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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #1050 on: June 22, 2014, 11:26:27 am »
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The reaction you quoted at B requires acid, whereas I only see aluminium sulfate in your solution (which doesn't have H+ as far as you're meant to know from the question; oddly enough aluminium sulfate is acidic which would actually change the entire question but you haven't been told that)
Even so, a bit of fiddling around with Nernst equations suggests that even at neutral pH, reduction of sulfate is more favourable than reduction of water. Sigh.

As for D, the only issue I can see is potentially whether or not you have 1 M sulfate ions there. Otherwise your answer should be ok.

Thankyou :)

Sorry yes the question did state that the concentration of both solutions was 1.0 M.

Okay I see what you mean, because H+ isn't present in the aluminium sulfate solution then the reduction reaction that I wrote for B can't occur. Or are you saying it still can? (We havent done Nernst equations).

Not quite sure if you are saying my answers for B and D are right or not....... Haha.
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #1051 on: June 22, 2014, 05:20:38 pm »
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Thankyou :)

Sorry yes the question did state that the concentration of both solutions was 1.0 M.

Okay I see what you mean, because H+ isn't present in the aluminium sulfate solution then the reduction reaction that I wrote for B can't occur. Or are you saying it still can? (We havent done Nernst equations).

Not quite sure if you are saying my answers for B and D are right or not....... Haha.

Yeah...I figured you hadn't look at the Nernst equation yet. Basically it allows you to find non-standard electrode cell potentials. Given that this is a Fundamentals subject, I'd say your answer to B is wrong because you don't have acid and that the given answer is correct. In reality, your reaction given in B would occur, but MUCH slower than the one given.

And I just realised the problem with D. 1 M sulfuric acid doesn't have 1 M sulfate ions; H2SO4 is a strong acid but HSO4- isn't. Hence your answer to D is wrong as well.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #1052 on: June 22, 2014, 05:25:15 pm »
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Yeah...I figured you hadn't look at the Nernst equation yet. Basically it allows you to find non-standard electrode cell potentials. Given that this is a Fundamentals subject, I'd say your answer to B is wrong because you don't have acid and that the given answer is correct. In reality, your reaction given in B would occur, but MUCH slower than the one given.

And I just realised the problem with D. 1 M sulfuric acid doesn't have 1 M sulfate ions; H2SO4 is a strong acid but HSO4- isn't. Hence your answer to D is wrong as well.

Cool man thanks. That all makes sense why now. I think that's way more complicated than anything we would get in an exam but that's cool. You learn more by doing problems beyond your level! Great - thanks again.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #1053 on: June 23, 2014, 02:32:10 pm »
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Hi everyone. 

Could anyone help me with some MCQ questions?

1. Which of the following acids (together with their sodium salts) is the best choice to buffer a solution at pH 5.0?
A. chloroacetic acid (Ka = 1.4 x 10-3)
B. propanoic acid (Ka = 1.3 x 10-5)
C. benzoic acid (Ka = 6.4 x 10-5)
D. hypochlorous acid (Ka = 3.5 x 10-8)
E. carbonic acid (Ka = 4.3 x 10-7)

I've subsequently discovered that you solve this by saying pH = pKa, and pKa = -log10Ka, but I don't know where this comes from and if it always holds true?  So I can solve this problem, I just don't know why we use pH = pKa.

2. Calcium carbonate, CaCO3, has a solubility product, Ksp, of 2.8 × 10-9. What is the concentration of Ca2+ ions in a saturated solution of CaCO3?
A. 1.4x10-9 M
B. 2.8x10-9 M
C. 5.6x10-9 M
D. 5.3x10-5 M
E. 1.1x10-4 M

Is this just because at equilibrium, stoichiometry says that, [Ca2+] = [CO32-] = √Ksp ?  Or is there something more to it?

