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March 29, 2024, 07:11:02 am

Author Topic: VCE Chemistry Question Thread  (Read 2313381 times)  Share 

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Stick

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Re: VCE Chemistry Question Thread
« Reply #375 on: February 22, 2014, 04:04:43 pm »
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The answer is B because bonds are always vibrating - however infrared radiation can alter their degree of vibration. Option A implies that bonds are static unless infrared radiation is applied.
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Bronzebottom64

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Re: VCE Chemistry Question Thread
« Reply #376 on: February 22, 2014, 05:47:23 pm »
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Hi all, I was doing some preparations for a SAC on Monday about Redox titrations. I was wondering if anyone could just give me run through of any example involving a dilution. Thanks
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Re: VCE Chemistry Question Thread
« Reply #377 on: February 22, 2014, 05:55:01 pm »
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Hi all, I was doing some preparations for a SAC on Monday about Redox titrations. I was wondering if anyone could just give me run through of any example involving a dilution. Thanks

The thing to remember is to find number of moles of X in conical flask containing 20mL aliquot. Then, to find n(X) in say the 250mL volumetric flask, you multiply n(X) by (250/20). Work from there :)

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Re: VCE Chemistry Question Thread
« Reply #378 on: February 22, 2014, 06:47:41 pm »
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The answer is B because bonds are always vibrating - however infrared radiation can alter their degree of vibration. Option A implies that bonds are static unless infrared radiation is applied.

That makes sense. Thanks for that Stick :)
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Re: VCE Chemistry Question Thread
« Reply #379 on: February 22, 2014, 07:34:21 pm »
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Can someone pls explain the uses of each spectroscopy technique briefly?
eg: uv spectroscopy is used to determine the concentration of a substance

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Re: VCE Chemistry Question Thread
« Reply #380 on: February 22, 2014, 07:41:51 pm »
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Hey guys

How much do we need to know about Biochemical fuels?

Thanks

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Re: VCE Chemistry Question Thread
« Reply #381 on: February 23, 2014, 02:07:07 pm »
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pls exlain briefly the chemical principles of AAS. What is it and how does it work?
My teacher has explained it a number of times but I still dont get it :(
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Re: VCE Chemistry Question Thread
« Reply #382 on: February 23, 2014, 02:35:58 pm »
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pls exlain briefly the chemical principles of AAS. What is it and how does it work?
My teacher has explained it a number of times but I still dont get it :(

Electrons in atoms are arranged in shells. To move an electron from one shell to a outer shell, you need to input a specific amount of energy (exactly X joules, anything just under or over it will not work), since it takes energy to pull an electron away from the positively charged nucleus.

Different elements have different distances between their shells. This is because every element has a different nuclear charge (eg. sodium has a nuclear charge of +11), which tends to pull all the electrons towards the nucleus. Also, different elements have a different electron configuration (eg. sodium has a 2, 8, 2 configuration). The more electrons on the outer shell, the further from the nucleus the shell tends to go, because the electrons repel one another.

If different elements have different distances between their shells, the energy required to move electrons from one shell to a higher shell will be different for different elements. Hence, different elements can absorb only specific energies.

In AAS, the energy is delivered as a light, which is basically a bunch of photons (light particles). Each photon has an energy, which is proportional to its frequency (E = hv, where E = energy, v = frequency and h is a constant). Different elements therefore absorb different specific frequencies of light - which is how one can differentiate between elements in AAS.
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Re: VCE Chemistry Question Thread
« Reply #383 on: February 23, 2014, 02:48:00 pm »
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Electrons in atoms are arranged in shells. To move an electron from one shell to a outer shell, you need to input a specific amount of energy (exactly X joules, anything just under or over it will not work), since it takes energy to pull an electron away from the positively charged nucleus.

Different elements have different distances between their shells. This is because every element has a different nuclear charge (eg. sodium has a nuclear charge of +11), which tends to pull all the electrons towards the nucleus. Also, different elements have a different electron configuration (eg. sodium has a 2, 8, 2 configuration). The more electrons on the outer shell, the further from the nucleus the shell tends to go, because the electrons repel one another.

