VCE Chemistry Question Thread

[Sine]

Question on spectroscopy

What should we know about double beam scanning?, How doesnit compensate for changes in output of radiation source?

How does IR radiation bend and stretch bonds?

In UV vis spectroscopy how is wavelength and hence wave number changed to produce accurate curves?

Also,do we need to know the intricacies of the instrumentation or just a generalised view of how it works?

Thanks

[keltingmeith]

What should we know about double beam scanning?, How doesnit compensate for changes in output of radiation source?

Nothing.

How does IR radiation bend and stretch bonds?

It excites the molecules which move based on how many degrees of vibrational freedom they have. For non-linear molecules, there are 3N-6 modes, where N=the number of atoms. This does not need to be known for VCE.

In UV vis spectroscopy how is wavelength and hence wave number changed to produce accurate curves?

Wavelength is changed by changing the colour of the light that we shine through the analyte.

Also,do we need to know the intricacies of the instrumentation or just a generalised view of how it works?

Just the basic basics. (eg, AAS works because each metal has its own specific absorption spectrum. Done.)

[Biology24123]

I was hoping someone could help me out

For part b ii would this get full marks as its different working to VCAA( dont mind sig figs)

C(MnO4)=35 mg/L
In 100ml 35/10=3.5 mg
3.5 x 1000/25=140mg
(55/119) x 140 = 64mg (55/119 in percentage composition of Mn in MnO4)

Thanks

[_inspire45]

Guys,

Can anyone please go through a run through of the uses of different spectroscopic techniques? Like when a q says "what would you use to analyse a blood sample", sometimes I don't know which one to use because some of them overlap
AAS, UV VIS, Colorimetry, IR, NMR, AES, FLAME TESTS

THANKS!!

[Peanut Butter]

Can someone please explain how to do this question:

What volume of 0.100 M sulfuric acid would be required to neutralise a solution containing 0.500 g of sodium hydroxide and 0.800 g of potassium hydroxide?

Spoiler

Answer is 0.134 L

Thank you

[Biology24123]

Can someone please explain how to do this question:

What volume of 0.100 M sulfuric acid would be required to neutralise a solution containing 0.500 g of sodium hydroxide and 0.800 g of potassium hydroxide?

Spoiler

Answer is 0.134 L

Thank you

n(NaOH)=0.500/40.0=0.0125
n(H2S04)= 0.0125/2= 0.00625 (balanced equation)

n(KOH) = 0.800/56=0.014285 mol
n(H2S04)= 0.014/2=0.0071425

0.00625+0.0071425=0.0134 mol

V(H2S04)= 0.0134/0.1 = 0.134 (Not bothered with sig figs)


So basically find mole of H2SO4 in separate reactions and add them together. Then use n=cv

[knightrider]

For this question attached

shouldn't the final answer be given to 4 sig figs.

in the book the answer has 3 sig figs?

[knightrider]

For this question attached

shouldn't the final answer be given to 4 sig figs.

in the book the answer has 3 sig figs?

Can anyone help ? 

[Splash-Tackle-Flail]

For this question attached

shouldn't the final answer be given to 4 sig figs.

in the book the answer has 3 sig figs?

m(jam+water)=30.95-20.22=10.73g
m(jam)=26.49-20.22=6.27g
m(water)=10.73-6.27=4.46g
you can do the rest
^since addition/subtraction goes to decimal places, that's where your 3 sig figs come from

[knightrider]

m(jam+water)=30.95-20.22=10.73g
m(jam)=26.49-20.22=6.27g
m(water)=10.73-6.27=4.46g
you can do the rest
^since addition/subtraction goes to decimal places, that's where your 3 sig figs come from

Thanks Splash-Tackle-Flail   

Do you know a place where i can learn about all the rules related to sig figs for unit 3/4 chem ?

[keltingmeith]

Thanks Splash-Tackle-Flail   

Do you know a place where i can learn about all the rules related to sig figs for unit 3/4 chem ?

This guy's series is pretty good. Would recommend/10

[Splash-Tackle-Flail]

Thanks Splash-Tackle-Flail   

Do you know a place where i can learn about all the rules related to sig figs for unit 3/4 chem ?

Hmm I think I learned mine from watching Crash Course haha.

Really all you need to know (iirc), is that sig figs are a measure of the accuracy of a value/measurement, and that in multiplication/division, you go to the lowest sig figs used in the calculation, while addition/subtraction goes to the lowest decimal place (which is rational as 14.9 and 4.9 would be expected to be to the same accuracy despite their different sig figs).

Then consider when a 0 is before a value, for example in 0.0059g. The zeros merely describe the magnitude of the value, and don't give an indication of its accuracy. This is demonstrated when you rewrite it in scientific notation: 5.9*10^(-3)g, which shows there is, in fact only 2 sig figs in this value. Also note that 0s after a decimal but after a number as well, e.g. 5.05 are significant, so this value has 3 sig figs.

There's more cases but I'd just look at your nearest website such as http://www.sciencegeek.net/Chemistry/taters/Unit0SigFigCounting.htm

hope it helps!

Edit: ninja'd but here's more places to learn about the magic of sig figs lol

[knightrider]

Hmm I think I learned mine from watching Crash Course haha.

Really all you need to know (iirc), is that sig figs are a measure of the accuracy of a value/measurement, and that in multiplication/division, you go to the lowest sig figs used in the calculation, while addition/subtraction goes to the lowest decimal place (which is rational as 14.9 and 4.9 would be expected to be to the same accuracy despite their different sig figs).

Then consider when a 0 is before a value, for example in 0.0059g. The zeros merely describe the magnitude of the value, and don't give an indication of its accuracy. This is demonstrated when you rewrite it in scientific notation: 5.9*10^(-3)g, which shows there is, in fact only 2 sig figs in this value. Also note that 0s after a decimal but after a number as well, e.g. 5.05 are significant, so this value has 3 sig figs.

There's more cases but I'd just look at your nearest website such as http://www.sciencegeek.net/Chemistry/taters/Unit0SigFigCounting.htm

hope it helps!

Edit: ninja'd but here's more places to learn about the magic of sig figs lol

This guy's series is pretty good. Would recommend/10

Thanks so much Splash-Tackle-Flail  and EulerFan101 

[knightrider]

How would you write a balanced equation for the following question?

[Splash-Tackle-Flail]

How would you write a balanced equation for the following question?

I'd write NaCl(aq) + AgNO3(aq)->AgCl(s)+NaNO3(aq)

You know that the initial compound is table salt from the question, then you know that a substance consisting of some Ag is added. Since NO3 is a common soluble companion to Ag (is companion the right word lol), I'd use AgNO3.

NB:spectator ions not required

Edit: AgNO3 state to aqueous