VCE Chemistry Question Thread

[keltingmeith]

Hi was wondering if someone can please help with this question:

Calculate the final pH of the solution when 200ml of 0.3M hydrochloric acid is added to 500ml of 0.7M sulfuric acid

Thanks

What are you struggling with?

[Coolgalbornin03Lo]

Why is it wrong to say that polysaccarides are broken down in the body with the use of enzymes? In the textbook it says they are (i.e cellulose with cellulase) but in my formative test I have been marked incorrect.

[Evolio]

Why is it wrong to say that polysaccarides are broken down in the body with the use of enzymes? In the textbook it says they are (i.e cellulose with cellulase) but in my formative test I have been marked incorrect.

Basically, humans don't possess the cellulase enzyme, meaning we can't catalyse the hydrolysis of cellulose (the polysaccharide). I assume you're talking about cellulose specifically?

[Coolgalbornin03Lo]

Basically, humans don't possess the cellulase enzyme, meaning we can't catalyse the hydrolysis of cellulose (the polysaccharide). I assume you're talking about cellulose specifically?

Haha yes that makes sense I should know that. But it also said glycogen and starch
Do all R side chains with NH2 have the potential to be proton acceptors? Or only certain ones? I don't understand why COO- is affected the most by decrease in pH because it becomes COOH. But what about the NH2 I have circled becoming NH3+?

I have attached the question

Mod edit(PF): Merged double post

[Evolio]

Do all R side chains with NH2 have the potential to be proton acceptors? Or only certain ones? I don't understand why COO- is affected the most by decrease in pH because it becomes COOH. But what about the NH2 I have circled becoming NH3+?
I have attached the question

This is because COO- has a negative charge and so it is able to attract the H+ proton because that's positively-charged (opposite charges attract). However, NH2 is neutral and so it doesn't attract the H+.

Haha yes that makes sense I should know that. But it also said glycogen and starch

Ohhh. Well, yeah, starch IS broken down. But I could see why they marked glycogen as wrong because it's an energy store that isn't really broken down by enzymes, because it stays in your body. But, it is broken down when energy is needed...

EDIT: Thank you keltingmeith for the correction!

[keltingmeith]

This is because COO- has a negative charge and so it is able to attract the H+ proton because that's positively-charged (opposite charges attract). However, NH2 is neutral and so it doesn't attract the H+.

Slight correction - amines will, 100%, attract H⁺. That's why ammonia is such a good base. There are two reasons why the amine isn't the right choice here:

1. As you've said, the acetate group (COO^-) has a negative charge. Negative charges are ALWAYS gonna be more attractive than a lone pair of electrons. It's like being asked if you want a coke or a pepsi - sure, both of them are cola flavoured and will do the job, but you've probably got a preference.

2. There aren't amines - they're amiDes. While amines are good and decent bases, amides are horrific bases. The explanation for way is university level (it's resonance, the answer to all organic chemistry), but essentially it's because the lone pair isn't exactly centred on the nitrogen, it's partly attracted to the C=O group, and so the amide isn't as good at accepting protons as an amine, where that lone pair is dead-centre on the nitrogen.

[keltingmeith]

How we find the pH of the question?

Well, that equation is quite simple:



If you're confused by the little 10, your calculator's "normal" log button should do the same thing. Using log is just like using sin, cos, and tan. And the square brackets simply mean the concentration of what's inside the brackets

[amanaazim]

can someone suggest 2 solutions that could be mixed to produce a precipitate of calcium carbonate. i need to write it in a full and ionic equation including states and name the spectator ions.

i did CaO + H^2O which equals to Ca(OH)^2 (can someone tell me if this is correct)

If not what would some suggestions be.

[Chocolatepistachio]

Calcium chloride and sodium carbonate  form the precipitate of calcium carbonate.

Calcium nitrate and sodium carbonate form the precipitate of calcium carbonate.

[keltingmeith]

So what would I do if the question was calculate the final pH of the solution when 200ml of 0.3M hydrochloric acid is added to 500ml of 0.7M sulfuric acid?

You would use the formula I just gave to you.

