VCE Chemistry Question Thread

[keltingmeith]

Such as the molecule OF2

Alright, cool, I was just a little unsure of the question. So, the rules for "oxidation states" are based on electronegativity. If you know electronegativity, you can forget oxidation rules...

Basically, F(neg)>O(neg), so the electrons are going to drawn to the F. However, each flourine can only hold 1 valence electron, so they'll each take one. This puts both the F ions at an electronegativity of -1. Since we have a formal charge of 0, the oxygen must have a +2 oxidation state.

[Kel9901]

When assigning oxidation numbers, the rules state that oxygen always has an oxidation number of -2, except in peroxides such as H2O2 and BaO2.

The oxidation rules also say that the most electronegative element in a compound has the negative oxidation number.

So if you had a compound including oxygen and another element with a greater electronegativity (such as fluorine), is it not certified that oxygen would have an oxidation state of -2?

Thanks

Yeah, the exceptions to the -2 oxygen state are molecular oxygen (0), peroxides (-1) and F2O (+2)

[chocolate.cake.1]

Hi 
Would you guys recommend writing your answers in scientific notation (e.g. 1.72 x 10^(-4)) or writing out all the numbers to the correct number of sig figs (e.g. 0.000172)

For some reason I like to write out all the numbers, but I don't know if it would be better for SACs/exams...

[RazzMeTazz]

In a redox titration, if I2 was being reduced to I- ions , and starch indicator was used - would it be correct to say that, initially the solution will be blue/black (due to the presence of I2) but then will become colourless, once all the I2 has been reduced to I- ?

Thanks

[lzxnl]

Yeah, the exceptions to the -2 oxygen state are molecular oxygen (0), peroxides (-1) and F2O (+2)

Oh there are a few more...try assigning oxidation numbers to the superoxide and ozonide ions...

[RazzMeTazz]

I have a question about significant figure:

A 2.0gram sample of impure limestone is treated with 100 mL of 0.925 mol L-1 HCl. After the reaction is complete, 19.53 mL of 1.5 mol L-1 sodium carbonate is required to neutralise the left-over acid. Calculate the amount of HCl reacted with the CaCO3 in the limestone.

So, the n (HCl) = 100 × 10^-3 ×  0.925 = 0.0925 mol HCl. I would then give this to 3sf which would also be 0.0925mol HCl.

Then, n(Na2CO3) = 19.53 × 10^-3 × 1.5 = 0.029295mol Na2CO3
                                                                 = 0.029mol Na2CO3 (2sf)

n(Na2CO3):n(HCl) = 1:2
∴ n(HCl reacted with Na2CO3) = n(excess HCl) = n(Na2CO3)  × 2 = 0.05859mol excess HCl
                                                                                                         = 0.059mol excess HCl (2sf)


n(HCl reacted with CaCO3) = n(HCl) - n(excess HCl)
                                           = 0.0925 - 0.05859
                                            = 0.03381mol HCl But how many decimal places would I give this final value to? I know I need to give it to the least number of decimal places of the numbers involved in the subtraction, but would I consider these numbers before they were put in significant figure form or after? 

 Apologies for the really long question!

Also, on the chemistry exam, do the examiners look for all the values in your calculations to be expressed in correct significant figures or do they only look for the correct number of sig figs in the final answer?

Any help would be great!

[Splash-Tackle-Flail]

What are we actually supposed to write in an introduction for a prac report? Do we expelling the method we completed, or talk about the process and why we do it? THis is for gravimetric analysis and honestly I have no idea what to do!

[Kel9901]

Oh there are a few more...try assigning oxidation numbers to the superoxide and ozonide ions...

probably yeah lol i'm not that advanced xD, but they are the ones relevant to VCE chem

[wunderkind52]

I have a question about significant figure:

A 2.0gram sample of impure limestone is treated with 100 mL of 0.925 mol L-1 HCl. After the reaction is complete, 19.53 mL of 1.5 mol L-1 sodium carbonate is required to neutralise the left-over acid. Calculate the amount of HCl reacted with the CaCO3 in the limestone.

