Post by: kenhung123 on December 13, 2009, 03:48:57 pm
The molar mass (M) is found by taking the atomic, molecular or formula mass and expressing it in grams. For example, one mole of CO2 gas has a mass of 44g mole^-1.
Post by: m@tty on December 13, 2009, 04:07:53 pm
The relative atomic mass of Carbon was set at 12, there is no unit for RAM, then the mole was found so that one mole of Carbon-12 weighed 12 grams, hence all RAM can be expressed in grams as the weight which comprises one mole.
Post by: kenhung123 on December 13, 2009, 04:28:24 pm
Post by: monokekie on December 13, 2009, 04:56:01 pm
firstly, you could find CO2's molar mass through the use of atomic masses. atomic mass is the sum of the number of protons and neutrons found in the nucleus of an atom.You could find them on the periodic table e.g, carbon=12, Hydrogen=1, etc
so,atomic mass of one carbon =12, and atomic of one oxygen =16,
=> in carbon dioxide, there are one carbon and two oxygens, 12+16+16= 44 g, i.e, the molar mass of CO2
can you follow me?
Post by: kenhung123 on December 13, 2009, 05:01:51 pm
Post by: monokekie on December 13, 2009, 05:10:44 pm
note: this would do for the VCE standard as their difference is actually above the scope of our course.
"Molecular mass differs from more common measurements of the mass of chemicals, such as molar mass, by taking into account the isotopic composition of a molecule rather than the average isotopic distribution of many molecules. As a result, molecular mass is a more precise number than molar mass; however it is more accurate to use molar mass on bulk samples. This means that molar mass is appropriate most of the time except when dealing with single molecules." - wikipedia
do you get this part?
Post by: kenhung123 on December 13, 2009, 05:13:45 pm
Post by: monokekie on December 13, 2009, 05:26:33 pm
formula mass doesn't necessary equal to atomic mass..
Post by: kenhung123 on December 13, 2009, 05:31:04 pm
Post by: kenhung123 on December 15, 2009, 06:02:31 pm
Post by: Ilovemathsmeth on December 15, 2009, 06:18:59 pm
Post by: kenhung123 on December 15, 2009, 06:36:18 pm
Formula mass is used for ionic substances; molecular for covalent substances like CO2.Oh ok such as NaCl (formula) CO2 (molecular)?
I also want to ask is it important to know conversions in chemistry? Such as 1m^3=1000 000mL?
Post by: Ilovemathsmeth on December 15, 2009, 08:10:10 pm
Post by: kenhung123 on December 15, 2009, 08:12:57 pm
Hmm I think you need to know them, especially miligram and microgram, they sometimes come up on exams...Ahh, I find them confusing because sometimes I think something can be converted into another when it can't. Are you referring to conversion like mg>g and ug>g? Because I am worried about converting in 1 unit to another.
Post by: m@tty on December 15, 2009, 08:14:31 pm
Post by: kenhung123 on December 15, 2009, 08:22:57 pm
Post by: Edmund on December 15, 2009, 08:25:38 pm
You are referring to like cm>m>km or like grams to milli litres? Something like grams to milli litres is hard as there are possible many possibilities.Just remember:
1L = 1kg
or
1L = 1000g
or
1mL = 1g
EDIT: Assuming this is water we are looking at, since density of water is
Post by: kenhung123 on December 15, 2009, 08:27:50 pm
1cm=1g
Also, so is that all..?
Post by: Edmund on December 15, 2009, 08:30:50 pm
1m^3=1000000gNo, you can't convert length units (cm) to mass units (grams). Just the ones I mentioned above where they could ask you to convert litres to grams...
1cm=1g
Also, so is that all..?
Post by: kenhung123 on December 15, 2009, 08:36:17 pm
Post by: Edmund on December 15, 2009, 08:43:33 pm
And this might be helpful:
EDIT: Fixed typo
Post by: kenhung123 on December 15, 2009, 08:46:55 pm
Post by: Edmund on December 15, 2009, 08:49:34 pm
Ok, so that is all the conversion from 1 unit to another??Yep, the conversions in my previous posts are essential. You should be able to easily convert between those units
Post by: kenhung123 on December 15, 2009, 08:53:12 pm
Did you mean
And this might be helpful:
Post by: Edmund on December 15, 2009, 08:54:41 pm
Aha yes, sorry for typoDid you mean
And this might be helpful:?
Post by: m@tty on December 15, 2009, 08:57:04 pm
And this might be helpful:
Wrong way around?
Should it be;
EDIT: beaten
Post by: kenhung123 on December 17, 2009, 01:24:10 am
Post by: kenhung123 on December 17, 2009, 01:25:04 am
Find the amount in mol of H3PO4 that reacted with 5g NaOH (Mr=40) from the above reaction.
