HSC Chemistry 2017

[Zainbow]

For 13, isn't it the strongest acid that would require the most amount of NaOH? I chose H2SO4 because it is diprotic and hence would require the most amount of NaOH

Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react, being triprotic, so a higher volume of NaOH is needed

[Happy Physics Land]

Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react so a higher volume of NaOH is needed

Definitely agree with you Zainbow! Good explanation!

[angelahchan]

20. D (moles of Ba(OH)2 = 0.02 x 0.02 = 0.0004 moles, moles of HCl = 0.05 x 0.04 = 0.002 moles. When you write out the balanced chemical equation, the ratio of Ba(OH)2 to (HCl) is 1:2. Hence HCl is in excess by 0.002 - (0.0004 x 2) = 0.0012 moles. pH = -log(0.0012) = 2.9)

for 20 I don't remember what answer I picked, but don't you have to divide the moles by the total volume or something to find concentration? (before putting it into the -log calculation I mean)

[mbdtHSC]

for 20 I don't remember what answer I picked, but don't you have to divide the moles by the total volume or something to find concentration?

Yeah you need to do c=n/v1 +v2 to get the final concentration, then pH from that

[maria19990]

Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react, being triprotic, so a higher volume of NaOH is needed

but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??

Hey there! NaOH being a strong base will drive one way reaction and will force the citric acid to ionise completely.

[tripat23]

don’t you have to divide by the volume for the concentration of the solution in question 20??

[maria19990]

Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react, being triprotic, so a higher volume of NaOH is needed

but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??

Hey there! Because NaOH is a strong base, it is going to drive one-way reaction and make the citric acid ionise completely even thought it is a weak acid.

[angelahchan]

Yeah you need to do c=n/v1 +v2 to get the final concentration, then pH from that

do you remember what answer you picked? because i forgot if I remembered to divide the moles by 2

[tripat23]

I got 1.6 I think

[Paul.I]

Now correct me if l am wrong, but for question 1 wasn't it based off accuracy, hence wouldn't a burette be more accurate than a measuring cylinder, therefore, A. And for q3 wasn't that the one about naming, if l counted correctly l thought there were 4 carbons, a fluorine on the first carbon and a chlorine on the second, therefore, also A. But thanks for posting those solutions

[mbdtHSC]

do you remember what answer you picked? because i forgot if I remembered to divide the moles by 2

Not sure from memory, 1.8 or something maybe?

[Zainbow]

but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??

Yes citric is weak. But the titration is a reaction and not simply just a dissociation, so the OH ions force all able protons to react

[Paul.I]

Also with 20, did you forget to divide by 0.07 L to get the concentration, which would make the answer C

[angelahchan]

Not sure from memory, 1.8 or something maybe?

thanks, I put it in my calculator and that seems like the right answer... do you remember which option that was?

[mbdtHSC]

thanks, I put it in my calculator and that seems like the right answer... do you remember which option that was?

C I think?