If different elements have different distances between their shells, the energy required to move electrons from one shell to a higher shell will be different for different elements. Hence, different elements can absorb only specific energies.

In AAS, the energy is delivered as a light, which is basically a bunch of photons (light particles). Each photon has an energy, which is proportional to its frequency (E = hv, where E = energy, v = frequency and h is a constant). Different elements therefore absorb different specific frequencies of light - which is how one can differentiate between elements in AAS.

Thanks for that. I get it but I dont understand how the process occurs. What is the need for the flame and the other instruments such as a monochromator?
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Limista

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Re: VCE Chemistry Question Thread
« Reply #384 on: February 23, 2014, 03:33:42 pm »
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Thanks for that. I get it but I dont understand how the process occurs. What is the need for the flame and the other instruments such as a monochromator?

Purpose of these instruments = to carry out Atomic Absorption Spectroscopy.

The only things you need to know about these instruments in 3/4 chemistry, is a brief definition of how they function and the order in which they function.



By 'order', I also mean that you should be asking yourself the steps involved in AAS to determine the concentration of the sample, and then how you would obtain the concentration of an unknown sample (in this case, you would plot a graph with concentrations of different known samples on y axis, absorbance readings on x axis, then draw a line of best fit. Use the absorbance reading of the unknown sample and apply to graph to determine concentration of unknown sample).
« Last Edit: February 23, 2014, 03:36:35 pm by Starfish »
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Rishi97

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Re: VCE Chemistry Question Thread
« Reply #385 on: February 24, 2014, 07:44:22 pm »
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Help pls:)

A group of students dissolved a 1.028g sample of a Group 2 metal ion chloride anhydrous solid in water so that the total volume was 250.0mL. They then titrated 20.00ml aliquots of this solution with a 0.1074 M aqueous silver nitrate solution to precipitate the silver chloride. The average titre was found to be 13.78mL. The reaction between silver and chloride ions can be represented by:
Ag++Cl---> AgCl(s). Both solutions are aqueous.
The group 2 metal ion present in the sample would be:
a) Mg2+
b) Ba2+
c) Sr2+
d) Ca2+
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Re: VCE Chemistry Question Thread
« Reply #386 on: February 24, 2014, 08:36:36 pm »
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Find no of mol of AgNO3
n=cv
  =0.1074(0.01378)
 =0.00148mol
n of Ag+=0.00148mol
n of Cl-in aliquot =0.00148mol
n of Cl- in250ml =250/20 (0.00148)
                         =0.0185mol
n of MCl2 = 1/2 x 0.0185mol
               =0.00925mol
Mass/ no of mol = molar mass
Molar mass= 1.028/0.00925=111.1g/mol
111.1-2(35.5)=40.1= molar mass of the metal ion which is Ca2+


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Re: VCE Chemistry Question Thread
« Reply #387 on: February 24, 2014, 09:02:59 pm »
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What is co-precipitation exactly?
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Re: VCE Chemistry Question Thread
« Reply #388 on: February 25, 2014, 07:04:07 am »
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In infrared spectrosopy, how do you distinquish the different bonds when the peaks in an infrared spectrum are really similar?

For example, I was asked to identify what bond is undergoing a vibrational change in ethanol at 2950cm-1cm
So
The O-H bond occurs at a wavenumber of 2500-3300
The C-H bond occurs at 2850 - 3300

I know both the bonds exist in ethanol and the wave numbers fit for both.. so why is it C-H and also how do they know that it's "streching"?
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Re: VCE Chemistry Question Thread
« Reply #389 on: February 25, 2014, 07:45:39 am »
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This is just means that you don't get the one precipitate that you are looking for.  You might get other precipitates forming that will add to the mass on the filter paper.

Things like Carbonates, Hydroxides, Phosphates can all create precipitates, by the addition of an acid this lowers their likely hood of forming and thus messing up your analysis.

What is co-precipitation exactly?
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