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[monicak]

I have a quick question about significant figures...
For example if they give you 4 sets of data with:
1. 2 sig figs
2. 3 sig figs
3. 3 sig figs
4. 4 sig figs
And in part (a), you use data #1 and #2. Your result in part (a) will therefore be to 2 sig figs. Then in part (b) you have to use data #3 and #4 AND the result you got in part (a).. Would your answer to part (b) be to 2 sig figs as you used that data in part (a) and then indirectly in part (b), or to 3 sig figs as you directly used data #3 and 4? I guess what I am actually asking is, do you determine the significant figures from the data that you have used in that part of the question or do you take into account parts of the question you have previously completed and need to use  to determine part (b)?
Hope that makes sense. Thanks

[bec]

The answer says 'no effect on calculated result.' But, since there's water left over in the conical flask, wouldn't the concentration of phosphate be less than what I calculated, since I used 20 mL as my volume and the real volume is higher than that? Wouldn't the calculated result be too low?

no, you're getting confused with volumetric analysis i think. it doesn't matter if there's water in the conical flask, because all you're doing in this experiment is adding something that reacts with phosphate ions (and not water), then measuring the precipitate that's produced.

having water there won't have any effect on the amount of precipitate produced, and you can still use 20mL as your volume because that's how much fertiliser solution there is.


also, i've got another question...somewhere, in a practice exam i think, i remember coming across a multiple choice question with a graph of enzyme activity. the activity was low at low temperatures, then low again at high temperatures. the explanation for this was that the enzyme was denatured at high temperatures but "not yet activated" (? i think??) at low temperatures. what does this mean? has anyone seen the question?

[Glockmeister]

Enzymes have an optimal temperature to run in. If it is too low, less collision happen between the enzyme and the substrate, and therefore the rate is decreased.

[Matt The Rat]

Enzymes have a range of operational temperatures. Going well above the temperatures causes bonds to break and denaturation will occur. Going to a much cooler temperature causes the enzyme to not work, as it falls below it's minimal operational temperature, but does not necessarily result in denaturation. Re-warming the enzyme to an appropriate temperature generally results in the function returning to the enzyme.

*N.B. Prolonged exposure to the cold can result in the enzyme not being able to warmed and then work properly*

[vce01]

The answer says 'no effect on calculated result.' But, since there's water left over in the conical flask, wouldn't the concentration of phosphate be less than what I calculated, since I used 20 mL as my volume and the real volume is higher than that? Wouldn't the calculated result be too low?

no, you're getting confused with volumetric analysis i think. it doesn't matter if there's water in the conical flask, because all you're doing in this experiment is adding something that reacts with phosphate ions (and not water), then measuring the precipitate that's produced.

having water there won't have any effect on the amount of precipitate produced, and you can still use 20mL as your volume because that's how much fertiliser solution there is.


also, i've got another question...somewhere, in a practice exam i think, i remember coming across a multiple choice question with a graph of enzyme activity. the activity was low at low temperatures, then low again at high temperatures. the explanation for this was that the enzyme was denatured at high temperatures but "not yet activated" (? i think??) at low temperatures. what does this mean? has anyone seen the question?

okay, thanks

aand i guess your question has already been answered too. (good to know that i knew it too haha)

umm has anyone had a look at the 2002 VCAA exam? the 1st 4 questions, are they relevant to unit 3? according to this http://www.cea.asn.au/Relevant%20questions%20from%20past%20VCAA%20examinations%20for%202008.pdf
they're not. i tried them, got em all wrong and theres no worked out solutions anywhere!!

[Glockmeister]

1, 2 and 4 are

3 is not

[polky]

Remember for pH, a change in pH in either direction will cause the enzyme to denature.  For temperature, though, only an increase in the temperature (that exceeds the optimal temperature) will cause the protein to denature.  A cooler temperature does decrease activity but will not denature it!

[chem-nerd]

umm has anyone had a look at the 2002 VCAA exam? the 1st 4 questions, are they relevant to unit 3? according to this http://www.cea.asn.au/Relevant%20questions%20from%20past%20VCAA%20examinations%20for%202008.pdf
they're not. i tried them, got em all wrong and theres no worked out solutions anywhere!!

Question 1
NaCl (D) would have a pH closest to 7 as all the other options would act as an acid/base/or both and hence change pH

Question 2
equilibrium --> Unit 4

Question 4
2H+ + 2e- ---->  H2   thus reduction
2Br-  ---->  Br2 + 2e-  thus oxidation

thus H2SO4 is acting as an oxidant

[lanvins]

Whats the
The oxidation number of oxygen in HOF
and  The oxidation number of nitrogen in  NH4 NO?

[Mao]

Here's the full question:
The pH of a 10^(-9)M solution of HCl is closest to:
a) 4
b) 6
c) 7
d) 9
And the answer, on the answer sheet, is c...

the self ionisation of water creates about 10E-7 mols of OH- and equal amount of H+

so the OH- will react with the 10E-9 mol of H+ from HCl, and with equilibrium, ends up at about pH=7

[bucket]

are you sure its 0 in HOF?
Wouldn't it be like +1??
Or is the oxidation number of F always -1?

[Mao]

are you sure its 0 in HOF?
Wouldn't it be like +1??
Or is the oxidation number of F always -1?

F is most electronegative
H is most electro"positive" if that makes sense

O is stuck in the middle = 0

[bucket]

Yeah I know that, but wouldn't F assume the charge of -2 like Oxygen does when it is most electronegative.

[Matt The Rat]

Look where F is in the Periodic Table; Group 17.

[lanvins]

Write a balanced half equation for the conversion of ethanol into its conjugate
organic product.