HSC 2012-2015 Chemistry Marathon (archive) (1 Viewer)

[bleakarcher]

re: HSC Chemistry Marathon Archive

I'm a little unsure with the bolded part and Sy do you make these questions up? I could use a few for practice lol.

By definition,
pH=-log[H+]
pOH=-log[OH-]
At 25 degrees C, [H+][OH-]=10^(-14)
Taking the common log of both sides:
log[[H+][OH-]]=log[10^(-14)]
log[H+]+log[OH-]=-14
-pH-pOH=-14
pH=14-pOH

 

[Aysce]

re: HSC Chemistry Marathon Archive

By definition,
pH=-log[H+]
pOH=-log[OH-]
At 25 degrees C, [H+][OH-]=10^(-14)
Taking the common log of both sides:
log[[H+][OH-]]=log[10^(-14)]
log[H+]+log[OH-]=-14
-pH-pOH=-14
pH=14-pOH

Alright, thanks for clarifying.

From what I have seen its harder for me personally to make questions for Chemistry since the numbers need to work in real life (I don't want to associate random numbers to the chemicals)
So at the moment I am just getting these questions from somewhere. But that may change =)

Can you tell me what this "somewhere" is?

I've been trying to look for practice calculation questions online but I'm not finding many things that are relevant/useful to me.

 

[Sy123]

re: HSC Chemistry Marathon Archive

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Balanced eqn (without states): H2SO4 + 2KOH -> K2SO4 + 2H2O

n(H2SO4) = [H2SO4] x V(H2SO4) = 0.5moles
n(KOH) = [KOH] x V(KOH) = 0.5 x 3 = 1.5moles

n(H2SO4) : n(KOH) = 1:2
Therefore, this reaction will need 0.5moles of sulfuric acid and 1 mole of potassium hydroxide

Therefore, there will be excess potassium hydroxide (0.5moles)

[KOH) = n(KOH)/V(KOH) = 0.5/4 = 0.08M

KOH -> K+ + OH- (from internet, KOH is a strong base and will ionise completely, hence obtaining 100% yield of H+)
pH = -log(h+) = -log(0.08) = something

 

[Sy123]

re: HSC Chemistry Marathon Archive

Balanced eqn (without states): H2SO4 + 2KOH -> K2SO4 + 2H2O

n(H2SO4) = [H2SO4] x V(H2SO4) = 0.5moles
n(KOH) = [KOH] x V(KOH) = 0.5 x 3 = 1.5moles

n(H2SO4) : n(KOH) = 1:2
Therefore, this reaction will need 0.5moles of sulfuric acid and 1 mole of potassium hydroxide

Therefore, there will be excess potassium hydroxide (0.5moles)

[KOH) = n(KOH)/V(KOH) = 0.5/4 = 0.08M

KOH -> K+ + OH- (from internet, KOH is a strong base and will ionise completely, hence obtaining 100% yield of H+)
pH = -log(h+) = -log(0.08) = something

Looks completely correct - nice work

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

Looks completely correct - nice work

ok

I forgot the question that i was going to post up, but i remember the key points

25.0 mL and 0.01mol/L of CH3COOH is diluted to 500.00mL w/ distilled water. Take 10.omL of that sol'n and dilute to 500.00mL. Find the pOH given that CH3COOH has 10% ionisation

 

[Sy123]

re: HSC Chemistry Marathon Archive

ok

I forgot the question that i was going to post up, but i remember the key points

25.0 mL of CH3COOH is diluted to 500.00mL w/ distilled water. Take 10.omL of that sol'n and dilute to 500.00mL. Find the pOH given that CH3COOH has 10% ionisation

I think you may be missing concentration of the CH3COOH


===========================

 

[Aysce]

re: HSC Chemistry Marathon Archive

The bonding of the hydronium ion consists of covalent bonds between the oxygen and three hydrogen atoms. Since this ion carries a positive charge, this consequently means that there is one less electron ie. The total number of electrons in the ion is 8 not 9. One hydrogen atom loses its electron whilst the other two covalently bond with two electrons in the oxygen atom. The third hydrogen atom is attached by the O atom using a pair of its unbonded electrons to form a coordinate covalent bond.

*insert diagram*

CORRECT ANSWER

 

[HeroicPandas]

re: HSC Chemistry Marathon Archive

The bonding of the hydronium ion consists of covalent bonds between the oxygen and three hydrogen atoms. Since this ion carries a positive charge, this consequently means that there is one less electron ie. The total number of electrons in the ion is 8 not 9. The three hydrogen atoms covalently bond with 3 of the valence electrons of the oxygen atom, whilst the oxygen atom has a remaining pair of unbonded electrons thus being able to reach a stable gas configuration.

