iron and water, or iron and something else, in which Iron LOSES electrons (ie. oxidises). The cell between Iron and water has a specific voltage, that you can calculate using the table of standard potentials. This is a 'path' of corrosion. However, with excess H+ ions in the water, Iron may 'prefer' to set up a cell with the H+ instead! This cell will have a lower voltage (again, see the table of standard potentials) and thus will be EASIER to move forward. So, the overall effect is that it becomes easier for Iron to corrode; the same amount of energy will corrode MORE iron! I hope that makes sense, let me know if I can clarify anything!

Jake

[tennis1]

I didn't do shipwrecks but Jake did so he can have those.

H₂O does nothing because like said, it is a liquid and hence no concentration change.
And for a similar reasons - solids have no concentration.
If a solid dissolves, note that it becomes aqueous and thus WILL have a concentration

With that part c), post up the rest of the question please or we cannot be sure of what we're doing

ok thanks Also here is the first part of part c)
In one commercial brand of aspirin, each ‘300 mg tablet’ is claimed to contain 100% aspirin. To determine the actual percentage by mass of aspirin in an aspirin tablet, the following procedure, involving a back titration, was used.
Step 1: Step 2:
Three aspirin tablets, each with a mass of 300.0 mg, were crushed and dissolved in excess sodium hydroxide solution. Exactly 100.0 mL of 0.204 mol L–1 solution of sodium hydroxide was used. The mixture was boiled to ensure complete reaction.
The excess sodium hydroxide solution was titrated with hydrochloric acid as follows: 20.0 mL of the solution from step 1 was pipetted into a conical flask and 0.125 mol L–1 hydrochloric acid was placed in the burette. The indicator, phenolphthalein, was used and an average titre of 17.89 mL of hydrochloric acid was required to reach the end-point.

[tennis1]

Hey! There are a whole bunch of ways to think about this; I think the quantitative method is easiest. Remember, corrosion is essentially when an electrolytic cell is set up between Iron and water, or iron and something else, in which Iron LOSES electrons (ie. oxidises). The cell between Iron and water has a specific voltage, that you can calculate using the table of standard potentials. This is a 'path' of corrosion. However, with excess H+ ions in the water, Iron may 'prefer' to set up a cell with the H+ instead! This cell will have a lower voltage (again, see the table of standard potentials) and thus will be EASIER to move forward. So, the overall effect is that it becomes easier for Iron to corrode; the same amount of energy will corrode MORE iron! I hope that makes sense, let me know if I can clarify anything!


Jake

Yeah that makes sense! thank you

[albert_c]

Hi, I'm not sure if this has been asked or not but in HSC 2009 Question 18 (5 Marker)

"There has been an increase in the concentration of the oxides of nitrogen in the atmosphere as a result of combustion.
Assess both the evidence to support this statement and the need to monitor these oxides"

What do I need to talk about to ensure 5 marks in this response?

Thanks in advance

[onepunchboy]

Hey guys for the Q : The heat of combustion of ethanol is 1360 KJ mol^-1. What mass of ethanol needs to be burnt to raise the temp of 250g of water through 40 degrees, if 55% of the heat released by ethanol is lost to the surroundings?

After calculating -mcAt, why do you divide by 0.45 instead of times 0.45?

Thanks in adv

[RuiAce]

Hi, I'm not sure if this has been asked or not but in HSC 2009 Question 18 (5 Marker)

"There has been an increase in the concentration of the oxides of nitrogen in the atmosphere as a result of combustion.
Assess both the evidence to support this statement and the need to monitor these oxides"

What do I need to talk about to ensure 5 marks in this response?

Thanks in advance

Look at the marking criteria first.

And also plan your response. Two examples to start you off, however
- Incidents of photochemical smog
- Acid rain

Hey guys for the Q : The heat of combustion of ethanol is 1360 KJ mol^-1. What mass of ethanol needs to be burnt to raise the temp of 250g of water through 40 degrees, if 55% of the heat released by ethanol is lost to the surroundings?

After calculating -mcAt, why do you divide by 0.45 instead of times 0.45?

Thanks in adv

[onepunchboy]

wait so when do you times the heat loss with the enthalpy and when do you divide, because alot of questions you need to times... i get confused

[RuiAce]

wait so when do you times the heat loss with the enthalpy and when do you divide, because alot of questions you need to times... i get confused

Break down the question. Every question is a different case.

Although here I apologise, since I think I stuffed it up at the start.


If we want enough heat to COUNTERACT the change, we need mass to be greater. Hence it should be what I am about to edit above.
In other words, the mass must be LARGE enough (higher than usual) to counteract the heat loss

I treat these questions the same way I treat relativity in physics. I just use logic and  common sense to figure out what's right.