Look, I'm not going to spoon feed you the answer - you're not going to learn that way. If there's a concept in this question you don't understand, tell us, and we'll help clear the air for you. But I'm not just going to answer the question for you - that will help you answer this question and questions like it, but it won't improve your understanding of the underlying chemistry going on

I think you work out the number of moles  first I don’t know if this is right
n(HCl) =cv
= 0.3x 0.2
= 0.06 mol
n(H2SO4)= 0.7 x 0.5
= 0.35 mol
HCl is the limiting reagent

c(HCl)=0.29/0.7
= 0.41
PH= -log 0.41

This will not give the right answer, sorry to say

[Chocolatepistachio]

is the answer -log 1.08571429

Is it possible to have a negative PH

[keltingmeith]

Is it possible to have a negative PH

Yes - pH is not a scale of 1-14, as many seem to believe (I have no clue where this misconception even started, tbh). Hell, a pH of 7 isn't always neutral - it's only neutral if the water temperature is at 25 degrees Celsius, in the presence of strong acids and bases. Neutralising acetic acid with sodium hydroxide will cause the pH to be somewhere around 8-9, to memory.

pH is just the negative logarithm of the concentration of hydronium ions (or free protons in solution). If that concentration is 10^(-15) (which is stupidly small), then the pH is 15. If you have a metric ton of pure water (=1,000 L), then add only 0.03646 nanograms of HCl, you will have a pH of 15. That's right - more than 14, and using an acid, not a base. So, to have a pH of less than 1 - or even negative - all you have to do is really concentrate that acid - you're going to need a concentration of more than 1 mol/L. But even then, it's not that simple, because the more free protons there are in solution, the more likely they'll want to re-bind to free chloride in solution - and that's not easy to measure. In fact, calculating pH in this only really makes sense as a measure in low concentrations as a result - if you want to use high concentrations, you're going to want to start using Ka to calculate what the true pH is. Unfortunately, that's no longer in the curriculum

[Chocolatepistachio]

For this question
A strontium hydroxide solution has a pH= 9.15 . Calculate the concentration of this strontium hydroxide solution

pOH=4.85
[OH]= 0.0000141
[sr(OH)2] = 7.06 x 10^-6
Is this right because the answer said it was 7.05

[SmartWorker]

For Chemistry 3/4 do we need to memorise which compounds form a precipitate and which ones are soluble in a solution and do we have to remember the charges of ions for example SO₄^2-

Thank you

[keltingmeith]

For this question
A strontium hydroxide solution has a pH= 9.15 . Calculate the concentration of this strontium hydroxide solution

pOH=4.85
[OH]= 0.0000141
[sr(OH)2] = 7.06 x 10^-6
Is this right because the answer said it was 7.05

Do you mean the answer should be 7.05 *10^(-6)? Because if that's the case, it's just a rounding error somewhere in the calculation and not worth noticing. If they're saying it's straight 7.05, then they're wrong long

For Chemistry 3/4 do we need to memorise which compounds form a precipitate and which ones are soluble in a solution and do we have to remember the charges of ions for example SO₄^2-

Thank you

No on solubility. Tbh solubility laws are garbage anyway lol. You will never be asked, "is X or Y soluble in water", or similar. The only time this might be an issue, is if you have to write out a chemical equation - since you need to write the physical states of the chemicals in that equation, you will need to know your solubility rules to "predict" those states. However, you can normally figure out what the state is by material supplied in the question. For example, if you're doing a titration, everything should be staying in solution and so they are dissolved. If a precipitate forms, that means the precipitate is not soluble, and so is in the solid state. I have never seen a question asked by VCAA where one of the compounds isn't soluble and you're not told that it is. So like, if you're really scared, sure, memorise your solubility rules - but tbh IMO it's a waste of time.  The one caveat to this, is VCAA seems to like asking questions about the solubility of random things (vitamins, fats, carbs, etc.) in the body, but the rule for that one is simple - does it have a long hydrocarbon chain without many functional groups? It's fat soluble. Is it a small, charged molecule, or does it have lots of polarised functional groups (think alcohols, amines, carboxylic acids, etc.)? It's water soluble. There ARE molecule that break this rule (my favourite is sodium dodecyl sulphate), but they're not covered in VCE.

Is it worth memorising charges of common ions? Honestly, just a flat yes. Tbh, it becomes very easy to predict them (eg, PO₄. P forms 5 bonds, which means that only one of those 4 oxygens will bond to the P twice - and oxygen by itself should be charged, so that's a charge of 3 minus. SO₄. S forms 6 bonds, so by the same logic has a charge of 2 minus. NO₃. N forms 5 bonds, so by the same logic has a charge of 1 minus.), so it's worth looking for trends like that to help you predict what's going on. Also, you can figure out straight from the periodic table for monoatomic ions - fluorine will always be 1 minus, sulphide will always be 2 minus, etc.