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Multiple choice-Q14 (1 Viewer)

Answer to Q14 multi

  • a)

    Votes: 3 2.3%
  • b)

    Votes: 60 45.1%
  • c)

    Votes: 53 39.8%
  • d)

    Votes: 17 12.8%

  • Total voters
    133

fishbulb

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Kearnzo said:
n(H+ in HCl)= 0.02 x 0.08 = 1.6x10^-3

n(OH- in NaOH)= 0.03 x 0.05 = 1.5x10^-3

That is right

But 0.0016 - 0.0015 is 0.0001

This is number of moles of H+ ions left in the solutions

Ph = -log10[H+]

c = n/v

volume is 20mls +30 mls = 0.05 L

therefore [H+] is 0.0001/0.05 = 0.002

Ph = -log10[0.002] is equal to 2.69... = 2.7
This is perfect working!
 

wLym

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@who said:
damn. i put d. 7 . lol i tried the calculations and came out with a weird answer so i just said since they are both strong acid / base then it must neutralise . fark me
hahahahaHAHAHAHA lol i did the exact same thing except i didnt even try calculations only looked at that question with 10 mins to go and i was like fuck it theyre both strong acids so they must neutralise to form neutral salts.....how unscientific of me xD
 

friction

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Im so stupid i just looked at it and went they are approx the same concentration overall and they are strong acid and strong base so they must be approximately pH of 7. I didnt even think about it. So stupid.
 

iEdd

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lol, funny how the people who put C are now trying to argue that acids cannot be diluted and defy the laws of chemistry. You do realise that if the answer is C, the volumes are completely ignored, and adding any amount more water to the solution will not change the pH (by your logic)?

The teacher that thinks it is C either made a mistake, as we all do, or is a moron and should be fired.

pH = -log[concentration of H+] You do this, you get B.
pH = -log[absolute number of H+] - do this, which is incorrect, you get C. And as I said, if you ignore volumes and don't work with concentrations, then you are effectively arguing that there is no such thing as diluting or concentrating a substance and the world as we know it is fucked as simple concepts are apparently too hard.
 

friction

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iEdd said:
lol, funny how the people who put C are now trying to argue that acids cannot be diluted and defy the laws of chemistry. You do realise that if the answer is C, the volumes are completely ignored, and adding any amount more water to the solution will not change the pH (by your logic)?

The teacher that thinks it is C either made a mistake, as we all do, or is a moron and should be fired.

pH = -log[concentration of H+] You do this, you get B.
pH = -log[absolute number of H+] - do this, which is incorrect, you get C. And as I said, if you ignore volumes and don't work with concentrations, then you are effectively arguing that there is no such thing as diluting or concentrating a substance and the world as we know it is fucked as simple concepts are apparently too hard.
Im not sure becuase i havent looked at the working but amaybe they think ur double counting the volume. I really dont care i know i got it wrong and that answer looks right cause all the rest are whole numbers there to throw stupid people like me off that like nice answers.
 

iEdd

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Well the working has been shown. (See Kearnzo)

Concentrations -> numbers of ions -> concentrations -> pH

The only way it can work.
 

Daniel-08

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Aaron.Judd said:
My chem teacher (JRAHS) says it is C.
your teachers credability goes out the window.

you have to find the concentration first by dividing by 0.05 litres. you cant just log the number of moles. are people just ignorant?
 

timmiitippii

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Aaron.Judd said:
My chem teacher (JRAHS) says it is C.
I find it quite funny that people are using comments like these as definite proof and reasoning for an answer ==

Also its funny because then that teacher ain't quite so good..
When you find pH, its -log [H+] i.e. u use the concentration of H+ ions.
20mL of 0.08 mol L-1 HCl = 0.0016 moles of HCl
30 mL of 0.05 mol L-1 NaOH = 0.0015 moles of NaOH
Therefore 0.0015 moles of H+ reacts with 0.0015 moles of OH- leaving behind
0.0001 moles of H+ ions.
Now since u added the two solutions together, the new volume is 50 mL
so concentration = moles/volume = .0001 / .05 = 0.002 mol L-1 H+
-log (.002) = 2.7 the answer is B

Don't embarrass the poor chem teacher from Jame Ruse anymore by bringing up her mistake again.
 

mick135

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your teacher says its c.

well, chuck norris says its d
and those of you who disagree will get a hi-jump-kick to the side of the face

your working is no match for the size 46 shoe of chuck norris.
he eats acid/base reactions for breakfast!
 

inflatablerozza

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well just for people's interest, here's a bit of the email my chem teacher sent me:

I got 2.7 = B

.0016 mol HCl - .0015 mol Na OH = excess .0001 mol HCl in .05L of water


Therefore [H+] = .0001/.05 = .002 molar

pH = -log10(.002) = 2.7

And my teacher is the chuck norris of chemistry and physics.
 
Last edited:

FatJ

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Can somebody who thinks the answer was C put some working behind their answer? I got B and I'm absolutely sure that that was the answer.
I find it incredible that people argue over the questions I find the easiest.
 

iEdd

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Sorry that won't happen. Anyone that put C has either seen it was a mistake, by the correct working here, OR is slightly special and will not show logic or reasoning, just "I think it's C".
 

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