(I got 0.0412mol)
Post by: Edmund on December 17, 2009, 02:20:35 pm
Post by: kenhung123 on December 17, 2009, 02:21:50 pm
Post by: m@tty on December 17, 2009, 02:25:45 pm
Post by: kenhung123 on December 17, 2009, 02:30:02 pm
0.300 is 3 sig fig
0.003 is 1 sig fig?
Post by: Edmund on December 17, 2009, 02:32:49 pm
In summary, only zeros that are between non zero digits and after are significant right? E.g. 0.303 is 3 sig figYeah
0.300 is 3 sig fig
0.003 is 1 sig fig?
http://vcenotes.com/forum/index.php/topic,17173.0.html
Post by: kenhung123 on December 17, 2009, 02:42:33 pm
Post by: m@tty on December 17, 2009, 02:58:34 pm
Post by: Edmund on December 17, 2009, 03:21:15 pm
Isn't it 3?Yep, 300 has 3 significant figures. If you have to express 300 in 1 decimal place, then it would be
Hope this helps...
Post by: kenhung123 on December 17, 2009, 03:30:58 pm
Post by: Edmund on December 17, 2009, 03:46:25 pm
Post by: kenhung123 on December 17, 2009, 05:09:34 pm
In fact the decimal point presence does matter!
E.g. 4000 in scientific form is 4x10^3 1 sig fig
4000. in scientific form is 4.000x10^3 4 sif figs!
4000.00 in scientific form is 4.00000x10^3 6 sig figs!
So just change the answer into scientific form remembering the decimal point and you can work out the sig figs easily. Other than that remember non zero digits, zero between numbers, zeros after non zero numbers both on the right of decimal point are sig. Zeros left of a decimal point are sig but if no decimal point it is not sig!
Post by: Edmund on December 17, 2009, 05:22:02 pm
2 significant figures:
3 significant figures:
4 significant figures:
Quote
About the decimal point thing, was that non existent? 400 as 1 sif fig and 400. as 3?Both have 3 significant figures.
Post by: kenhung123 on December 17, 2009, 05:30:14 pm
1 significant figure:Yea. I guess the decimal is sig. :)= 500
2 significant figures:
QuoteAbout the decimal point thing, was that non existent? 400 as 1 sif fig and 400. as 3?Both have 3 significant figures.
2 sig figs are not reversable btw. 5.0x10^2=500. but does not equal 5.00x10^2...
Post by: kenhung123 on December 17, 2009, 05:51:10 pm
An impure sample of iron (III) sulfate, weighing 1.545g, was treated to produce a precipitate of Fe2O3. If the mass of the dried precipitate was 0.315, calculate the percentage of iron in the sample.
Mr(Fe2O3)=159.6 Mr (Fe)=55.8
Post by: Edmund on December 17, 2009, 08:02:40 pm
Then find the number of moles of Iron (Fe) in this precipitate:
Now find the mass of iron (all the iron came from the impure sample):
Therefore the percentage is:
Post by: kenhung123 on December 18, 2009, 12:21:18 am
First find the moles of precipitate:I have found m(Fe2(SO4)3) why is it just finding Fe?
Then find the number of moles of Iron (Fe) in this precipitate:
Now find the mass of iron (all the iron came from the impure sample):
Therefore the percentage is:
Thanks Edmund
Post by: Edmund on December 18, 2009, 01:36:08 pm
Post by: kenhung123 on December 18, 2009, 01:46:44 pm
Post by: Greggler on December 18, 2009, 08:05:48 pm
In redox titrations reactants include things such as KMnO4.
However when wiriting/balancing out these complex equations with the oxygens and the water etc. it is simply written down as an ion (MnO4-)
The K simply disapears from the equation (i presume as a spectator ion?)
Why is this. I guess that this is because of something to do with solubility rules etc. But then why does this never happen in acid/base titrations? (or does it?)
Any info that could help clear this up would be greatly asppreciated :)
Post by: /0 on December 18, 2009, 08:09:12 pm
Post by: Greggler on December 18, 2009, 08:23:01 pm
By the way, is this just a thing with titrations?
Post by: longy1991 on December 19, 2009, 05:09:09 pm
Post by: kenhung123 on December 19, 2009, 08:41:33 pm
Post by: chem-nerd on December 19, 2009, 08:59:24 pm
If you rinse the conical flask with the solution you are adding additional and unknown amount of mole rather than just the accurate amount delivered by the pipette.
If you rinse the pipette with water it will no longer deliver an accurate volume of KNOWN concentration.
