O-Level Chemistry

PSLE marks the graduation of Primary school students and their entry into Secondary schools as teenagers. Discuss all issues about Secondary schooling here.
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sunnymoon
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Re: Chemistry Help Centre

Post by sunnymoon » Sun Jun 15, 2014 1:37 am

Thanks a lot hterbin. I really appeeciate your help n clear explanation..

hterbin
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Re: Chemistry Help Centre

Post by hterbin » Sun Jun 15, 2014 2:58 pm

No problem :) My pleasure

sunnymoon
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Re: Chemistry Help Centre

Post by sunnymoon » Sun Jun 29, 2014 4:40 pm

Can help me with this question
The pH of a lake polluted with acid can be raised by adding
1) calcium carbonate
2) potassium hydroxide

and can explain why you chose the answer. Thanks

hterbin
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Re: Chemistry Help Centre

Post by hterbin » Sun Jun 29, 2014 6:22 pm

Hi, before I start I would just like to point out that this question is a badly set question. Therefore, I am assuming that the lake needs to be specifically neutralised instead of just raising the pH level.

In this case, the answer would be Calcium Carbonate.

You see, all carbonates reacts with an acid to produce a salt, carbon dioxide and water. The carbonate itself is neutral.

When you add carbonate to an acidic solution, the carbonate reacts with the H+ ions, to form water. (E.g. CaCO3 + H2SO4 -> CaSO4 + H2O + CO2) As you can see, the H+ ions react with the oxide ions from the carbonate, thus decreasing the concentration of H+ ions, neutralising the solution. The solution would not become alkaline as all the acid has reacted with the carbonate.

On the other hand, when you add Potassium Hydroxide, the solution would then raise in the concentration of hydroxide ions. Yes indeed, the alkaline would react with the acid to form a salt and water. However, when added in excess, the alkaline would raise the pH beyond its neutral point. Because all the acid has reacted away and no H+ ions is available to counteract the hydroxide ions.

To sum it up, both solutions will raise the pH, but neutralising the solution is another key point. KOH will raise the pH beyond 7 (neutral) which means the lake will become more alkaline. However, CaCO3 would only bring the pH up to 7 (neutral). Hope that answers your question.

P.S. Do share this help page with ur other friends :)

sunnymoon
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Re: Chemistry Help Centre

Post by sunnymoon » Sun Jun 29, 2014 6:34 pm

Thanks for your clear explanation. Sure I will share this page with my friends


sunnymoon
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Re: Chemistry Help Centre

Post by sunnymoon » Sun Jun 29, 2014 6:34 pm

Thanks for your clear explanation. Sure I will share this page with my friends

Goodluck889
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Re: Chemistry Help Centre

Post by Goodluck889 » Wed Jul 02, 2014 10:30 pm

Can someone help me with the following questions
Q1) Why is Sodium Carbonate a base?

Q2) Will acid in organic solvents such as propanone ionize? How does it conduct electricity?

tutor76
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Re: O-Level Chemistry

Post by tutor76 » Fri Jul 04, 2014 1:44 pm

Something for others to try :)
Image

tutor76
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Re: O-Level Chemistry

Post by tutor76 » Sat Jul 05, 2014 4:17 pm

Having hard time preparing for o lvl chemistry? No problem .with an experienced tutor Collin Wong will guide you to your dream grades .interested please pm me or email to classicalerhu@gmail.com for more info .cheers
O lvl Chem and Physics Specialist

Zhou's Studio
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Preparing for IP chem and O level Chem

Post by Zhou's Studio » Wed Jul 09, 2014 5:36 pm

For sharing and to prepare students for some IP chem and O level questions :)

While coaching Sec 4 RI students yesterday, 50% of them can't solve this question: how many isomers for C4H9Cl? Can you solve it?

For O level students:

Which salt can't be prepared by acid-alkali titration method?

A) sodium sulfate
B) ammonium sulfate
C) potassium nitrate
D) iron (11) nitrate

Easy question I think students can guess correct but do you know why?

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