Some general chem queries. (1 Viewer)

[Aerlinn]

Things that made me scratch my head...
-- When you are told that compounds ie. in a solubility table are 'slightly soluble', how are you supposed to predict their states in a chemical equation?---In titrations, we know the 'known volume' goes in the conical flask, but is the exact volume measured out beforehand, before putting it all in the flask with the pipette, or is the volume measured against the markings of the flask as it fills up? An odd question, I know.
--When something says that reactants are 'stoichiometrically equivalent', does that mean that the number of mole of reactants are the same, eg. 2 mole of this and 2 mole of that.
--When balancing redox half equations... if there are some of the same ions, eg. H+, K+ on both sides, do they cancel like the electrons do? I havent come across any cases yet, but I heard it someplace, dun know if it's right. eg.
bla + bla + 4H+(aq) ----> bla + bla + 6H+(aq)
does it become...
bla + bla ----> bla + bla + 2H+(aq) ...?

Any help would be much welcomed.

:wave:

 

[xiao1985]

one question at a time then:

1) when something is slightly soluable, depends on the exact amount of the substance, the chemical maybe (aq) or a mixture of (aq) and (s) .... just imagine if you put too much salt in a beaker of water, you have lots of salt dissolved (aq) but some undissolved (s)

2) ALWAYS use pipitte if you need to know the volume of something precisely... markings on conical flask and beaker are NOT suppose to be used to measure volume... ever...

3) can't remember what does stoichmetric mean any more, but iirc:
say for equation 2H2 +O2 -> 2H2O, to have the two reactant stoichmetrically equivalent, you need to have twice as much of H2 in moles comparing to O2

4) yes, if that ever happen...

 

[Lucid Scintilla]

--When something says that reactants are 'stoichiometrically equivalent', does that mean that the number of mole of reactants are the same, eg. 2 mole of this and 2 mole of that.

can't remember what does stoichmetric mean any more, but iirc:

Stoichiometrically equivalent means that the reactants and products are the same in the no. of moles.
Basically it means balance the equation, so yes, it means that both sides have the same quantity of the same quality.

 

[Aerlinn]

Thanx guys.
---So, with things that are slightly soluble, it doesn't matter whether you put (s) or (aq) since some of the substances will be (s) and some will be (aq)...?
---So... stoichiometrically equivalent basically means you have to have reactant/ product ammounts in the mole ratio, so say with xiao's example, 2H2 +O2 -> 2H2O... there would need to be 2 moles H2 reacting with 1 mole O2 to produce 2 H2, is that basically what you are saying?

 

[Lucid Scintilla]

If you're going to write out a equation, you probably will need to write the state which it is in.
As for stoichiometry, 2H₂ + O₂ --> 2H₂O. Yeah, that's right. 4 hydrogen moles + 2 oxygen moles --> 2 moles of water (dihydrogen monoxide).

Edit:

there would need to be 2 moles H2 reacting with 1 mole O2 to produce 2 H2, is that basically what you are saying?

Pretty much.

 

[xiao1985]

@aerlinn: [edited out] 2 moles H2 reacting [edited out] to produce 2H2

well it's 2 H2O... (i know what you are saying, but just to be technically correct)

@lucid scintilla: i guess you are referring to moles of atoms instead of molecules in your last post... got me Oo for quite abit over there...

 

[Aerlinn]

Uh, k, so what I said before was more correct... 'stoichiometrically equivalent'= when you have x moles of a reactant to produce x moles of a product....? Rather than the phrase = amount of products/reactants you have according the mole ratio... ('cause a bit confusing as to why you editted out the O2)

 

[xiao1985]

re: edited out: apologies, my eyes playing trick on me... (i need more sleep... didn't seen the O at the end of H2O)

re: stoichmetric - i still hold doubt on your view: take 2 equivalent of hydrogen + 1 equivalent of oxygen to produce 2 equivalent of water for eg. stoichmetrically equivalent would mean you put in 2 mole of hydrogen and 1 more of oxygen into the reaction chamber... it can also mean you put in 200 moles of hydrogen and 100 moles of oxygen in to the reaction chamber...

does that sort make sense now?! OO

 

[Aerlinn]

Ok so this time you're saying you just get quantities of the reactants according to the mole ratio like I said before: 2 moles H2 reacting with 1 mole O2 to produce 2 H2.
Right? Or are you sayin its reactants only...
Are you saying the same thing each time, 'cause you seem to switch what you're saying...

 

[xiao1985]

sorry if i made it confusing... you are right now...

if i say: "mix two portions of hydrogen and oxygen in a stoichmetrically equivalent manner" i would mean mix 2 moles of hydrogen and 1 mole of oxygen together...

sorry for any confusions caused..

 

[Aerlinn]

Ok. That much is clear, so applying to reactants only or products as well?

 

[xiao1985]

i will say only reactants... what's the point of mixing product into the reaction chamber?!

for eg, if you want to produce water, from hydrogen and oxygen, why would you want to put water into the chamber containing hydrogen and oxygen?!