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March 30, 2024, 12:01:54 am

Author Topic: VCE Chemistry Question Thread  (Read 2313820 times)  Share 

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PsychoT

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Re: VCE Chemistry Question Thread
« Reply #315 on: February 08, 2014, 10:50:35 am »
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Hi, I did the Volumetric determination of Nitrogen content in Lawn fertilser back titration prac yesterday, someone care to help me out with a few of these questions?

already know:
Average titre of HCl = 5.5mL (concordant titre)
c(HCl) = 0.1406
n(HCl) = 0.0007733 mol
v(NaOH) = 0.02L
c(NaOH) = 0.097M
n(NaOH) = 0.00194 mol

Equations:
NH4+ + OH- ---> NH3 + H20 (Ammonium compounding in the fertiliser)
HCl + NaOH ---> NaCl + H20

I'm now stuck when I get to here.

2d. Find the amount, in mol of Sodium Hydroxide that did not react with the fertiliser solution in each conical flask (is this just 0.00194 - 0.0007733)?
3. Then I need to calculate the amount of mol of Sodium Hydroxide that did react with the fertiliser solution (seems pretty easy once I know if my 2d is correct)
4. Find the amount of NH4+ ions present in each 20mL aliqout
5. Calculate the amount, in mol of NH4+ ions that were originally present in the 250mL volumetric flask
6. I should be about to find out the mass of nitrogen and the percentage by mass once I know this.

Will supply more info if needed, cheers guys!
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #316 on: February 08, 2014, 01:07:34 pm »
+2
Hi, I did the Volumetric determination of Nitrogen content in Lawn fertilser back titration prac yesterday, someone care to help me out with a few of these questions?

already know:
Average titre of HCl = 5.5mL (concordant titre)
c(HCl) = 0.1406
n(HCl) = 0.0007733 mol
v(NaOH) = 0.02L
c(NaOH) = 0.097M
n(NaOH) = 0.00194 mol

Equations:
NH4+ + OH- ---> NH3 + H20 (Ammonium compounding in the fertiliser)
HCl + NaOH ---> NaCl + H20

I'm now stuck when I get to here.

2d. Find the amount, in mol of Sodium Hydroxide that did not react with the fertiliser solution in each conical flask (is this just 0.00194 - 0.0007733)?
3. Then I need to calculate the amount of mol of Sodium Hydroxide that did react with the fertiliser solution (seems pretty easy once I know if my 2d is correct)
4. Find the amount of NH4+ ions present in each 20mL aliqout
5. Calculate the amount, in mol of NH4+ ions that were originally present in the 250mL volumetric flask
6. I should be about to find out the mass of nitrogen and the percentage by mass once I know this.

Will supply more info if needed, cheers guys!

The amount of sodium hydroxide that did not react with the ammonium ions is simply the amount that reacted with the HCl, which is 0.0007733 mol.
The amount that DID react with the fertiliser is 0.00194 - 0.0007733 mol.
Then you can work out n(NH4+) etc.
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PsychoT

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Re: VCE Chemistry Question Thread
« Reply #317 on: February 08, 2014, 01:24:06 pm »
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The amount of sodium hydroxide that did not react with the ammonium ions is simply the amount that reacted with the HCl, which is 0.0007733 mol.
The amount that DID react with the fertiliser is 0.00194 - 0.0007733 mol.
Then you can work out n(NH4+) etc.

That's perfect mate. Exactly what I was stuck on, cheers.
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Sanguinne

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Re: VCE Chemistry Question Thread
« Reply #318 on: February 08, 2014, 03:04:40 pm »
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Having a bit of trouble with the theory behind atomic absorption spectroscopy.

What does the monochromator actually do to the light after some of it has been absorbed by the metal atoms?
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Ancora_Imparo

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Re: VCE Chemistry Question Thread
« Reply #319 on: February 08, 2014, 05:06:18 pm »
+1
The monochromator essentially filters out all the unwanted wavelengths of light, such as those given off by the flame, leaving only a specific wavelength or a very narrow band of wavelengths to be passed to the recorder.
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Sanguinne

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Re: VCE Chemistry Question Thread
« Reply #320 on: February 08, 2014, 05:21:00 pm »
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The monochromator essentially filters out all the unwanted wavelengths of light, such as those given off by the flame, leaving only a specific wavelength or a very narrow band of wavelengths to be passed to the recorder.

