VCE Chemistry Question Thread

[Champ101]

"titration for volumetric analysis"
Erm... They're two different terms for the exact same thing. You've basically said "titration for titration".

Anyway, for a titration, we have two moles - , for the acid, and , for the base. They can be calculated like so:



Now, once we hit the equivalence point, these moles are equal, so we can simplify to:



So, now we get to your question.
Let's say we lower the concentration of the acid - this means, we need one of two things:

a) The numerator to also lower, since that'll give us a smaller number.
b) The denominator to raise, since that will also give us a smaller number.

To lower the numerator, we either use a less concentrated base, or we use less base in the burette. (this assumes the acid is in the conical flask, and the base is in the burette) Similarly, we could also put more acid in the conical flask.

Does that explain it at all?

Awesome, thanks EulerFan!

You said once they react to reach equivalence point the number of moles are equal, but isn't it only proportionately equal. For instance with 1H2SO4 + 2NaOH. If this is the case how can we say n_a = n_b?
Thanks!

[keltingmeith]

Awesome, thanks EulerFan!

You said once they react to reach equivalence point the number of moles are equal, but isn't it only proportionately equal. For instance with 1H2SO4 + 2NaOH. If this is the case how can we say n_a = n_b?
Thanks!

The only difference is that there's going to be some constant involved, but the idea is exactly the same. Let's say that instead of a one-to-one ratio, instead the mole ratio is some constant k (so in this case, the ratio could be 2 or 1/2 depending on which way you work). So, now we require that:



to reach the equivalence point. So, this equation translates to:



And the exact some logic applies as earlier. Except, now you can also make some inferences from before, based on the mole ratio between the acid and base. This is an exercise I leave to the reader.

[cosine]

Can someone please explain limiting and excess reactants to me? Thanks

[Redoxify]

Can someone please explain limiting and excess reactants to me? Thanks

Give us an example

[cosine]

Give us an example

Calculate the mass of silver bromide that can be formed if a solution containing 15.0g of silver nitrate is allowed to react with a solution containing 10.0g of calcium bromide - please explain in simple terms for me, thank you, redoxify

[Redoxify]

Calculate the mass of silver bromide that can be formed if a solution containing 15.0g of silver nitrate is allowed to react with a solution containing 10.0g of calcium bromide - please explain in simple terms for me, thank you, redoxify

Okay in the most simplest terms. If you get two masses, one of them will completely react, while the other is still not finished, so in that case put them into an equation, work out the mole ratios, then work out the number of mole for each compound. so example if you get 1:1 ratio, and you get one to = 0.1 and one to = 0.2, obviously the 0.1 is the limiting reagent, therefore you use that number of mole to work out usually the other parts of the question, idk maybe not simple enough let me know

[cosine]

Okay in the most simplest terms. If you get two masses, one of them will completely react, while the other is still not finished, so in that case put them into an equation, work out the mole ratios, then work out the number of mole for each compound. so example if you get 1:1 ratio, and you get one to = 0.1 and one to = 0.2, obviously the 0.1 is the limiting reagent, therefore you use that number of mole to work out usually the other parts of the question, idk maybe not simple enough let me know

Just the part bolded I don't understand, if you could reiterate it, thanks

[Redoxify]

Just the part bolded I don't understand, if you could reiterate it, thanks

Hahaha, okay umm, just watch this video, it's very helpful https://www.youtube.com/watch?v=rESzyhPOJ7I

[jessss0407]

Hi guys!

I have 2 questions regarding sigfigs

1) Evaluate (2.03*10^-3 - 1.20*10^-4)/0.02412
Apparently the answer is 0.0792 - can someone please explain why it is that number of sigfigs/decimal places?

2) Evaluate -log₁₀(0.0192*0.9022)=1.761
again, could someone explain why it is 4 sigfigs and 3 dp?

Thanks!

[keltingmeith]

Hi guys!

I have 2 questions regarding sigfigs

1) Evaluate (2.03*10^-3 - 1.20*10^-4)/0.02412
Apparently the answer is 0.0792 - can someone please explain why it is that number of sigfigs/decimal places?

2) Evaluate -log₁₀(0.0192*0.9022)=1.761
again, could someone explain why it is 4 sigfigs and 3 dp?

Thanks!

The first one is 3 sigfigs because the lowest amount of sigfigs in your data is to 3 sigfigs.

The second one should be 3 sigfigs as well, because above reason.

[cosine]

What's the difference between a full balanced chemical equation and an ionic equation?

[keltingmeith]

What's the difference between a full balanced chemical equation and an ionic equation?

A full balanced chemical equation mentions everything that's happening:



An ionic equation only mentions the things that we care about (in this case, the acid and base reaction):



We often decide what we "care about" by turning all aqueous solutions into ions, and getting rid of things that appear on both sides. For the above:



In this case, the chloride and sodium ions appear on both sides, but everything else is only visible once. So, we "cancel those out", and are left with the ionic equation I showed earlier.

[cosine]

A full balanced chemical equation mentions everything that's happening:



An ionic equation only mentions the things that we care about (in this case, the acid and base reaction):



We often decide what we "care about" by turning all aqueous solutions into ions, and getting rid of things that appear on both sides. For the above:



In this case, the chloride and sodium ions appear on both sides, but everything else is only visible once. So, we "cancel those out", and are left with the ionic equation I showed earlier.

Thanks Euler!

In your first balanced equation, when something's aqueous, say HCl in this case, what does this mean, in terms of the molecular stage? Like i know it means the the solution is submerged into water, but speaking in terms of ions etc what's going on ? Thanks

[warya]

when something's aqueous, say HCl in this case, what does this mean, in terms of the molecular stage? Like i know it means the the solution is submerged into water, but speaking in terms of ions etc what's going on ? Thanks
[/quote]

It means the ions have dissociated because they've dissolved in water (more attracted to water molecules than themselves, grossly simplified). Look through your year 11 notes there was like a whole chapter on solubility and ionic equations, chap 12 I think (heinemann)

[grannysmith]

Hey guys.

Say we've got a strong base (NaOH) being titrated with a strong acid (HCl) and they're concentrations are identical at 0.1M. The equivalence would be at a pH of 7. Would the choice between methyl red and bromothymol blue as an indicator make any difference?

Because I'm thinking that methyl red would be less suitable given that it has a pH range between 4.2-6.3. If it is used as the indicator, would this mean that the end point occurs after the equivalence and so the titre volume would be higher than the actual value?