3. Iodide reacts with peroxodisulphate in aqueous solution to produce iodine and sulphate according to the equation:
 2I-(aq) + S2O82-(aq) → I2(aq) + 2SO42-(aq)

If the rate at which I2 is produced is 2.0 x 10-6 Ms-1, then:


A. S2O82- is consumed at 1.0 x 10-6 Ms-1
B. I- is consumed at 2.0 x 10-6 Ms-1
C. SO42- is formed at 2.0 x 10-6 Ms-1
D. S2O82- is consumed at 2.0 x 10-6 Ms-1
E. SO42- is formed at 1.0 x 10-6 Ms-1

The provided answer says D, which makes sense, but I don't know why E isn't also true (by stoichiometric ratios)?  i.e. D is true because the stoichiometric ratios of I2 and S2O82- are 1:1, but isn't E also true because SO42- will form at half the rate of I2 because it is producing twice the amount?

4. Samples of chlorine, neon and nitrogen are contained in three identical 1litre flasks. All are at the same temperature. The mass of gas in each flask is the same. Which of the following is TRUE?

A. They are all at the same pressure.
B. The density of chlorine is higher than the other two.
C. The density of nitrogen is lower than the other two.
D. The number of Ne atoms is half the number of Cl atoms. The pressure is highest in the flask of neon.
E. The pressure is highest in the flask of neon.

I got the answer E, because of the following rationale, but the provided answer said B?
Atomic masses: N < Ne < Cl
Molecular mass of gas: Ne < N2 < Cl2
Therefore Ne flask has most number of molecules and Cl2 has the least (to make the same mass).

  • A is false because n is different for each flask.
    B is false because Cl2 has fewest molecules, thus is LEAST dense.
    C is false because N2 is in the middle.
    D is false because for 1 gram mass, n(Ne) = 0.05 mol and n(Cl) = 0.028 mol.
    E is true because Ne contains the most molecules, maximizing PV=nRT.
So why is B the answer?

Thanks everybody :) 
« Last Edit: June 23, 2014, 02:35:41 pm by hobbitle »
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #1054 on: June 23, 2014, 04:15:57 pm »
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Hi everyone. 

Could anyone help me with some MCQ questions?

1. Which of the following acids (together with their sodium salts) is the best choice to buffer a solution at pH 5.0?
A. chloroacetic acid (Ka = 1.4 x 10-3)
B. propanoic acid (Ka = 1.3 x 10-5)
C. benzoic acid (Ka = 6.4 x 10-5)
D. hypochlorous acid (Ka = 3.5 x 10-8)
E. carbonic acid (Ka = 4.3 x 10-7)

I've subsequently discovered that you solve this by saying pH = pKa, and pKa = -log10Ka, but I don't know where this comes from and if it always holds true?  So I can solve this problem, I just don't know why we use pH = pKa.

2. Calcium carbonate, CaCO3, has a solubility product, Ksp, of 2.8 × 10-9. What is the concentration of Ca2+ ions in a saturated solution of CaCO3?
A. 1.4x10-9 M
B. 2.8x10-9 M
C. 5.6x10-9 M
D. 5.3x10-5 M
E. 1.1x10-4 M

Is this just because at equilibrium, stoichiometry says that, [Ca2+] = [CO32-] = √Ksp ?  Or is there something more to it?

3. Iodide reacts with peroxodisulphate in aqueous solution to produce iodine and sulphate according to the equation:
 2I-(aq) + S2O82-(aq) → I2(aq) + 2SO42-(aq)

If the rate at which I2 is produced is 2.0 x 10-6 Ms-1, then:


A. S2O82- is consumed at 1.0 x 10-6 Ms-1
B. I- is consumed at 2.0 x 10-6 Ms-1
C. SO42- is formed at 2.0 x 10-6 Ms-1
D. S2O82- is consumed at 2.0 x 10-6 Ms-1
E. SO42- is formed at 1.0 x 10-6 Ms-1

The provided answer says D, which makes sense, but I don't know why E isn't also true (by stoichiometric ratios)?  i.e. D is true because the stoichiometric ratios of I2 and S2O82- are 1:1, but isn't E also true because SO42- will form at half the rate of I2 because it is producing twice the amount?

4. Samples of chlorine, neon and nitrogen are contained in three identical 1litre flasks. All are at the same temperature. The mass of gas in each flask is the same. Which of the following is TRUE?

A. They are all at the same pressure.
B. The density of chlorine is higher than the other two.
C. The density of nitrogen is lower than the other two.
D. The number of Ne atoms is half the number of Cl atoms. The pressure is highest in the flask of neon.
E. The pressure is highest in the flask of neon.

