Chemistry 3/4 2013 Thread

[SocialRhubarb]

Can a pH value be less than 0 or above 14, particularly within VCE/VCAA guidelines? If I received an answer outside of the 0 to 14 range, should I be crapping my pants? I recently came across a question where the answer was 14.4pH.

Consider the following reaction:
CH3COOH(aq)+ NaHCO3 (s) -> NaCH3COO(aq)+ CO2 (g) + H2O(l)

I chose A. However, why is the answer D?

I think the issue is the failure to describe the volume of CO₂ over time. The rate at which CO₂ is evolved does give you a good measure of the reaction rate, however the volume of CO₂ evolved will only tell you the extent of the reaction, not the rate.

Surface area is a determining factor in rate of reaction, so a measure of surface area, assuming things like concentration of reactants and temperature are the same, would give you a measure of rate of reaction.

To be honest, I don't really like this question.

In response to your pH question, there is no problem with negative pH or a pH greater than 14.

[lolipopper]

i got 68/73 on VCAA 2009 chem unit 3. is that good?

[Edward21]

Hey, could a smart cookie  tell me their process for working this out? I got my mol of butane, used the heat of combustion value to get the kJ released, converted it to J. Then used the 4.18 specific heat capacity formula solving for mass (assuming 1g/ml) and got answer D. BUT it asks on each of the days?? Wouldn't that be half, so cumulatively it's answer D, but for each day it would be half of that if it's two days? The actual answer is D which is why I am confused here. Any help would be greatly appreciated 

[Stevensmay]

Hoping this isn't a VCAA question as there are a few inconsistencies with it.

• Boiling point of water in these conditions is 112, not 100. Pretty sure this conversion is not on the course.
• Don't know if the canister is also pressurised to 1.5 atm, so we have to assume it is not.
• Density of water will not be 1g/mL at 10C
• SHC of water in these conditions will be 4.19 not 4.18[\li]
  

There is a reason we have SLC!

I know these are all minor, but they will affect the answer to the question by some degree.

I do agree with your way of thinking, that we should be halving 1970. But seeing as if we do this, we don't get any of the options we have to go with D. I feel that it should be 985.

[Patches]

i got 68/73 on VCAA 2009 chem unit 3. is that good?

Would be a mid to high A+

[Edward21]

Hoping this isn't a VCAA question as there are a few inconsistencies with it.

• Boiling point of water in these conditions is 112, not 100. Pretty sure this conversion is not on the course.
• Don't know if the canister is also pressurised to 1.5 atm, so we have to assume it is not.
• Density of water will not be 1g/mL at 10C
• SHC of water in these conditions will be 4.19 not 4.18[\li]
  

There is a reason we have SLC!

I know these are all minor, but they will affect the answer to the question by some degree.

I do agree with your way of thinking, that we should be halving 1970. But seeing as if we do this, we don't get any of the options we have to go with D. I feel that it should be 985.

Oh thankyou! Yeah, well it was a free TSFX question.. hmmm 

[thushan]

Would be a mid to high A+
[/quote

i got 68/73 on VCAA 2009 chem unit 3. is that good?

Yeah, with a solid Unit 4 exam on VCAA 2009 that would net you a 50. That year had a hard midyear.

[lzxnl]

Heyyyya,
I don't really understand these questions from a chemology exam.
Detailed explanations would be great!

Ch33rz

This is just from my point of view.
(a) The steel post, the carbon, acts as the cathode where the oxygen and water attack. As it is positive, it would repel any iron(II) ions formed, so they would form further away from the steel post. Insert equations here. Meanwhile, the iron is reactive enough to react with the air. Insert relevant equations here.
(b)Copper CAN rust...it's just not favoured. Over a long period of time, copper is still going to rust. The electrochemical series doesn't really work when there IS no copper(II) ions.
(c) Fire=hotter. Faster reaction.

Hoping this isn't a VCAA question as there are a few inconsistencies with it.

• Boiling point of water in these conditions is 112, not 100. Pretty sure this conversion is not on the course.
• Don't know if the canister is also pressurised to 1.5 atm, so we have to assume it is not.
• Density of water will not be 1g/mL at 10C
• SHC of water in these conditions will be 4.19 not 4.18[\li]
  

There is a reason we have SLC!

I know these are all minor, but they will affect the answer to the question by some degree.

I do agree with your way of thinking, that we should be halving 1970. But seeing as if we do this, we don't get any of the options we have to go with D. I feel that it should be 985.

Hahahaha conversions of boiling points at different temperatures. It's a real shame we can't do that in VCE.

[Jaswinder]

Barium hydroxide is soluble in water. The ph at 25 degree of a 0.005M solution of Ba(OH)2 is?
Idk how to do this could someone show me their steps? Answer is 12

[Jaswinder]

c(OH-)= 2 x 0.005M = 0.01M

I ALWAYS get confused with this step. I thought that in one mol of barium hydroxide there's two OH- therefore 0.005 should be divided by two?

[lzxnl]

In one mole of barium hydroxide, you have two moles of hydroxide ions.
Therefore n(Ba(OH)2) = 1
n(OH-) = 2 = 2n(Ba(OH)2)

Write it out and it'll make sense.

[barydos]

I always forget, but should only our final answers be in the correct significant figures, or should our working out also be in significant figures (and so there will be cumulative errors)?

[lzxnl]

Just in the final answer

[barydos]

Just in the final answer

Thanks

[thushan]

I always forget, but should only our final answers be in the correct significant figures, or should our working out also be in significant figures (and so there will be cumulative errors)?

I'd write working out to correct number of sig figs/decimal places, whilst keeping value in calculator - mainly because it makes navigating sig figs much easier since all you need to do is look at the previous step. And also because you can be sure you won't make mistakes when switching between sig figs and decimal places.