VCE Chemistry Question Thread

[JellyBeanz]

Guys quick question please help out,

Guys if i have 250ml Of 0.1M standard solution of Na2CO3 in a volumetric flask, and i make four 20ml aliquots in conical flasks, what will the amount of sodium carbonate in mol be, present in each conical flask?

[Swagadaktal]

Guys quick question please help out,

Guys if i have 250ml Of 0.1M standard solution of Na2CO3 in a volumetric flask, and i make four 20ml aliquots in conical flasks, what will the amount of sodium carbonate in mol be, present in each conical flask?

n(Na2CO3) = cV = 0.02 * 0.1 (as concentration stays same)
therefore 0.002 mol Na2CO3 in each flask

[JellyBeanz]

n(Na2CO3) = cV = 0.02 * 0.1 (as concentration stays same)
therefore 0.002 mol Na2CO3 in each flask

Thanks bro, always see you answering questions, another one, then another one, then another one.

Nah but thanks bro seriously

[tony2k]

You half the n(Al3+) to obtain the n(Al2(SO4)3) as two atoms of Al are required to make aluminium sulphate, in a sense.

It's probably easier to think if you have ten atoms of Al3+ floating around, you'd only have enough to make 5 particles of Al2(SO4)3

Ooh ok thanks for that!
Does that always apply to any equation? As I remember I could usually just multiply the mol by whatever ratio I needed to without regarding the amount of atoms, i.e. the analogy

[Sine]

The concentration of hydrogen peroxide-based bleach was determined by titration with 0.01980M potassium permanganate solution. 20.00mL of the peroxide solution was diluted to 250.0mL. A 20.00mL aliquot of the diluted solution was acidified and titrated with the potassium permanganate solution. An average titre of 19.12mL was required. What was the concentration of the hydrogen peroxide in the bleach?

How are we meant to know the products of the reactions to form a chemical equation? Or do we assume MnO4- ---->Mn2+

[keltingmeith]

Ooh ok thanks for that!
Does that always apply to any equation? As I remember I could usually just multiply the mol by whatever ratio I needed to without regarding the amount of atoms, i.e. the analogy

You should use mol ratios with atoms if that's what you're choosing to work with. In this case, you decided to go by an atom route, so you must use the mol ratios of the atoms. Another thing you could've done is considered the overall equation:

Al2(SO4)3 + 6Cl- -----> 2AlCl3+3SO4

and used molecular mole ratios from that.

The concentration of hydrogen peroxide-based bleach was determined by titration with 0.01980M potassium permanganate solution. 20.00mL of the peroxide solution was diluted to 250.0mL. A 20.00mL aliquot of the diluted solution was acidified and titrated with the potassium permanganate solution. An average titre of 19.12mL was required. What was the concentration of the hydrogen peroxide in the bleach?

How are we meant to know the products of the reactions to form a chemical equation? Or do we assume MnO4- ---->Mn2+

This is actually a pretty complicated reaction, and I'd imagine that you would be told the equation in an exam. I'd imagine they wanted you to use the electrochemical series, but I can't find Mn^7+ or MnO_4^- on there, so you wouldn't have any way of knowing which is the stronger oxidant. If something like this happens again, though, you don't get many oxidants stronger than Mn^7+, and Mn (VII) ----> Mn(II) can be quite a safe assumption if you think it's a redox reaction (transition metals generally like to be in the 2+ oxidation state for various reasons, so you can usually safely assume that if they're reacting, they'll end up there - not a safe assumption past VCE, but for now...)

[Splash-Tackle-Flail]

{you can usually safely assume that if they're reacting, they'll end up there - not a safe assumption past VCE, but for now...)

There always seems to be this sense of wonder in me. As if VCE Chemistry is hiding the true chemical secrets of the world. Whether if be under the façades of "oh tis not in the study design" or "just assume boom reaction", or simply that we're just taking such a shallow (but broad?) course, I'm proud to say, that as university starts and the mysteries of Chem that you and |nzxl| always marvel about unravel before me:



"my body is ready"

[lzxnl]

The concentration of hydrogen peroxide-based bleach was determined by titration with 0.01980M potassium permanganate solution. 20.00mL of the peroxide solution was diluted to 250.0mL. A 20.00mL aliquot of the diluted solution was acidified and titrated with the potassium permanganate solution. An average titre of 19.12mL was required. What was the concentration of the hydrogen peroxide in the bleach?

How are we meant to know the products of the reactions to form a chemical equation? Or do we assume MnO4- ---->Mn2+

Yeah just accept that.
Note: H2O2 is being oxidised to give oxygen gas here.

There always seems to be this sense of wonder in me. As if VCE Chemistry is hiding the true chemical secrets of the world. Whether if be under the façades of "oh tis not in the study design" or "just assume boom reaction", or simply that we're just taking such a shallow (but broad?) course, I'm proud to say, that as university starts and the mysteries of Chem that you and |nzxl| always marvel about unravel before me:



"my body is ready"

Yep. VCE really doesn't teach you much. You get to uni and you realise how insignificant VCE is. Then you graduate and find out how insignificant uni is.

[knightrider]

How would you do this question ?

Many antacids fizz when dissolved in a glass of water.One such brand lists among its ingredients sodium hydrogen
carbonate and citric acid. Write an ionic equation for the reaction between HCO3- (aq) and H3O+ (aq) responsible for the ‘fizz’.

[keltingmeith]

How would you do this question ?

Many antacids fizz when dissolved in a glass of water.One such brand lists among its ingredients sodium hydrogen
carbonate and citric acid. Write an ionic equation for the reaction between HCO3- (aq) and H3O+ (aq) responsible for the ‘fizz’.

One of these is an acid, the other a base. Identify which is which and then do a proton transfer.

[amraa345]

If anyone can share some tips for VCE chemistry (units 1 and 2) or the fundamentals you're expected to know, that would be great

[knightrider]

One of these is an acid, the other a base. Identify which is which and then do a proton transfer.

the answer says  HCO3- (aq) + H3O+ (aq) → CO2 (g) + 2H2O (l)

shouldnt it be HCO3- (aq) + H3O+ (aq) → H2CO3 (aq) + H2O (l)

how do they get this ?

[keltingmeith]

the answer says  HCO3- (aq) + H3O+ (aq) → CO2 (g) + 2H2O (l)

shouldnt it be HCO3- (aq) + H3O+ (aq) → H2CO3 (aq) + H2O (l)

how do they get this ?

The H2CO3 can undergo further reaction to H2O and CO2 - you see it all the time in soft drink, in fact. I'm guessing the book just prefers that reaction - probably not too bad an assumption to have carbonate break down into CO2 if you ever see it like that.

If anyone can share some tips for VCE chemistry (units 1 and 2) or the fundamentals you're expected to know, that would be great

For 1/2, they should assume nothing. Having said that, your school might assume you know some things, so it's worth asking your teacher to find out.

[Biology24123]

Are the answers to these correct? Thanks

[knightrider]

The H2CO3 can undergo further reaction to H2O and CO2 - you see it all the time in soft drink, in fact. I'm guessing the book just prefers that reaction - probably not too bad an assumption to have carbonate break down into CO2 if you ever see it like that.

Thanks EulerFan101