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March 29, 2024, 04:31:47 am

Author Topic: VCE Chemistry Question Thread  (Read 2313298 times)  Share 

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Sanguinne

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Re: VCE Chemistry Question Thread
« Reply #345 on: February 12, 2014, 08:59:09 pm »
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q12 of the extended response in chemical analysis in checkpoints

Copper metal does dot dissolve in HCL, but can be dissolved by reaction with concentrated nitric acid, HNO3. The products are, Cu2+, NO2, and water. Give balanced ionic equations for

b) the reduction process
How would i know if its HNO3 or NO3- that goes under reduction. I thought it was HNO3 but the answer is NO3-

thanks
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scribble

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Re: VCE Chemistry Question Thread
« Reply #346 on: February 12, 2014, 09:04:38 pm »
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HNO3 is a very strong acid and usually ionises completely into H+ and NO3-.
I wouldn't say it matters hugely if you used HNO3 in your equation over NO3- though.

PsychoT

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Re: VCE Chemistry Question Thread
« Reply #347 on: February 13, 2014, 05:05:18 pm »
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I need an easy way to remember what Polar and Non-Polar molecules are. Can anyone explain it in a way that I might remember.
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jgoudie

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Re: VCE Chemistry Question Thread
« Reply #348 on: February 13, 2014, 08:00:22 pm »
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Generally non-polar molecules are symmetrical and polar molecules are asymmetrical.  In reality we it is not a black and white thing as in polar and non-polar, we deal with varying degrees polarity.  Here are few key things to remember:

If the molecule is only carbon it is highly non-polar.
If a molecule contains OH,NH2,NH,COOH then the degree of polarity depends on the length of the carbon chains and the amount of OH.  the longer the carbon chain the less polar it is, the more functional groups the more polar it is.

Hope this helps out a little.

I need an easy way to remember what Polar and Non-Polar molecules are. Can anyone explain it in a way that I might remember.
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PsychoT

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Re: VCE Chemistry Question Thread
« Reply #349 on: February 13, 2014, 08:31:29 pm »
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Generally non-polar molecules are symmetrical and polar molecules are asymmetrical.  In reality we it is not a black and white thing as in polar and non-polar, we deal with varying degrees polarity.  Here are few key things to remember:

If the molecule is only carbon it is highly non-polar.
If a molecule contains OH,NH2,NH,COOH then the degree of polarity depends on the length of the carbon chains and the amount of OH.  the longer the carbon chain the less polar it is, the more functional groups the more polar it is.

Hope this helps out a little.

Cheers Jason! Helps alot.

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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #350 on: February 13, 2014, 08:34:46 pm »
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A 150mL aqueous NaCl solution contains 0.0045g NaCl.
Calculate the concentration in ppm

easy way to do this?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #351 on: February 13, 2014, 09:42:30 pm »
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A 150mL aqueous NaCl solution contains 0.0045g NaCl.
Calculate the concentration in ppm

easy way to do this?

Yep. Very easy way, actually.
Parts per million is a mass fraction. Originally, it means 1 g solute per million grams solvent, but given that 1 L water is pretty close to 1000 g, we can say 1 ppm is really 1g/1000L. Or, rephrased, 1 ug/mL
So, we have 0.0045g/150 mL = 4500 ug/150 mL = 30ug/mL = 30 ppm
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #352 on: February 14, 2014, 12:01:42 am »
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So we don't need to calculate the concentration?
While I was doing the question, it felt like it took longer than It needed. :/
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Edward21

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Re: VCE Chemistry Question Thread
« Reply #353 on: February 14, 2014, 12:20:22 am »
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So we don't need to calculate the concentration?
While I was doing the question, it felt like it took longer than It needed. :/
Ppm is a particular unit ratio for concentration! For ease of use, we simply use either mg/L and ug/mL as they will both give you the ppm. But essentially it's telling you how many parts of what you're looking at, per 10^6 (or a million) parts of solvent, so this is used for small concentrations.

