Post your Q here (2 Viewers)

[BlackJack]

Originally posted by stuchie
ok... i have a question

what is the equation for the leclanche cell

each of my textbooks say a different thing so i dunno which one i should learn....

Just learn one of them. Different companies might use different formulas in their batteries...
But here's one:
Zn -> Zn2+ + 2e-
NH4+ + MnO2 + H2O + e- -> Mn(OH)3 + NH3

Originally posted by doctor
in a titration which pieces of equipment have to be rinsed with the substance being placed in them and why is this important?

Pipette and burette. Because, you're interested in keeping the concentration of your acids and bases constant so you can do reliable calculations...

 

[McLake]

Originally posted by stuchie
ok... i have a question

what is the equation for the leclanche cell

each of my textbooks say a different thing so i dunno which one i should learn....

Just pick one, they will have to take it as correct ...

 

[McLake]

Originally posted by doctor
in a titration which pieces of equipment have to be rinsed with the substance being placed in them and why is this important?

What BJ said. Also note flask rinsed with distilled H2O, dosn't effect concentration. Important to insure no residue from last titration/wash wich may contain chemicals ...

 

[sleepy]

man
reading all this stuff is makin me soooo dizzy...
i reckon anything that was in last yrs hsc is not gonna be in this yrs one!!! smart eh??

which is good coz i dun have to know shit abt batteries but bad coz none of the mandatory pracs were asked last yr and i dunno any of them so yeah..

i mite just add im screwed for biochem my option but it seems that no one does it (huge sigh) anyway i mite go back to my sleep i mean study..

 

[Dumbarse]

Q. which of the following dissolves in water to form a basic solution?
a) potassium citrate
b) sodium nitrate
c) ammonium chloride
d) aluminium sulfate

can someone please explain how u are supposed to work this out?!

 

[Dumbarse]

another one...

In an aqueous solution of the weak acid nitrous acid, HNO2, which of the following species is present in the highest concentration?

a) HNO2
b) H30+
c) 0H-
d) NO2-

how do u work it out, explanation please

 

[BlackJack]

Relative strengths of acids and bases...
Look at all the ions. Assume these were all produced from neutralisation of acids and bases. Which one, in its original form was a weak acid/base? The only weak one is citric acid.
Therefore, when you put the citrate ion into the water it will start grabbing some H+ from water... making OH-

hence, A.

Man, I'm going to get beaten by a lot of people I teach... I'm not feeling well... (speaking of which, who's the one who did ask me to teach him so that he'll beat me in physics? Well?)

 

[spice girl]

Originally posted by Dumbarse
another one...

In an aqueous solution of the weak acid nitrous acid, HNO2, which of the following species is present in the highest concentration?

a) HNO2
b) H30+
c) 0H-
d) NO2-

how do u work it out, explanation please

A

In the equilibrium HNO2 -> H+ + NO2-

Equilibrium lies to the left, because HNO2 is a weak acid. Thus, A is the answer

 

[BlackJack]

Since you DID put in an acid into neutral water (assumed), it's not going to be OH-... as for H3O+ and NO2-, they will be in the same concentration and cannot be both true.

Hence, HNO2.

Or another way, all of the weak acids taught in the syllabus have a dissolution rate of <10% or something... so you can always assume HNO2 will be higher than its products.

 

[Dumbarse]

Thanx guys

i still dont really get the first one

and spice girl for the second one, did u get that equilibrium
HNO2 -> H+ + NO2-
because the ions dissociate in water??
will it always be an equilibrium like that?

 

[spice girl]

Yes, in fact it should be a dblarrow like this: <->

 

[BlackJack]

The first one would be a higher band q anyway...

Look at the acid/conjugate base theory...
If an acid is strong, then it'll fully disociate in water. It's conjugate base will be very weak.

All of the answers are salts. Salts are pairs of ions... You know that if you add Na+ to water there is no effect becase NaOH is a strong base (thus its conjugate acid, Na+, is very weak). Apply this to most of the other ions...

As for aluminium however, I tihnk it does do something with the water, but it's only going to lower pH, as it's Al3+ and ca't grab any more H+.

As for citrate ions.... citric acid forms citrate ions in water. But citric acid is a weak acid... therefore its conjugate base, citrate, is not going to stay as a citrate ion in solution...
CH2O-CHO-CH2O- + 3H2O <-> CH2OHCHOHCH2OH + 3OH-

 

[Dumbarse]

kool, thanx BJ

another question. In a titration u know how u can get a electrical conductivity graph in a V shape, why does the bottom of the V not reach a zero value??

 

[Dumbarse]

Originally posted by Dumbarse
another one...

In an aqueous solution of the weak acid nitrous acid, HNO2, which of the following species is present in the highest concentration?

a) HNO2
b) H30+
c) 0H-
d) NO2-

how do u work it out, explanation please

hey for this question., if it had been a strong acid, say H2SO4, then it wouldnt be equilibrium, what would happen then spicegirl?

would it just be H2SO4 ---> 2H(+) + SO4(2-)

then how would u work out what has the highest conc? would it be the H+ cause there is 2 mols of them

 

[spice girl]

Originally posted by Dumbarse


hey for this question., if it had been a strong acid, say H2SO4, then it wouldnt be equilibrium, what would happen then spicegirl?

would it just be H2SO4 ---> 2H(+) + SO4(2-)

then how would u work out what has the highest conc? would it be the H+ cause there is 2 mols of them

Yep

 

[Ozz^E]

Basically: Even at the endpoint there are ions in solution.

Take the example of having HCl and adding NaOH into it. As we add the NaOH, we are essentially replacing hydrogen ions with sodium ions in solution:

H+ + Cl- + Na+ + OH- --> H2O + Na+ + Cl-

At the equivalence point, our solution of Hcl is now a soultion of NaCl. Sodium ions are larger than hydrogen ions and so have a smaller conductivity. Therefore conductance falls from the start to the endpoint at which there are still ions in solution and hence THE CONDUCTANCE IS NOT ZERO (however, if our salt in insoluble than we will get much closer to zero). After endpoint, we are essentially adding NaOH to our NaCl solution: adding extra ions, so conductance increases.

 

[Dumbarse]

haha at last making some progress
u explain in like one message what my teacher tried to explain all year
, thanx spicegal, ozee and bjack

 

[aby]

How do sulfate reducing bacteria cause acidic conditions around shipwrecks? Sea water is naturally alkaline.... how would it change to acidic ??? thnks!

 

[superhubert]

anerobic bacteria don't need oxygen. they survive off sulphate. they reduce the sulphate ions and they react with water molecules to form sulphuric acid H2SO4. and oxygen gas. because they accumulate around ships. the water around the ships becomes slightly acid and aids corrosion. (i think.)

 

[McLake]

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