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August 12, 2020, 04:24:03 pm

Author Topic: Tricks of the trade  (Read 13427 times)  Share 

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Martoman

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Tricks of the trade
« on: June 04, 2010, 10:34:25 pm »
+5
I'd like to start a thread will little hints and tricks that people have found useful whilst doing chem.

I'll start it off.

 - When converting things use dimensional analysis (ie see them as fractions and things cancelling). like 100ml of 8.9 g/ml then if you multiply them together the mL will cancel with the denominator because its on the numerator and the denominator (ie: you don't have to think).

 - A change in pH is a dilution by a factor of 10 each time. Say you started from pH 1 and a 50mL solution.
You wanna get to pH 2.
 Your solution has to be made up to 50*10 so you need to add 450mL.

- If you have H ion concentration in the form of then according to the *double decker bus* principle the pH = x.


 - Multiplicity applies only when there are two different environments splitting an environment. Otherwise... n+1 rules the roost. If you have a case in which you have n hydrogens in a neighbouring environment and m hydrogens in another then:

(n+1)(m+1) peaks on High resolution NMR.

 - Think of things in proportion to one another. Good example is:

Which of the following gases occupy biggest volume at STP?
A 10g of CO2
B 10 g of NO2
C 10g of SO2
D) 10g of O2

How i think is that n = v/Vm

so we are interested in V = n*Vm

Vm is a constant here. so really all we are interested in is how V varies with mol and we can clearly see that it is directly proportional, ie: a bigger volume will be the one with the biggest mol.

Now how do we maximise mol? well n = m/M, but m is constant here.

So we have to minimise M (as if you have a small denominator you get a big answer).

Now directly you just go for D as all the others have (O2 + another element) so the lowest molar mass and thus highest mol and thus greatest volume.

This is just an example to explain what i mean by thinking proportional.


- When doing gas equations get a feel for them.

ie: A fixed mass of gas has a volume of 900mL under certain conditions. The pressure and tempreature are both doubled. What is the volume of gas after these changes?

Visually if you have a fixed amount of pure gas, you have a fixed amount of mol. This means that you have a fixed volume (thereabouts). So I see a mess of particles inside a canister (representing volume). If you increase the temp by a factor of two, so will the volume by 2. So the particles are moving around more craazy. But if you increase pressure by 2 then the volume must deacrease by 2 because it needs to hit the walls in a certain ratio.

So its still. 900mL.


Let's see what the rest of the community has in their toolbox! (i'll add more when i can think of any)



edit: thanks to chansthename for correcting my fail 900 = 800
« Last Edit: June 04, 2010, 11:17:04 pm by Martoman »
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Richiie

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Re: Tricks of the trade
« Reply #1 on: June 04, 2010, 10:37:57 pm »
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=O
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chansthename

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Re: Tricks of the trade
« Reply #2 on: June 04, 2010, 10:44:15 pm »
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TrueTears

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Re: Tricks of the trade
« Reply #3 on: June 04, 2010, 10:46:51 pm »
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ie: A fixed mass of gas has a volume of 900mL under certain conditions. The pressure and tempreature are both doubled. What is the volume of gas after these changes?

Visually if you have a fixed amount of pure gas, you have a fixed amount of mol. This means that you have a fixed volume (thereabouts). So I see a mess of particles inside a canister (representing volume). If you increase the temp by a factor of two, so will the volume by 2. So the particles are moving around more craazy. But if you increase pressure by 2 then the volume must deacrease by 2 because it needs to hit the walls in a certain ratio.

So its still. 800mL.



i like this, its unit 4 material, and ur thought process was exactly like mine when i did this in unit 4, although ive forgotten all about chem now.
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Martoman

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Re: Tricks of the trade
« Reply #4 on: June 04, 2010, 10:51:27 pm »
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Its also from unit 2 which can be asked in unit 3
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2011: Holidaying, screw school.
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chansthename

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Re: Tricks of the trade
« Reply #5 on: June 04, 2010, 10:52:17 pm »
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can someone please also explain multiplicity (I can't find it here) and am not sure what it is.

naved_s9994

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Re: Tricks of the trade
« Reply #6 on: June 04, 2010, 10:52:46 pm »
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ie: A fixed mass of gas has a volume of 900mL under certain conditions. The pressure and tempreature are both doubled. What is the volume of gas after these changes?

Visually if you have a fixed amount of pure gas, you have a fixed amount of mol. This means that you have a fixed volume (thereabouts). So I see a mess of particles inside a canister (representing volume). If you increase the temp by a factor of two, so will the volume by 2. So the particles are moving around more craazy. But if you increase pressure by 2 then the volume must deacrease by 2 because it needs to hit the walls in a certain ratio.

So its still. 800mL.



i like this, its unit 4 material, and ur thought process was exactly like mine when i did this in unit 4, although ive forgotten all about chem now.


The explanation of her's is exactly like using the p1v1/t1=p2v2/t2
and using up/down arrows with factors, as pronumerals.

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Martoman

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Re: Tricks of the trade
« Reply #7 on: June 04, 2010, 10:53:20 pm »
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Yeah boooiiiiiiii
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kakar0t

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Re: Tricks of the trade
« Reply #8 on: June 04, 2010, 10:55:27 pm »
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AAS = METALS

WHEN YOU SEE AAS ALWAYS THINK METALS!!!!

azn_dj

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Re: Tricks of the trade
« Reply #9 on: June 04, 2010, 11:09:50 pm »
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Another trick I worked out that I posted in another topic (but just so its in one place)
Titration errors:
If you add more water to the unknown, your calculated concentration is lower.
If you add more water to the known, your calculated concentration is higher
That knocks half the titration errors on the head.

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stonecold

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Re: Tricks of the trade
« Reply #10 on: June 04, 2010, 11:27:57 pm »
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Another trick I worked out that I posted in another topic (but just so its in one place)
Titration errors:
If you add more water to the unknown, your calculated concentration is lower.
If you add more water to the known, your calculated concentration is higher
That knocks half the titration errors on the head.


I like!
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naved_s9994

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Re: Tricks of the trade
« Reply #11 on: June 04, 2010, 11:32:07 pm »
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Another trick I worked out that I posted in another topic (but just so its in one place)
Titration errors:
If you add more water to the unknown, your calculated concentration is lower.
If you add more water to the known, your calculated concentration is higher
That knocks half the titration errors on the head.


I like!


Me too, because it keeps a clear head :)
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kakar0t

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Re: Tricks of the trade
« Reply #12 on: June 04, 2010, 11:55:55 pm »
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Another trick I worked out that I posted in another topic (but just so its in one place)
Titration errors:
If you add more water to the unknown, your calculated concentration is lower.
If you add more water to the known, your calculated concentration is higher
That knocks half the titration errors on the head.


I like!


So does that rule apply if you add more water to the conical flask where the 'unknown' is placed?
Me too, because it keeps a clear head :)

azn_dj

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Re: Tricks of the trade
« Reply #13 on: June 04, 2010, 11:59:52 pm »
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Quote
So does that rule apply if you add more water to the conical flask where the 'unknown' is placed?
Ok. There you go. An exception. If you add water to conditions that are not the norm......
is that better?




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stonecold

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Re: Tricks of the trade
« Reply #14 on: June 05, 2010, 12:02:20 am »
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Can anyone confirm if this is correct.

Adding more mol to known = adding water to the unknown
Adding more mol to unknown = adding water to the known
Rinsing unknown with known = adding water to unknown
Rinsing known with unknown = adding water to known

I just made it up, not sure if it is correct...
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