VCE Chemistry Question Thread

[cosine]

To draw the Lewis Structure of , the answer has N double bonded to only one P atom. I understand how and why this works, but I do not understand why N can not form two double bonds with two O atoms, and form a single bond with the remaining O atom. If this is the case, the two double bonded O atoms will have 6 valence electrons, and the single bonded atom would have 7 electrons which explains the negative charge on the ion, and the N atom would have its usual 5 valence electrons.

Thank you

[lzxnl]

Nitrogen can't make five bonds. Ever.
In simple form, if it did, it'd gain 5 electrons and have 10 valence electrons, which isn't allowed.

In a slightly more complicated form, there are 4 2s+2p orbitals in total for nitrogen to use for bonding, which in combination with the atomic orbitals from the oxygen leads to 8 molecular orbitals, four pairs of bonding and four pairs of antibonding molecular orbitals. Antibonding molecular orbitals are, as per the name, unstable if filled (they're less stable than the original atomic orbitals). If nitrogen were to make five covalent bonds, there would be ten electrons that would be in molecular orbitals. This means that there would be four bonding and one antibonding filled, which isn't stable. Instead, the electrons in the antibonding orbital would leave and return to their original atoms where they are more stable (chemically the same as breaking an extra bond).

[Maz]

hey, can i please get some help with this question?
when you suffer from an acid stomach, the fluid in your stomach can contain hydrochloric acid at
a concentration of 2.5x10^-4 mol L^-1
to reduce the amount of acid in your stomach you are advised to take an antacid tablet that contains 5mg of aluminium hydroxide and 5mg of magnesium hydroxide. if at this time you have 1.5L of fluid in your stomach calculate the concentration of hydrogen ions and hydroxide ions in you stomach after you have taken the tablet.
if you took a second antacid tablet what would the hydrogen and hydroxide ion concentration be now?
i'd really appreciate the help...
thankyou

[Mishmosh]

Why is paper chromatography more efficient for polar and water soluble compounds while TLC is better for less polar compounds?

[Elizawei]

Hey, got a quick MC question:

Which of the following measurements would be most useful in identifying an unknown component separated by a gas chromatograph?

A- distance moved by the component through the stationary phase
B- width of the component's peak on the recorder chart
C- time taken for the component to pass through the instrument
D- height of the component's peak on the recorder chart


I'm hesitating between C and D, but I'm not very sure which one is right.
Thanks in advance!!! 

[Sine]

Hey, got a quick MC question:

Which of the following measurements would be most useful in identifying an unknown component separated by a gas chromatograph?

A- distance moved by the component through the stationary phase
B- width of the component's peak on the recorder chart
C- time taken for the component to pass through the instrument
D- height of the component's peak on the recorder chart


I'm hesitating between C and D, but I'm not very sure which one is right.
Thanks in advance!!! 

A. no - thats paper and TLC
B. no - kinda quantitative (not really)
C. yes - Retention times can be compared to known standards run at same conditions or compared to known Rt values(run in same conditions)
D. no   - also more quantitative

So C is the correct answer

[vox nihili]

A. no - thats paper and TLC
B. no - kinda quantitative (not really)
C. yes - Retention times can be compared to known standards run at same conditions or compared to known Rt values(run in same conditions)
D. no   - also more quantitative

So C is the correct answer

Agree with your answer, but just wanted to add that the width actually gives you more of an idea about the ability of the set-up to discriminate between different molecules than anything else. Certainly not quantitative though.

[Sine]

Agree with your answer, but just wanted to add that the width actually gives you more of an idea about the ability of the set-up to discriminate between different molecules than anything else. Certainly not quantitative though.

how would that work?

would it be like; greater width -->bad ability  and narrow--->good ability

because i thought width just meant how "seperated" parts of one component was from each other just per chance.

[kingy123]

Can someone please help me with this question:

When 120 ml of a gaseous hydrocarbon was burnt in 400mL Oxygen, which was more than sufficient, 280ml of gas was present at the end of the reaction after the products were cooled to STP. The gas mixture was bubbled through concentrated LiOH, which absorbed all the CO2 present, leaving 40ml of gas. What was the molecular formula of the hydrocarbon, if all gas volumes had been measured at STP?

The correct answer is C2H4 , but how do you get there?

[Adequace]

http://imgur.com/a/z1nCc

I'm having trouble with a and b for some reason, got the rest of them correct though.
The answer has 5 and 2.5 mol for a and b respectively. I'm not sure how? I'm fairly confident in my working, but just need reassurance or corrections. Thanks.

[@#035;3]

Do we have to know about co-factors and co-enzymes for VCE CHEM?

[Shax]

For chemical analysis, is it required to memorize all the theory behind the techniques and how the instruments obtain the results? It is just that the textbook contains lots of details about the steps of the analyses with little emphasis on how the results are interpreted. Does this comply with vcaa standards?
Any help, guys??   

[ally12579]

For chemical analysis, is it required to memorize all the theory behind the techniques and how the instruments obtain the results? It is just that the textbook contains lots of details about the steps of the analyses with little emphasis on how the results are interpreted. Does this comply with vcaa standards?
Any help, guys??   

This is from the chemistry study design:
 principles and applications of spectroscopic techniques (excluding features of instrumentation and
operation), and interpretation of qualitative and quantitative data
So you must know when the instrument is used and how to use the results from the instrument but you don't need to know how they work.

Do we have to know about co-factors and co-enzymes for VCE CHEM?

I don't think you need to learn about co-factors or co-enzymes in VCE Chem

[ankita]

Hey
I'm a year 9 student (not VCE) but could someone simply yet in detail explain to me about chemical bond, nuclear decay and reaction rates?
Thankyou so much

[@#035;3]

Can someone please explain to me why the melting point of propane is lower then that of methane and ethane?
cheers