Post by: kenhung123 on December 19, 2009, 09:32:35 pm
Post by: chem-nerd on December 19, 2009, 09:43:27 pm
Post by: kenhung123 on December 19, 2009, 10:13:38 pm
Post by: kenhung123 on December 19, 2009, 10:29:18 pm
If you titrate this with a solution in burette, that means there will be less requirement because less is needed to react. Therefore the calculated mole with n=CV is accurate.
Post by: Potter on December 20, 2009, 01:17:28 am
My observation: If you add water into pipette, you will have the same volume but less mole and concentration.
I'm confused if you're talking about a Burette or a pipette..
Also, If you add water into the thing the volume and concentration will change. The Mol will be constant.
Post by: kenhung123 on December 20, 2009, 10:07:50 am
Post by: natty on December 20, 2009, 10:20:44 pm
If there is some water in the pipette then the volume is already increased and less number of moles can enter right? Also the concentration much change.
Adding water= decreases concentration, but same number of mole.
If there is water in the pipette before adding the substance, less moles will be delivered to the flask as less substance will be added. The concentration may not change hugely but it can affect the calculations.
Post by: kenhung123 on December 21, 2009, 03:07:20 am
Why does the pH change rapidly when the equivalence point is reached?
Post by: monokekie on December 21, 2009, 03:27:12 am
Thanks!
Why does the pH change rapidly when the equivalence point is reached?
pH is calculated on a logarithmic scale, for example, a pH of 1 will have 10 times the hydronium concentration of pH 2. So it gradually takes less amount of base to neutralise the acid as the pH increases from 1 to the equivalance point. Once the pH of the solution is near the equivalence point, the change by a factor of 10 occurs really quick, thus the pH changes rapidly. This is basically why the graph is usually very steep at the equivalence point :)
Post by: kenhung123 on December 21, 2009, 03:29:50 am
Post by: stonecold on December 21, 2009, 05:04:27 am
Don't understand sorry :S
i am a horrible teacher :-[
I got it :)
Ken, go over acid/base reactions from the text and it should be clearer. I think the particular section is called Bronsted Lowry Theory. But yeah, as pH is calculated on a logarithmic scale, if you can imagine a log graph, there is the part where the gradient changes very rapidly, and there are other parts where the gradient doesn't change a great deal. This is what monokekie was talking about in relation to why the pH changes at different rates. It depends on the concentration of hydronium ions present in the solution.
Post by: monokekie on December 21, 2009, 05:26:45 am
yayyy :) someone understands ~Don't understand sorry :S
i am a horrible teacher :-[
I got it :)
Ken, go over acid/base reactions from the text and it should be clearer. I think the particular section is called Bronsted Lowry Theory. But yeah, as pH is calculated on a logarithmic scale, if you can imagine a log graph, there is the part where the gradient changes very rapidly, and there are other parts where the gradient doesn't change a great deal. This is what monokekie was talking about in relation to why the pH changes at different rates. It depends on the concentration of hydronium ions present in the solution.
Post by: kenhung123 on December 21, 2009, 11:21:00 am
No I am just dumb :)Don't understand sorry :S
i am a horrible teacher :-[
Post by: kenhung123 on December 21, 2009, 12:07:29 pm
Thanks, does this mean that the rapid increase will always be constant? This is because the rule is -log[H3O+]Don't understand sorry :S
i am a horrible teacher :-[
I got it :)
Ken, go over acid/base reactions from the text and it should be clearer. I think the particular section is called Bronsted Lowry Theory. But yeah, as pH is calculated on a logarithmic scale, if you can imagine a log graph, there is the part where the gradient changes very rapidly, and there are other parts where the gradient doesn't change a great deal. This is what monokekie was talking about in relation to why the pH changes at different rates. It depends on the concentration of hydronium ions present in the solution.
Post by: stonecold on December 21, 2009, 12:58:35 pm
What is the question? Is this from Heinemann Chem 3/4?
Post by: kenhung123 on December 21, 2009, 01:01:28 pm
Post by: stonecold on December 21, 2009, 01:13:08 pm
e.g. 0.2 M HNO3 has the same pH as 0.1 M H2SO4
Also note that as volume increases, the concentration decreases.
Post by: kenhung123 on December 21, 2009, 02:06:14 pm
Hope this helps for future people!
