Chemistry 3/4 2013 Thread

[Edward21]

Thanks, makes sense

This really sums it up neatly, 

[swampman]

I'm getting a bit confused for finding limit and excess reagent. If the equation was 2NaOH + H2SO4 --> Na2SO4 + 2H2O (all aqueous states except water: liquid), I just put divide the no. of mol by their coefficient, but what about an equation like 2 Al (s) + 6HCI (g)---> 2AlCl3 (g) + 3H2 (g)? How do I find which one is limiting reagent and which is excess?

Thanks!

[teletubbies_95]

Hi !
You do the same thing as you did with the reaction of NaOH and H2SO4!

So for example , if we had 4 mols of Al and HCl .
We divide each by the coefficient , therefore there would be Al= 2 mols, HCl = 2/3 mols.
Therefore , the reactant in excess is Al and limiting reactant is HCl .

Hope that helped!!

[swampman]

and when it says mole of gas present in container, does it included the excess mols left over? (because it said sealed container....)

[teletubbies_95]

Yes! That's if a the reactant which is a gas is in excess!
For example,
If there are 2 mols of Al and 12 mols of HCl.
In this case , HCl would be in excess , right?

If the question is asking how much gas is produced at the end of the reaction, always include the excess gas !
So in this example, our excess would be 12 - ( 6/2 x 2) = 6 mols in excess , along with 2 mols of AlCl3 and 3 mols of H2 , which is a total of 11 mols of gas!

Hope that helped!

[lzxnl]

If you really get stuck, think of it this way.
2 Al + 6 HCl => 3 H2 + 2 AlCl3

2 mol Al, by mole ratio, only reacts with 6 moles HCl. Therefore you know that HCl is in excess.
If you had 5 mol Al and 13 mol HCl, you could do the same thing and see that 5 mol Al reacts with 15 mol HCl. We do not have enough HCl for that, so Al is in excess.

[09Ti08]

Hello

Can someone show me how to write semi-structural formulae for organic cmpound please? I gave it a search but nothing much came up, and it seemed that we did not have unique convention for this... Normally, I just follow the longest carbon chain and then add hydrogen and any other elements, but I doubt if we have a convention for alkenes, which have a double bond.

Many thanks!

[Daenerys Targaryen]

Q24.
The mineral cobaltite is mined for the production of cobalt. Ore containing cobaltite may also contain trace quantities of nickel. A sample of ore was analysed by AAS to determine the concentration of nickel present. 5.0 g of the ore was dissolved in 25 mL concentrated nitric acid, then diluted to 100 mL. A concentrated stock solution containing 1000 ppm nickel was also prepared. 10 mL of the ore sample solution was pipetted into four 100 mL flasks and 1, 2, 4 and 6 mL of the concentrated stock nickel solution was added to the flasks. A ‘standard addition’ calibration curve was prepared using 10 ppm, 20 ppm, 40 ppm and 60 ppm added nickel, giving the standards listed in the table:
   Concentration of nickel (ppm)   Absorbance
Standard 1   x + 10   0.25
Standard 2   x + 20   0.34
Standard 3   x + 30   0.44
Standard 4   x + 40   0.53
Sample   x   0.15
The absorbance readings for the standards and sample were determined by using light of wavelength 325.4 nm. The value of x, the concentration of nickel in ppm, was found from the intercept of the calibration curve with the x-axis at absorbance 0.
a   Plot the absorbance values for the standards and sample on the axes provided (Figure 7.53). Insert a line of best fit through the points and determine the concentration of nickel in the sample solution.
 Answer is 16ppm
b   What mass of nickel was present in 5.0 g of ore?
c   What is the percent by mass of nickel in the ore sample?
d   Why is it preferable to use a standard addition calibration technique instead of preparing the standards in distilled water?
e   Arsenic is present in equal concentrations to that of cobalt in cobaltite. Why does it not interfere in the calculation of the cobalt or nickel?

[itsdanny]

Hi,

Going through my revision of AoS1, may I ask how do you determine if a reaction will have a limiting or excess reagent in the first place?
And if so, how would you determine which species is the limiting or excess reactant?

i.e. Given:
http://i.imgur.com/1VNa64b.png

Looking at the mol ratios or coefficients obviously won't tell you anything right? Is it such that you need to actually calculate the amount of mol of each reactant firstly? So once you do that, do you just divide by the corresponding coefficient of the species to find out which one is limiting or excess?

Just need a confirmation, thanks

[teletubbies_95]

Yes your right!
So ...In your problem:
The n( AgNO3) = 15/Mr(AgNO3) =0.08828 mol
n( CaBr)= 10/Mr(CaBr)= 0.0840
But as AgNO3 has a coefficient of 2 , we divide the n(AgNO3 ) by 2 = 0.04414mol

This shows that CaBr is in excess and AgNO3 is the limiting reagent.
Hope that helps

@HatersGonnaHate:
I remember that question being on this thread. I'll find it and ill link it .

[itsdanny]

Yes your right!
So ...In your problem:
The n( AgNO3) = 15/Mr(AgNO3) =0.08828 mol
n( CaBr)= 10/Mr(CaBr)= 0.0840
But as AgNO3 has a coefficient of 2 , we divide the n(AgNO3 ) by 2 = 0.04414mol

This shows that CaBr is in excess and AgNO3 is the limiting reagent.
Hope that helps

Yes — that's what I thought, thank you for verifying that for me. 
Also, so how would you know whether a limiting or excess reagent exists (thus has to be determined) in a question? i.e. some questions state in the stem that an excess amount of x is added, etc, but your calculations don't actually have to involve the limiting or excess reactants at all. So it's kind of confusing. Is there an obvious way of telling that you need to or not?

[Edward21]

Proton NMR question, every question has been fine with the n+1 rule, n being the no. of H protons on the adjacent atom, ect...
I get a question where, in the peak splitting, there's 8! I was like, Ok does that mean that there's 7 H on adjacent atoms?
The answer says: "Peak B consists of 8 lines or two quartets." and in this structure there's 3 H on one adjacent atom, and 1 on another adjacent atom, how does this work I've never been so confused, I was getting every question right 

[thushan]

Proton NMR question, every question has been fine with the n+1 rule, n being the no. of H protons on the adjacent atom, ect...
I get a question where, in the peak splitting, there's 8! I was like, Ok does that mean that there's 7 H on adjacent atoms?
The answer says: "Peak B consists of 8 lines or two quartets." and in this structure there's 3 H on one adjacent atom, and 1 on another adjacent atom, how does this work I've never been so confused, I was getting every question right 

Ignore that question. The question you got is out of the course. All questions you get in VCAA will use the n+1 rule

[lzxnl]

Multiplets! I remember those devilishly tricky things...
VCAA, taking stuff out of the course bit by bit. Soon, students won't even have to work out what peak splitting means.

[clıppy]

Let's say I have a blue solution
If i was trying to find the concentration of this solution using colorimetry I would set up several standard solutions, plot a calibration graph, etc. etc. but when i am shining a light into the blue solution, am i shining a blue light and detecting how much light is being transmitted through, or a complimentary light (e.g yellow) and seeing how much is being transmitted through?

On another note, is colorimetry specific colours or specific wavelengths, and if we are talking about those boxes with the RGB LED's, are those just referred to as colours or wavelengths?