Multiple Choice (3 Viewers)

[sam5]

2009 HSC
1. C
2. C
3. D
4. C
5. A
6. D
7. B
8. A
9. C
10.B
11.D (don't like question as pH adjustment can also be done to assist with flocculation)
12.A
13.B
14. B
15. A

I didn't think that they were too tricky
As usual - stupid mutli-step calculations question worth 1 mark (Q14,15)

Can anyone plz write up the mc (even if its a very half arsed job)?

 

[blabla91]

2009 HSC
1. C
2. C
3. D
4. C
5. A
6. D
7. B
8. A
9. C
10.B
11.D (don't like question as pH adjustment can also be done to assist with flocculation)
12.A
13.B
14. B
15. A

I didn't think that they were too tricky
As usual - stupid mutli-step calculations question worth 1 mark (Q14,15)

I got exactly the same as this except I put B for q.11.... Lime is added in the mixing tank prior to flocculation in order to create alkaline conditions... surely that has to be a correct answer??? also Im sure that any other pH adjustment comes AFTER chlorination since the hypochlorite ion changes the pH again...

 

[buildabridge]

Mine is:
1.C
2.C
3.D
4.C
5.A
6.D
7.B
8.A
9.C
10.B
11.B
12.A
13.B
14.B
15.A
LOL there are 2 threads for MC

I got the same as you

 

[HD09]

1:C
2:C
3: D
4:C
5:A
6: D
7:B
8:A
9:C
10:B
11:B --> need appropriate pH conditions before flocculation and coagulation occurs is that right?
12:A
13:B
14:B
15:A

 

[C-BAS]

I can't remember what I put, all I know is I didn't get many of them. The only thing I feel reasonably confident about is the Option (Industrial Chemistry). I think I may have gotten around 20/25 or about that.

I'll be happy if I can get 60%. Yep, that's how much I fucked it.

 

[Izla]

14 could have been a short answer worth three marks :l

 

[niloony]

Talking to my bro who does chem at uni the answers were...C C D C A D B A C B C A C B A

I got 11 (PH has to be adjusted for alum to work), 13 (Why did i times by 2 instead of 6 >< ) and 14 wrong.

Thats if he's correct >>

 

[Tronester]

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.

 

[walk]

This comes from the Hunter Water web site....
"Dungog TP
The Dungog plant currently provides a peak supply capacity of 90 megalitres per day and treats water from Chichester Dam.

Dungog Water Treatment Plant uses a direct filtration method.

Alum and polyelectrolyte are rapidly mixed into the water, causing coagulation and flocculation. The floc is filtered through a filter bed composed of sand, anthracite and gravel, using contact filtration. After filtration water is chlorinated, fluoridated and dosed with lime and carbon dioxide for pH correction and buffering, before being returned to the Chichester Main."

Not clear the in order in which the chlorination/fluoridation/pH dosing occurs... have read other texts where pH adjsutment is last?

 

[TechnIx]

Talking to my bro who does chem at uni the answers were...C C D C A D B A C B C A C B A

I got 11 (PH has to be adjusted for alum to work), 13 (Why did i times by 2 instead of 6 >< ) and 14 wrong.

Thats if he's correct >>

Your brother is way off.

 

[walk]

Aslo from the Hunter Water site.....

"Grahamstown TP

Grahamstown water is treated by a conventional filtration process. The plant has a peak supply capacity of 265 megalitres per day and is Hunter Water’s largest water treatment plant.

Coagulation is achieved by the addition of alum and poly-electrolyte, followed by sedimentation, then filtration, followed by the addition of lime, chlorine and fluoride (refer to figure below).

A powdered activated carbon dosing plant has also been constructed at Grahamstown Dam, which serves two main purposes. It:

removes objectionable taste compounds that may be present in the water
assists in removing blue-green algal toxins, if the need ever arises.
Figure 1 Grahamstown Water Treatment Plant Process Flow Diagram"

This one has the lime before the chhlorine......so......?????

 

[dekimasu]

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.

n(NaOH) = [NaOH] x L = 0.55 x 0.0295 = 0.016225 mol

Equation is H3X + 3NaOH ---> Na3X + 3H20
1 : 3 : 1 : 3

So 3n(H3X) = n(NaOH)
= (1/3)x0.016225
= 0.0054083...
[H3X] = mol/L
= .0054... / 0.25
= 0.2163.. mol/L
mol/L x g/mol = 0.2163 x 192.12
= 41.56 g/L

 

[Mr-Morten]

My answers were:
1 C
2 C
3 D
4 C
5 A
6 D
7 B
8 A
9 C
10 B
11 A
12 A
13 B
14 B
15 A

I've checked and from what i can tell they're all right. Here's hoping *crosses fingers*

 

[Mr-Morten]

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.

Nah bromine is non-polar. For one thing the two bromine atoms in Br2 have the exact same electronegativity as each other and therefore it can't be polar (feel free to correct me if i'm wrong)

 

[TechnIx]

My answers were:
1 C
2 C
3 D
4 C
5 A
6 D
7 B
8 A
9 C
10 B
11 A
12 A
13 B
14 B
15 A

I've checked and from what i can tell they're all right. Here's hoping *crosses fingers*

Question 11, I'm almost 100% sure that it's actually D.
pH adjustment is a late step, after all the treatment.

After all, they'd want to get rid of the shit before they add other chemicals to balance the pH right?

As for the rest, they seem correct.

I wish someone would explain 9 to me though. I got B, but everyone says its C.

 

[TechnIx]

Nah bromine is non-polar. For one thing the two bromine atoms in Br2 have the exact same electronegativity as each other and therefore it can't be polar (feel free to correct me if i'm wrong)

That was my reasoning as well.

 

[untouchablecuz]

hahahaha

i guessed B for 14

how right i was

=]

 

[Mr-Morten]

Question 11, I'm almost 100% sure that it's actually D.
pH adjustment is a late step, after all the treatment.

After all, they'd want to get rid of the shit before they add other chemicals to balance the pH right?

As for the rest, they seem correct.

I wish someone would explain 9 to me though. I got B, but everyone says its C.

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

 

[TechnIx]

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

I'm reading my Excel (latest edition). It doesn't say anything about pH, Calcium Hydroxide, or anything of the sort. I'm not saying you're lying, I just think that it's annoying how these sources all contradict each other! =P

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

Thanks

 

[Mr-Morten]

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