Post by: stonecold on December 21, 2009, 02:21:35 pm
Post by: kenhung123 on December 21, 2009, 02:32:37 pm
Write an equation for the reaction that occurs during the titration
Calculate the amount in mole of hydrochloric acid used to reach the end point
Calculate the percentage by mass of magnesium carbonate in the tablet Mr MgCO3-84
What is the function of the substances that make up the remaining mass of the tablet
Why do people often burp after taking an antacid tablet
Post by: chem-nerd on December 21, 2009, 03:09:16 pm
n(HCl) = 0.02775 x 1.00
n(MgCO3) = 1/2 n(HCl)
m(MgCO3) = n(MgCO3) x 84
use this to work out % by mass (m(MgCO3)/1.30) X100
Other substances: binding agents, fillers, preservatives etc
Burping due to the CO2 produced
Post by: kenhung123 on December 21, 2009, 03:25:46 pm
A sample for air emitted from a car exhaust was taken and analysed for its sulfur dioxide (SO2) content. The gas sample was bubbled through 50ml of acidified 0.08M K2Cr2O7. The SO2 reacted with K2Cr2O7 to produce Cr 3+ and SO4 2-. The remaining amount of Cr2O7 2- was titrated with 0.2M SnCl2 solution.
Cr2O7 20 + 14H+ + 3Sn 2+ => 2Cr 3+ + 3Sn 4+ +7H2O
A titre of 10.2ml was required
The Redox equation is 3SO2+6H2O+Cr2O7 2- +2H+=> 3SO4 2- + 12H+ +2Cr 3+ + 7H2O
Find the amount of Cr2O7 2- in excess after reacting with SO2
Find the mass of SO2 present in the sample of air initially.
Post by: Edmund on December 21, 2009, 05:14:05 pm
Therefore the amount of
Total amount of
Amount
Amount of
Therefore the mass of
Post by: kenhung123 on December 21, 2009, 05:14:50 pm
Post by: kenhung123 on December 21, 2009, 08:53:10 pm
Post by: kenhung123 on December 21, 2009, 08:58:24 pm
Post by: Edmund on December 21, 2009, 09:27:01 pm
E.g.
Reduction is gain:
E.g.
Is this reduction: 2I- => I2+2e ?Oxidation
Post by: kenhung123 on December 21, 2009, 09:32:06 pm
Oxidation is the loss of electrons:Thank ya
E.g.
Reduction is gain:
E.g.Is this reduction: 2I- => I2+2e ?Oxidation
Post by: Ilovemathsmeth on December 22, 2009, 01:04:55 am
Post by: stonecold on December 22, 2009, 02:49:01 am
Oxidation is Loss of Electrons and occurs at the anode.
Reduction is Gain of electrons and occurs at the cathode.
Best 4 words you will ever learn! :D
Post by: Ilovemathsmeth on December 22, 2009, 11:33:55 am
Post by: kenhung123 on December 22, 2009, 01:02:26 pm
E.g.
X + Y + 7H+ => Y + X + 2H+
Would it be:
X + Y + 5H+ => Y + X ?
Post by: Potter on December 22, 2009, 01:17:16 pm
How does the H+ cancellation work?
E.g.
X + Y + 7H+ => Y + X + 2H+
Would it be:
X + Y + 5H+ => Y + X ?
Yeah, that's pretty much it, you do the same thing when you have water on both sides of the equation. Don't just apply that to H+'s
Post by: kenhung123 on December 22, 2009, 01:20:00 pm
Post by: kyzoo on December 22, 2009, 01:32:34 pm
Post by: kenhung123 on December 22, 2009, 02:30:52 pm
Post by: kenhung123 on December 25, 2009, 04:02:53 am
(I got Rf blue as 0.83 and Rf red as 0.58)
2. A sample of brown dye used in a brand of writing ink was found to contain blue and yellow components. The Rf values of these substrances using ethanol as a solvent are 0.59 and 0.19 respectively.
How far apart would the blue and yellow componenets be after the solvent front has move 8.0cm from the origin?
(I got 3.2cm)
Post by: Aden on December 25, 2009, 10:35:17 am
Post by: kenhung123 on December 25, 2009, 02:31:21 pm
Post by: kenhung123 on December 25, 2009, 03:53:01 pm
Is it 3?
Post by: Aden on December 25, 2009, 04:24:55 pm
Post by: kenhung123 on December 25, 2009, 04:59:24 pm
Post by: kenhung123 on December 25, 2009, 10:24:25 pm
Post by: enwiabe on December 25, 2009, 10:52:41 pm
From the graph, we see that the diluted solution's 250 arbitrary units of peak area corresponds to roughly 5% v/v ethanol.
But that's of the DILUTED solution. We now have to work out the dilution factor of the solution being tested.
Remember that they took 20 mL of whiskey and added 180 mL of water.
Remembering that
We know that
Thus, plugging this in,
and so,
Which means that the dilution factor is 10. This makes sense that if you dilute a sample from 20 mL to 200mL (increase volume by 10 times), then the concentration decreases by a factor of 10.
So as we worked out from the graph,
therefore,
If I got this wrong I am going to cry and just stick to the maths boards :(
Post by: kenhung123 on December 25, 2009, 10:53:51 pm
Thanks man!