## 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.
Skyed
BlackBelt
Posts: 776
Joined: Tue Nov 20,

### Re: O-Level Chemistry

Split. Your friend is correct if they ask number of molecules, not number of atoms.

Dr.Daniel
GreenBelt
Posts: 143
Joined: Sun Dec 26,
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### Re: O-Level Chemistry

KSP2013777 wrote:How many atoms are there in 2 moles of ammonia gas?

My friend say 2 X 6 X 10^23 = 12 × 10^23 atoms

But I think must break up NH3 first

Hydrogen : 2 × 6 × 10 ^ 23 = 12 × 10 ^ 23 hydrogen atoms
Nitrogen : 2 × 3 × 6 × 10 ^ 23 = 36 × 10^23 nitrogen atoms
total atoms = 48 × 10^23 atoms

We argue long time about this. Who correct? My friend or I?
You are correct that you have to break up the NH3 into atoms first. In your calculation you multiplied the number of nitrogen atoms by 3, but you need to do that for the hydrogen. I think you meant to say this:

Nitrogen : 2 × 6 × 10 ^ 23 = 12 × 10 ^ 23 nitrogen atoms
Hydrogen : 2 × 3 × 6 × 10 ^ 23 = 36 × 10^23 hydrogen atoms
total atoms = 48 × 10^23 atoms

The number of ammonia molecules is 2 X 6 X 10^23 = 12 × 10^23 molecules.

KSP2013777
OrangeBelt
Posts: 92
Joined: Fri Dec 28,

### Re: O-Level Chemistry

Thank you Dr Daniel for your explain. Yes, sorry I type too fast, little bit mix up.

I have new question. How many atoms are there in 64g of sulphur?
chemical formula for sulphur is S8 or S2?
If S8, then 64 ÷ (8×32) = 0.25 mol
0.25 X 6 X 10^23 = 1.5 × 10^23 sulphur atoms

Is this correct?

Pls help. Tks

Dr.Daniel
GreenBelt
Posts: 143
Joined: Sun Dec 26,
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### Re: O-Level Chemistry

Hi KSP – Sulphur is an element and it is a solid at room temperature, so there is no chemical formula for it. It’s just S1, although a chemist would not write S1, they would just write S. Sulphur is a yellow powder in it’s elemental form.

According to the Periodic Table of the elements sulphur has an atomic mass of 32, meaning 32 grams per mole. That means that 32 grams would be one mole and double that or 64 grams, would be 2 moles. That comes out to 12 X10^23.

If you have a periodic table just look up Aluminium and you’ll see that it has an atomic mass of 27. If I have 27 grams of Aluminium then I have 1 mole. So 27 grams of Aluminium has the same number of atoms as 32 grams of sulphur.

I have several samples of metals that are all one mole. I was showing these to some students just yesterday. It’s neat to see all the metals lined up with the same number of atoms in each sample.

There are times when an element has more than one atom and this would be the case for so called “diatomic gasses”. An example is the Oxygen we breathe. It pairs up as a gas into a molecule of O2. Oxygen has an atomic mass of 16, so if a question asked how many oxygen atoms are in 16 grams of oxygen gas, then the answer would be one mole. But if they asked how many oxygen molecules are in 16 grams of oxygen gas, then the answer would be half a mole.

HappieNewbie123
OrangeBelt
Posts: 97
Joined: Sat Oct 08,

### Re: O-Level Chemistry

Hi I require some help for these 2 redox reaction questions:

Q1)
Equations given:
1) Cl2(g) + H2S(g) --> 2HCl(g) + S(s)
2) SO2(g) + 2H2S(g) --> 2H2O(l) + 3S(s)
3) Cl2(g) + 2H2O(l) + SO2(g) --> 2HCl(aq) + H2SO4(aq)

a) From the equations given, deduce which of the three gases: chlorine, hydrogen sulfide and sulfur dioxide is the strongest
i) Oxidation agent
ii) Reducing agent

b) State how you arrive at your answers.

Q2) Disproportionation is a reaction in which the same element is both oxidized and reduced in a reaction.
Explain why the following reaction is a disproportionation reaction.
Cl2 + 2NaOH --> NaCl + NaOCl + H2O

(Oxidation state of NaOCl is not balanced? Na is +1, O is -2 and Cl is -1?? I find this a bit strange...)
Thank you!