I thought that the reason the cathode lamp was used was to produce wavelengths of light corresponding to the metal being analysed. If this is true, what is the difference between the lamp and the monochromator?
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Ancora_Imparo

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Re: VCE Chemistry Question Thread
« Reply #321 on: February 08, 2014, 05:45:09 pm »
+1
I thought that the reason the cathode lamp was used was to produce wavelengths of light corresponding to the metal being analysed. If this is true, what is the difference between the lamp and the monochromator?

That is indeed true. However, after the light has been passed through the flame where the metal atoms are being atomised, there will be interference from the flame itself. Thus the monochromator is used to ensure that these unwanted wavelengths are not passed through to the recorder.
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Sanguinne

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Re: VCE Chemistry Question Thread
« Reply #322 on: February 08, 2014, 06:33:32 pm »
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That is indeed true. However, after the light has been passed through the flame where the metal atoms are being atomised, there will be interference from the flame itself. Thus the monochromator is used to ensure that these unwanted wavelengths are not passed through to the recorder.

oh ok, thanks  :)
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darklight

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Re: VCE Chemistry Question Thread
« Reply #323 on: February 08, 2014, 09:56:17 pm »
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Hey guys,

In the Heinemann textbook, when showing how a chloroalkane can become an alkanol, it demonstrates that H+ (aq) and Cl-(aq) are separate in the solution. Why doesn't they join together to from HCl (aq). P. 149 for anyone who has the book :)
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #324 on: February 09, 2014, 10:16:08 am »
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For gas chromatography, is the stationary phase already inside the column? ..
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #325 on: February 09, 2014, 10:18:29 am »
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What is a dessicator and its function exactly?
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Ancora_Imparo

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Re: VCE Chemistry Question Thread
« Reply #326 on: February 09, 2014, 10:51:38 am »
+3
For gas chromatography, is the stationary phase already inside the column? ..

Yes.

What is a dessicator and its function exactly?

A desiccator is a container that is used to keep substances dry. It contains compounds known as desiccants which absorb water, thus keeping the substance in concern dry. 
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DJA

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Re: VCE Chemistry Question Thread
« Reply #327 on: February 09, 2014, 11:16:46 am »
+1
What is a dessicator and its function exactly?

Just to add to that definition: A desiccator is used to dehydrate – to remove water from substances – for example in gravimetric analysis where the water needs to be removed from the precipitate. Some common dehydrating agents that are placed in the bottom of the desiccator to remove the water are fused calcium chloride, concentrated H2SO4 or anhydrous copper sulphate.
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #328 on: February 09, 2014, 01:03:57 pm »
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thanks guys.

tsfx states that typical uncertainties in volumetric analysis are:
* 20ml pipette: +/- 0.05ml
* burette: +/- 0.02ml

I thought the pipette was more accurate than the burette? o.o
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #329 on: February 09, 2014, 01:17:03 pm »
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So ionic compounds, such as HCl, dissociate into their separate ions in aqueous solution. This is why solutions with ionic compounds are able to conduct electricity - there are always ions moving about, free to conduct electrical charge.

Thus, HCl actually exists as H+ and Cl- in solution and NaOH actually exists as Na+ and OH-, but for convenience, when writing chemical equations, we group them together.

Slight correction. HCl is a polar covalent compound. In its pure gaseous form, it is a covalent molecule. When dissolved in water, HCl reacts with water in an acid base reaction to form H3O+ (or H+(aq)) and Cl-. HCl is a strong acid as chloride ion is quite stable (full octet of electrons) so you only get H+ and Cl- in an aqueous solution of HCl.
If dissolved in liquid HF, however, HCl is protonated by the stronger acid (liquid HF is a strong acid unlike aqueous HF) and acts as a base. The ionization of HCl is a result of its reaction with water, not because it is ionic.

NaOH is ionic though so when that dissolves it really does split apart into ions.

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