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Multiple Choice (1 Viewer)

Mr-Morten

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I'm reading my Excel (latest edition). It doesn't say anything about pH, Calcium Hydroxide, or anything of the sort.
I've got the slightly older edition so that's probably the cause of that discrepency. I think this is just something where its really hard to tell who's right, especially when (like you said) different sources say different things. Gah, i hate ambiguity
 

Randomist

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Can anyone tell me what mark I got? I'm thinking about 10.

C
C
D
C
D

D
B
A
B
B
D
A
B
A
C

 

RandalThor

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Now this is where I shined. Apart from 15, I found these very easy in comparison to past years. Some were so straightforward it wasn't funny. :sun:
 

cdiddy7

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C
C
B - WRONG
A - WRONG
A
D
B
A
C
B
D
A
B
B - guessed
C - guessed


what would i have got
 

truestar92

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i hated MC i feel like this is the part of the exams where i lost the most marks ... sigh
 

kieranjackson91

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According to my Excell hsc book: "
  • Flocculation is the addition of electrolytes... blah blah blah... Calcium hydroxide is added to adjust the pH of the water.
  • Settling: blah blah blah"
I'm pretty sure that adjusting the pH before settling helps the colloidal particles to clump together better. As for balancing the pH (e.g. to kill bacteria etc.), isn't that the job of the chlorination stage? I just can't see how they could mark A as incorrect

As for Q9, in Cr2O7 the -2 on each oxygen adds up to -14. Since the total charge is -2, the Chromium ions must each contribute +6 (i.e. -14+6+6=-2). Therefore, in the reaction, the oxidation state of chromium has changed from +6 to +3. This corresponds to a gain of electrons and a lower oxidation state
Flocculation a.k.a. coagulation occurs AFTER the pH has been raised to precipitate aluminium ions or Fe3+, so the answer from excel would be B. The pH needs to be adjusted to have enough OH- ions to precipitate out the added cations which then flocculate . A could not be correct as it implies flocculation occurs before pH adjustment. But pH is adjusted to flocculate.
 

cdiddy7

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pretty sure the answers are

C C D C A D B A C B D A B B A

i could be wrong
this is just based on what everyones saying
and i no 11 has been a topic of discussion
so that could be wrong
but im fairly confident with those answers
tell me if u think ive made an error
 

Drewx

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Mate in the end none of us can be 100% certain unless you have the answer sheet.
 

Gibbatron

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Can someone show how to do 14, the fact that citric acid is triprotic confused me alot.

Also wouldnt Bromine be polar as it dissolves in water? This would make 6 A although I put D as many others have.
The way i did this question was to work out the moles of NaOH using n=CV. I then (and i think this is the part i did wrong) assumed that because citric acid is tripotic, then you would need three times as much NaOH to neutralise it. Using this assumption i worked out the moles of cirtric acid (divided moles NaOH by three) and then used the molar mass of citric acid(given) to determine the mass (m=nM). With the mass and the volume of acid (25mL), you could work out the concentration of the acid.

I think my final answer was B. The only problem is citric acid doesn't fully ionise so you wont need exactly three times as much NaOH to neutralise it, but i wasn;t sure how to tak that into account without more information.
 

walk

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It doesnt' matter that citiric acid is weak and doesn't fully ionise - when you do a titration this continually disturbs the equilibrium between the acid molecules and the ions, pulling the equilib to the right every time you add more base - so eventually all three acidic protons are removed from the acid molecule - so dividing the number of moles of NaOH by three is right.
 

Aerath

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pretty sure the answers are

C C D C A D B A C B D A B B A

i could be wrong
this is just based on what everyones saying
and i no 11 has been a topic of discussion
so that could be wrong
but im fairly confident with those answers
tell me if u think ive made an error
Nah, I reckon you're all correct. I put the same for question 11, but I'm not sure, but yeah. :p
 

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