0.0045g of NaCl in 150mL? Let's convert!

Ppm=mg/L=(0.0045X10^3 mg/0.150L)=30mg/L or 30ppm.
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Re: VCE Chemistry Question Thread
« Reply #354 on: February 14, 2014, 05:20:58 pm »
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I don't understand this question at alllllllllllllllllllllll

1.5 mol of zinc metal was requiored to reduce 1 mol of vanadium compound. The redox half equation for the reduction of vanadium would be:

a) VO2+(aq) + 2H+(aq) + e -> V3+ + H2O (l)

b) VO2+ (aq) + 4H+ (aq) + 3e -> V2+ (aq) + 2H2O (l)

c) V3+ + e -> V2+(aq)

d) VO2+ (aq) + 2H+ (aq) + 2e -> V2+ (aq) + H2O (l)
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thushan

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Re: VCE Chemistry Question Thread
« Reply #355 on: February 14, 2014, 06:13:41 pm »
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Think about how many moles of electrons 1.5 mol of zinc gives off when it is oxidised to Zn2+. That number of electrons would be mopped up by 1 mol of vanadium.
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #356 on: February 14, 2014, 06:41:26 pm »
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Think about how many moles of electrons 1.5 mol of zinc gives off when it is oxidised to Zn2+. That number of electrons would be mopped up by 1 mol of vanadium.

I'm so sorry but I still don't understand, what do you mean by 'how many moles of electrons'? I didn't know electrons have a mole number?

Is it because 2e are needed for the oxidation of Zn and since there is .5 more mole of zinc metal than of the vanadium compound that the amount of electrons are 1? ...? O.ooo
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Ancora_Imparo

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Re: VCE Chemistry Question Thread
« Reply #357 on: February 15, 2014, 12:09:46 pm »
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I'm so sorry but I still don't understand, what do you mean by 'how many moles of electrons'? I didn't know electrons have a mole number?

Is it because 2e are needed for the oxidation of Zn and since there is .5 more mole of zinc metal than of the vanadium compound that the amount of electrons are 1? ...? O.ooo

The word 'mole' just refers to Avogadro's number, which is approximately 6.02 * 1023. You can apply 'mole' to anything, not just atoms or molecules - the number of apples I have, the number of times I've failed at Chemistry etc. So if I have one mole of electrons, it just means that I have 6.02 * 1023 electrons.

So if we have 1.5 mol of zinc, and each zinc atom gives off 2 electrons when it is oxidised to Zn2+, we would have 1.5 * 2 = 3.0 mol of electrons.

Now we know that 3.0 mol of electrons are required to reduce 1 mol of the vanadium compound. That means the ratio between the electrons and the vanadium compound in the equation must be 3:1. Hence, the answer is equation b).
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Re: VCE Chemistry Question Thread
« Reply #358 on: February 16, 2014, 06:39:45 pm »
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Why do larger alcohol molecules become progressively less soluble?

My answer:
That the influence of the OH- group and its ability to make hydrogen bonds with water is diminished when the chain increases, in that the straight and unreactive C-C chain dominates. The increase in the alkanol homologous series leads to a decreasing solubility ability.

Please improve my answer  :)

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Re: VCE Chemistry Question Thread
« Reply #359 on: February 16, 2014, 09:57:04 pm »
+1
Why do larger alcohol molecules become progressively less soluble?

My answer:
That the influence of the OH- group and its ability to make hydrogen bonds with water is diminished when the chain increases, in that the straight and unreactive C-C chain dominates. The increase in the alkanol homologous series leads to a decreasing solubility ability.

Please improve my answer  :)

The hydroxyl group is neutral. It is best to just refer to it as a -OH group. Larger alcohol molecules have a longer non-polar end, the carbon chain, and the longer the carbon chain the more dominant the non-polar, and thus insoluble, part of the molecule becomes. Eventually, it is no longer favourable for the water molecules to bond to the alkanol; they'd rather bond to themselves as they're much more polar than the alkanol.
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