Dr.Daniel
GreenBelt
Posts: 143
Joined: Sun Dec 26,
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### Re: O-Level Chemistry

HappieNewbie123 wrote:Hi I require some help for these 2 redox reaction questions:

Q1)
Equations given:
1) Cl2(g) + H2S(g) --> 2HCl(g) + S(s)
2) SO2(g) + 2H2S(g) --> 2H2O(l) + 3S(s)
3) Cl2(g) + 2H2O(l) + SO2(g) --> 2HCl(aq) + H2SO4(aq)

a) From the equations given, deduce which of the three gases: chlorine, hydrogen sulfide and sulfur dioxide is the strongest
i) Oxidation agent
ii) Reducing agent

b) State how you arrive at your answers.

Q2) Disproportionation is a reaction in which the same element is both oxidized and reduced in a reaction.
Explain why the following reaction is a disproportionation reaction.
Cl2 + 2NaOH --> NaCl + NaOCl + H2O

(Oxidation state of NaOCl is not balanced? Na is +1, O is -2 and Cl is -1?? I find this a bit strange...)
Thank you!
I asked our new Chemistry tutor these questions. Here are the answers:

1(a)
Strongest Oxidising Agent: Cl2
Strongest Reducing Agent: H2S

(b) From the first two equations, we can see that S always oxidised from -2 in H2S to 0 in S by both Cl2 and SO2. Granted, its ease of being oxidised makes it the strongest reducing agent. Then we look at the 3rd equation, where Cl is still reduced from 0 in Cl2 to -1 in HCl, while S is oxidised from +4 in SO2 to +6 in H2SO4. This indicates that SO2 is more easily oxidised than Cl2 (and thus a stronger reducing agent). Hence, the strength of oxidising agents is as such: Cl2 > SO2 > H2S

2) The reaction is a disproportionation reaction because Cl is 0 in Cl2, and it is concomittantly reduced to -1 in NaCl and oxidised to +1 in NaOCl.

*The oxidiation state of Na is always +1, O is generally -2 (except in cases of peroxides and superoxides), and for Cl to fulfill charge neutrality it has to be +1 in NaOCl.

iamastudent
OrangeBelt
Posts: 44
Joined: Tue Aug 14,

### Re: O-Level Chemistry

Hi,
May I know what is the oxidation state of ions? Example like the equation:
(S2O3)2- + 4Cl2 + 5H2O --> 2(SO4)2- + 8Cl- + 10H+

What is the oxidation state of H in H+? Is it +1 or is it 0?
O in (SO4)2-? Is it -2? S in (SO4)2-? Is it +6? Thank you

dioprem
OrangeBelt
Posts: 50
Joined: Wed May 22,
Total Likes:1

### Re: O-Level Chemistry

iamastudent wrote:Hi,
May I know what is the oxidation state of ions? Example like the equation:
(S2O3)2- + 4Cl2 + 5H2O --> 2(SO4)2- + 8Cl- + 10H+

What is the oxidation state of H in H+? Is it +1 or is it 0?
O in (SO4)2-? Is it -2? S in (SO4)2-? Is it +6? Thank you
The oxidation state of simple ions is always the charge of the ion. For example, in the case of H+, its oxidation state would be +1. In the case of Cl-, its oxidation state would be -1.

In the case of polyatomic ions, such as the sulfate ion, the sum of all the individual oxidation states of the atoms present in an ion must add up to the charge of the ion. In the case of (SO4)2-, the oxidation state of the 1 sulfur atom and 4 oxygen atoms must add up to -2. Oxygen generally has an oxidation state of -2. So the total oxidation state of the 4O atoms would be -8. So in order for the sulfate ion to have a charge of 2-, the S atom must have an oxidation state of +6.

Note that oxygen can have other oxidation states. For example, it will have an oxidation state of -1 in peroxide ions (O2)2-. It can also have an oxidation state of -0.5 in the case of superoxides (O2)-

Hope this helps!

WhatsappTuitionSG
KiasuNewbie
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### Re: O-Level Chemistry

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Mojostar
YellowBelt
Posts: 16
Joined: Tue May 19,

### Re: O-Level Chemistry

Hi all,

I'm weak in electrolysis...any way to improve in this area? Any book to recommend? Thanks.