Chemistry 3/4 2013 Thread

[Stick]

Alright, so when you've been given two masses, we can no longer assume that both masses will be used up. It's more likely that one reactant will be in excess in comparison to the other. Using the mole ratio provided in the chemical equation, we can determine which reactant is the limiting reactant and we used the information regarding this substance to calculate information regarding other reactants or products.

If you're having trouble with this, consult Chapter 15 of the Heinemann Chemistry 1 textbook.

[Mr Keshy]

Btw, why is n(H2)=0.05? i thought it was 3/2*0.05? I'm horrible at chemistry, can you explain please?

Hopefully this doesn't confuse you.

n(H2)  = 3            3 and 6 are the co efficients (remember 6HCl 3H2)
_____   ___
n(HCl)     6


Now you rearrange it for n(H2)

n(H2) = 3/6 x 0.1 (remember we figured this out before)
          = 0.05 mol

then to get the mass n x M so 0.05 x (1x2) = 0.1 g


This might be confusing in hindsight hahaha

[Math Nerd]

Thanks for all your help guys! I didn't take unit 1&2 chemistry last year which is why i am trying my best to learn as much of it as possible so i don't fall behind this year.

[Stick]

Ah! I'd really recommend you get a tutor then. Units 1&2 Chemistry is an imperative foundation for Units 3&4.

[Mr Keshy]

Thanks for all your help guys! I didn't take unit 1&2 chemistry last year which is why i am trying my best to learn as much of it as possible so i don't fall behind this year.

Good man

I made a mistake at first as well! It's best to keep asking questions no matter how embarrassing you take it to be. With the commitment and effort I'm sure your putting in, you'll definitely be able to catch up.

Do you understand the previous question now though?

[Math Nerd]

Machan: Beautiful explanation. I understand it now! I kind of confused myself haha. 

[Mr Keshy]

Hehe, that's the first time I actually helped somebody in VCE I always received, but never gave. Haha

[Reckoner]

The last few pages of this thread make me really happy, keep it going guys! I wish I had something like this last year. Just quietly its making me question my career path, but that's not important/off-topic/irrelevant, just keep it up!

[Math Nerd]

Ok I am starting to understand stoichiometry now.
1. Balanced equation
2. Find amount (mol)  of substance
3. Use mole ratios (amount of substance required/amount of substance required)
    *limiting/excess reactants
4. Find mass 

Is it the same process in Unit 3/4 Chem?

[Stick]

Yep! It's basically the same. Just be careful though, the last step may not necessarily be finding a mass.

Keep asking your gravimetric analysis questions. In the mean time, here's a bit of a challenging question for you guys:

What 4 characteristics must a substance possess if it is to be used to precipitate an ion in solution? No consulting the textbook! Apparently, this question was asked in the exam a few years ago and hardly anyone got it, so our teacher made a point of the answer.

[teletubbies_95]

Okay, that's a great question!
I'm trying to relate to what I learnt in a standard solution to this:
1. Known formula
2. Solubility has to be low

Okay, I can't think of any more?

[thushan]

Yep! It's basically the same. Just be careful though, the last step may not necessarily be finding a mass.

Keep asking your gravimetric analysis questions. In the mean time, here's a bit of a challenging question for you guys:

What 4 characteristics must a substance possess if it is to be used to precipitate an ion in solution? No consulting the textbook! Apparently, this question was asked in the exam a few years ago and hardly anyone got it, so our teacher made a point of the answer.

Similar to that of primary standard - remember this precipitate has to be weighed.

3) Inert in air
4) Doesn't precipitate any other ions that could be in solution

[paulsterio]

It'd be the exact same conditions as the conditions required of a compound to make a standard solution, sort of?

[Mr Keshy]

3 is actually the only one I could remember I'm not in too much trouble right?

[Stick]

Similar to that of primary standard - remember this precipitate has to be weighed.

3) Inert in air
4) Doesn't precipitate any other ions that could be in solution

I learnt a different 3: stable when heated. :S