Chemistry 3/4 2013 Thread

[NoticeMeSenpai]

Hello

Can anyone help me with this question?
The volume, in mL, of pure water that must be added to 50.0mL of 0.0100 M HNO3, to produce a diluted solution of pH 4.00 is closest to:  (VCAA sample exam 2008)

Thank you.

[sin0001]

n(HNO3) = n(H+) =  cV = 0.05 x 0.01 = 5 x 10^(-4) mol
For a solution of pH 4, c(H+) = 10^(pH) = 10^(-4) M
You have the c(H+), the n(H+), plug em' into n=cv to find final volume:
V = n/c = (5 x 10^-4 mol)/(10^(-4) M) = 5 Litres (Be careful: this isn't the volume that needs to be added, it's your total volume)
Subtract 50 mL from your total volume, you get 4950 mL

[NoticeMeSenpai]

Thank you so much

[randomuser]

What's a partial equation?

[hungover]

a question said the following:

two students titrate a weak acid(ethanoic acid) and a weak base(Na2CO3)
i) what is the problem that would be encountered? (1 mark)
ii) how should they ensure good results are obtained? (2 mark)

My answers were:
i) the end point will not be distinguishable accurately and so a back titration will be required.
ii) perform a back titration; make sure the equipment is rinsed with appropriate liquids.

what marks should i be awarded?

[NoticeMeSenpai]

Isn't HNO3 an acid not a base?

[Limista]

What's a partial equation?

That's not a very discrete term.

I think that partial equation means either a reduction/oxidation half equation.

[hungover]

Isn't HNO3 an acid not a base?

sorry my bad.
its fixed now. it was supposed to be Na2CO3

[saba.ay]

a question said the following:

two students titrate a weak acid(ethanoic acid) and a weak base(Na2CO3)
i) what is the problem that would be encountered? (1 mark)
ii) how should they ensure good results are obtained? (2 mark)

My answers were:
i) the end point will not be distinguishable accurately and so a back titration will be required.
ii) perform a back titration; make sure the equipment is rinsed with appropriate liquids.

what marks should i be awarded?

I don't know whether this is right or not, but maybe include 'broad endpoint' somewhere in there? Also, not sure what ii) requires as it's worth 2 marks. Imo, your answers seem right, but you probably need to get someone more knowledgeable to check.

[lzxnl]

Mentioning that the reaction will not go significantly to completion would also help.

[Limista]

For question 4a) page 18 Cambridge checkpoints -

Does anyone know why we divide by 2 to get the number of moles of P₂O₅?

Thanks

[teletubbies_95]

Hi !
The mole ratio is 1/2 , therefore dividing by 2!

Hope that helps!

[Limista]

Hi !
The mole ratio is 1/2 , therefore dividing by 2!

Hope that helps!

I'm confused because it says:

* All the phosphorus has been precipitated (so shouldn't (number of mol of precipitate) = (number of mole of P2O5)?)

 

[lzxnl]

One mole of P2O5 will have two moles of P atoms.
Therefore, if you halve the number of moles of P2O5, you have the number of P atoms.

[Limista]

Ok get it now - thankyou!