So, the n (HCl) = 100 × 10^-3 ×  0.925 = 0.0925 mol HCl. I would then give this to 3sf which would also be 0.0925mol HCl.

Then, n(Na2CO3) = 19.53 × 10^-3 × 1.5 = 0.029295mol Na2CO3
                                                                 = 0.029mol Na2CO3 (2sf)

n(Na2CO3):n(HCl) = 1:2
∴ n(HCl reacted with Na2CO3) = n(excess HCl) = n(Na2CO3)  × 2 = 0.05859mol excess HCl
                                                                                                         = 0.059mol excess HCl (2sf)


n(HCl reacted with CaCO3) = n(HCl) - n(excess HCl)
                                           = 0.0925 - 0.05859
                                            = 0.03381mol HCl But how many decimal places would I give this final value to? I know I need to give it to the least number of decimal places of the numbers involved in the subtraction, but would I consider these numbers before they were put in significant figure form or after? 

 Apologies for the really long question!

Also, on the chemistry exam, do the examiners look for all the values in your calculations to be expressed in correct significant figures or do they only look for the correct number of sig figs in the final answer?

Any help would be great!

When doing addition and subtraction, it doesn't come down to significant figures; it becomes a question of decimal places. It's probably easiest to see if you're working without x10^...s, but make sure everything is in the same unit!  In that case, 0.0925 has the least number of decimal places, so your answer will have 4 decimal places, 0.0338 which is 3sf.

Significant figures don't play a large role on the exam, and it's only relevant to your final answer. If you vide the examiner's reports, at times there is one mark on the whole exam that is specifically dedicated towards obtaining the correct number of significant figures.

[mondray]

Spectroscopy Question:
The Potassium content of an apple was measured using AAS. 6.0g of apple was treated with nitric acid and the resulting solution made up to 100.0mL. Using several potassium standard solutions, a calibration curve was constructed. The absorbance of the apple solution was then determined to be 0.3

a) Apples contain metal ions other than K+. Explain why these other ions do not interfere with the analysis performed to determine the K+ content of the apple.

[keltingmeith]

Spectroscopy Question:
The Potassium content of an apple was measured using AAS. 6.0g of apple was treated with nitric acid and the resulting solution made up to 100.0mL. Using several potassium standard solutions, a calibration curve was constructed. The absorbance of the apple solution was then determined to be 0.3

a) Apples contain metal ions other than K+. Explain why these other ions do not interfere with the analysis performed to determine the K+ content of the apple.

What does an AAS measure? -hint: it's unique for all metals-

[grannysmith]

When working out, say, the number of moles of a substance, is it required that we show the formula (i.e. n=m/M) in our working? Will we lose marks for not showing n=cv, n=m/M etc.?

[bts]

i have a prac about making paracetamol, why is it important to ensure that clean and dry conical flasks are used for the  preparation and recrystalisation of paracetamol? It would make much of a difference because its distilled water and I won't be diluting anything?

[wunderkind52]

When working out, say, the number of moles of a substance, is it required that we show the formula (i.e. n=m/M) in our working? Will we lose marks for not showing n=cv, n=m/M etc.?

Haven't actually sat a chef exam, but I generally include it (it makes your working out clearer to you) but I think it's mostly irrelevant on the exam! Definitely in a 1-mark question a simple answer is sufficient! For more than one mark, as long as you show SOME working (i.e. n=0.2*55. or something I think that suffices as well.

[grannysmith]

Haven't actually sat a chef exam, but I generally include it (it makes your working out clearer to you) but I think it's mostly irrelevant on the exam! Definitely in a 1-mark question a simple answer is sufficient! For more than one mark, as long as you show SOME working (i.e. n=0.2*55. or something I think that suffices as well.

Yeah, those were my sentiments exactly. But I suppose it's better to play it safe when it comes to peculiar SAC marking