I got the answer E, because of the following rationale, but the provided answer said B?
Atomic masses: N < Ne < Cl
Molecular mass of gas: Ne < N2 < Cl2
Therefore Ne flask has most number of molecules and Cl2 has the least (to make the same mass).

  • A is false because n is different for each flask.
    B is false because Cl2 has fewest molecules, thus is LEAST dense.
    C is false because N2 is in the middle.
    D is false because for 1 gram mass, n(Ne) = 0.05 mol and n(Cl) = 0.028 mol.
    E is true because Ne contains the most molecules, maximizing PV=nRT.
So why is B the answer?

Thanks everybody :)

1. Buffer works best when [acid] = [conjugate base]
So if Ka = [H+][base]/[acid], then the [acid] and [conjugate base] cancel => Ka = [H+], pKa = pH

2. Exactly. There's nothing more to it.

3. Let's have a look at this carefully. For every mole of I2 formed, stoichiometric ratios suggest that 2 moles of sulfate are formed. Hence, if the rate of formation of I2 is 2 * 10^-6 M/s, the rate of formation of sulfate should be twice that, not half that.

4. I agree with your answer. The densities (mass/volume) are the same as the masses and volumes are all identical. I think the question is messed up.
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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #1055 on: June 23, 2014, 04:31:19 pm »
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Thanks lznxl!!!
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Reus

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Re: VCE Chemistry Question Thread
« Reply #1056 on: June 24, 2014, 09:18:27 pm »
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Generally the arrows for a reversible reaction looks like this


But is it okay if I wrote it like this?


Just have a tendency to do it that way when I'm writing fast, just wondering if it matters haha?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #1057 on: June 24, 2014, 09:28:53 pm »
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Well. You'll be unlucky if you lost a mark for that. I still wouldn't write them that way :P
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Re: VCE Chemistry Question Thread
« Reply #1058 on: June 25, 2014, 07:45:02 pm »
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Hi guys,
With amino acids in a neutral pH we would have NH3+ and COO- and we would call this amphiprotic as the carboy can donate a proton and amino can accept it. However, I was just wondering if the pH dropped to say 2 and we had NH3+ and COOH would this still be considered amphiprotic?  :)
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Re: VCE Chemistry Question Thread
« Reply #1059 on: June 25, 2014, 08:15:09 pm »
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Could some please explain this to be simply. I get 0.01=mass of solvent (x) / 1000 ( because 1000ml in litre and density is 1g ml-1). I get an answer of 10 micro grams, but answer is D

soNasty

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Re: VCE Chemistry Question Thread
« Reply #1060 on: June 27, 2014, 05:45:45 pm »
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is this an exothermic or endothermic reaction?

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Re: VCE Chemistry Question Thread
« Reply #1061 on: June 27, 2014, 06:15:21 pm »
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Could some please explain this to be simply. I get 0.01=mass of solvent (x) / 1000 ( because 1000ml in litre and density is 1g ml-1). I get an answer of 10 micro grams, but answer is D

1ppm = 1 mg/Litre (used for weight/volume questions)

0.01 ppm = 0.01 mg/ Litre.

Since it is asking for the mass of Cd in 1 Litre, you get a mass of 0.01 mg from direct conversion. For this question there is no need for working out. You just have to be able to recognise that 1 ppm = 1 mg/ L. Hence, D is the answer :)
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #1062 on: June 27, 2014, 09:51:20 pm »
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is this an exothermic or endothermic reaction?

I'm going to say endothermic. But this is really vague; wouldn't you typically be given the thermochemical equation (with the enthalpy) for the reaction, from which you can decipher whether the reaction is endo- or exothermic by looking at whether delta H is + or -?

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Re: VCE Chemistry Question Thread
« Reply #1063 on: June 29, 2014, 02:18:10 pm »
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What do we call reactions that keep donating H+? Like when the left side keeps losing H
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Re: VCE Chemistry Question Thread
« Reply #1064 on: June 29, 2014, 02:22:37 pm »
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What do we call reactions that keep donating H+? Like when the left side keeps losing H

Are you talking about redox reactions?
If so, I like to look more-so at the loss/gain of electrons!!