*insert diagram*

It must be a trick question! because covalent bonding is obvious

 

[Sy123]

re: HSC Chemistry Marathon Archive

The bonding of the hydronium ion consists of covalent bonds between the oxygen and three hydrogen atoms. Since this ion carries a positive charge, this consequently means that there is one less electron ie. The total number of electrons in the ion is 8 not 9. The three hydrogen atoms covalently bond with 3 of the valence electrons of the oxygen atom, whilst the oxygen atom has a remaining pair of unbonded electrons thus being able to reach a stable gas configuration.

*insert diagram*

Hmmm I would like a second opinion on this, its similar to what I had in mind.

Oxygen by itself originally has two pairs of non-bonding electrons, but in H3O+ we can observe as you pointed out that it only has 1 pair of bonding electrons.

What happened to the other pair?

 

[Aysce]

re: HSC Chemistry Marathon Archive

It must be a trick question! because covalent bonding is obvious

It sure as hell isn't ionic, metallic or coordinate covalent bonding

(My mistake, there is coordinate covalent bonding involved)

Awkies.

 

[Aysce]

re: HSC Chemistry Marathon Archive

Hmmm I would like a second opinion on this, its similar to what I had in mind.

Oxygen by itself originally has two pairs of non-bonding electrons, but in H3O+ we can observe as you pointed out that it only has 1 pair of bonding electrons.

What happened to the other pair?

Well since H3O+ is a cation, you know that it has lost an electron. So subtract an electron from the 6 valence electrons of the O atom, that leaves us with 5. So yeah you can see that since we've lost an electron hence a pair of non-bonding electrons is lost since the single one remaining from the unbonded pair is now covalently bonded with a hydrogen atom. dunno if that answers your question :/

 

[Sy123]

re: HSC Chemistry Marathon Archive

Well since H3O+ is a cation, you know that it has lost an electron. So subtract an electron from the 6 valence electrons of the O atom, that leaves us with 5. So yeah you can see that since we've lost an electron hence a pair of non-bonding electrons is lost since the single one remaining from the unbonded pair is now covalently bonded with a hydrogen atom. dunno if that answers your question :/

Where did the other electron come from though?
And remember if electrons are non-bonding they cannot themselves split up to bond with other stuff.

 

[Aysce]

re: HSC Chemistry Marathon Archive

It must be a trick question! because covalent bonding is obvious

can O hydrogen bond with a H+ ion>?

I don't think so unless the O atom is involved in a polar covalent bond with F or N, giving it a charge. The O atom alone has no charge so I don't see any attraction occurring between the two. Just what I think.

 

[Aysce]

re: HSC Chemistry Marathon Archive

Where did the other electron come from though?
And remember if electrons are non-bonding they cannot themselves split up to bond with other stuff.

What's the electron you're referring to? The one that was in the pair?

I agree with the second part though who's to say the non-bonding electrons THEMSELVES split up? If anything, "something" has taken an electron away from the pair - the electrons themselves did not split on their own accord.

 

[Sy123]

re: HSC Chemistry Marathon Archive

What's the electron you're referring to? The one that was in the pair?

I agree with the second part though who's to say the non-bonding electrons THEMSELVES split up? If anything, "something" has taken an electron away from the pair - the electrons themselves did not split on their own accord.

Yes, I can tell you that 1 pair of non-bonding electrons go somewhere, they do not split up but they go somewhere.
Think about the bonding from the 'perspective' of the H+ ions now.

 

[Aysce]

re: HSC Chemistry Marathon Archive

Yes, I can tell you that 1 pair of non-bonding electrons go somewhere, they do not split up but they go somewhere.
Think about the bonding from the 'perspective' of the H+ ions now.

Elaborate on "somewhere".

 

[deswa1]

re: HSC Chemistry Marathon Archive

It forms what's known as a co-ordinate covalent bond. I think you do this in module 3.

 

[Aysce]

re: HSC Chemistry Marathon Archive

It forms what's known as a co-ordinate covalent bond. I think you do this in module 3.

Idgi, there is no coordinate covalent bond from what I see in the hydronium ion.

 

[Sy123]

re: HSC Chemistry Marathon Archive

Elaborate on "somewhere".

It forms what's known as a co-ordinate covalent bond. I think you do this in module 3.

Yes it is known as co-ordinate covalent bonding (dative), I didn't know it was in module 3 though. But I wasn't looking for the name of the bond, I was looking for this specific sentence specifically:

Now when we add another H+ to H2O, H+ is simply a 'proton', it has no electrons to 'contribute' to sharing with the H3O+, thus the 2 non-bonding electrons from the O atom goes and forms a bond with the H+ and O, hence making H3O+