Chemistry: Post your doubts here!

[Casablanca]

yah but hw did u do it?

3FeO + 2Al -> 3Fe + Al2O3

Enthalpy change of formation of FeO: -266
Enthalpy change of formation of Al2O3: -1676

-Since FeO is breaking, not forming, you flip the sign. So it becomes +266. And since it's 3 molecules of FeO, you multiply it by 3, so it's +798.
-Al2O3 is forming, so it is -1676.

-1676 + 798 = -878

Therefore, the answer is B.

 

[KurayamiKimmi]

Q2) First of all, we know CS2 is the limiting reactant
So 10 cm3 CS2 needs 30 cm3 O2 (See molar ratio)
That means 50 - 30 = 20 cm3 O2 is left after the reaction
Further,
10 cm3 CO2 and 20 cm3 SO2 are produced (See molar ratio)
So total volume of gases left after burning = 20 cm3 O2 + 10 cm3 CO2 + 20 cm3 SO2 = 50 cm3

The answer must be either C or D

Adding NaOH will neutralise (or remove) acidic gases
It will remove 10 cm3 CO2 and 20 cm3 SO2 = 30 cm3 acidic gases
Gas left after NaOH is added = 50 - 30 = 20 cm3

So, C is correct!

Q4) you have add the atomic number of hydrogen carbonate (HCO3-)
H= 1, C=6, O=8
So number of electron of anion = 1+6+(8*3)+1 = 32
The extra 1 is because the anion has a one extra electron that's the reason it has an overall charge of -1 so you have to add that too.
Option C is correct

Q14)View attachment 8889

Thank you so much! ^_^

 

[lavanyamane]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s10_qp_11.pdf
Q.5, Q.11 please?

 

[KurayamiKimmi]

What is a dimer?

from wikipedia

A dimer is a chemical entity consisting of two structurally similar monomers joined by bonds that can be either strong or weak, covalent or intermolecular.

lemme expain:
example of a dimer is Al2Cl6
its a dimer of AlCl3
so you can think of a dimer as two same molecules bonded to each other for ex AlCl3+AlCl3=al2cl6

 

[KurayamiKimmi]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_qp_1.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_ms_1.pdf

does anyone know how to get answers to q no 3O and 38??
any help will be GREATLY appreciated ^_^

 

[smartangel]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_12.pdf
Q34..after 20 mins rxn is complete..mass of mg used is same so how can sukfuric acid produce more gas? statement 3 is correct according to ms..how?

 

[Casablanca]

from wikipedia

lemme expain:
example of a dimer is Al2Cl6
its a dimer of AlCl3
so you can think of a dimer as two same molecules bonded to each other for ex AlCl3+AlCl3=al2cl6

So... is C10H12 a dimer of C5H6?

 

[smartangel]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf
Q6..plz help me out with this one..

 

[KurayamiKimmi]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_12.pdf
Q34..after 20 mins rxn is complete..mass of mg used is same so how can sukfuric acid produce more gas? statement 3 is correct according to ms..how?

Answer for question 35
the reactions will be
Mg + 2 CH3COOH <=> Mg(CH3COO)2+ H2(g)
Mg + H2SO4 → MgSO4 + H2


1 After 2 minutes, the sulfuric acid is at a higher temperature than the ethanoic acid.
2 After 2 minutes, the sulfuric acid has produced more gas than the ethanoic acid.
this is because the sulfuric acid is a stronger oxidising agent and therefore will be quicker in forming the magnesium sulfate and hydrogen than ethanoic acid .and when it does form , bond forming is exxothermic and so it is at an higher temperature
3 After 20 minutes, the sulfuric acid has produced more gas than the ethanoic acid.
This is because the reaction between ethanoic acid and magnesium is in equilibrium which means the reaction never reaches completion.Moreover , the stoichichemtry of the equilibrium shows us that more reactants are present than products (reactants =3m products =2m)

 

[furry.panda]

17 Which statement explains the observation that magnesium hydroxide dissolves in aqueous ammonium chloride, but not in aqueous sodium chloride?

AThe ionic radius of the NH4+ ion is similar to that of Mg2+ but not that of Na+.
BNH4Cl dissociates less fully than NaCl.
CThe ions Na+ and Mg2+ are isoelectronic (have the same number of electrons).
DThe ion NH4+ acts as an acid.


Ans: D

Can someone please explainn? thanks in advance

 

[smzimran]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s10_qp_11.pdf
Q.5, Q.11 please?

AoA,
I have solved q5 in this thread before, search it

q11:
A is correct because when you acidify pool water, that means you add H+ ions and these H+ ions will react ith OH- ions
The concentration of OH- will decrease and equilibrium will shift towards right and more HOCL will be produced

 

[Osake]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_qp_1.pdf
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_ms_1.pdf

does anyone know how to get answers to q no 3O and 38??
any help will be GREATLY appreciated ^_^

yield = Actual yield/measured yield x100 right?
we know there is 30 g of ethanoic acid
so Measured yield = 30(98)/60 x100 = 45 , closest to 50 so answer is C

 

[KurayamiKimmi]

So... is C10H12 a dimer of C5H6?

yes

 

[nerdybookworm]

URGEEEEEENNNTTTTTTTT...OCTO NOV...2011 VARIENT 11 PAPER 1...Q 6,914,15,20,23,25,28,29...plzz....

 

[Casablanca]

17 Which statement explains the observation that magnesium hydroxide dissolves in aqueous ammonium chloride, but not in aqueous sodium chloride?

AThe ionic radius of the NH4+ ion is similar to that of Mg2+ but not that of Na+.
BNH4Cl dissociates less fully than NaCl.
CThe ions Na+ and Mg2+ are isoelectronic (have the same number of electrons).
DThe ion NH4+ acts as an acid.


Ans: D

Can someone please explainn? thanks in advance

MgOH is basic, so the ammonium chloride neutralizes it, since it's acidic.

 

[Casablanca]

yes

Okay, got it. Thanks. =^.^=

 

[smartangel]

Answer for question 35
the reactions will be
Mg + 2 CH3COOH <=> Mg(CH3COO)2+ H2(g)
Mg + H2SO4 → MgSO4 + H2


1 After 2 minutes, the sulfuric acid is at a higher temperature than the ethanoic acid.
2 After 2 minutes, the sulfuric acid has produced more gas than the ethanoic acid.
this is because the sulfuric acid is a stronger oxidising agent and therefore will be quicker in forming the magnesium sulfate and hydrogen than ethanoic acid .and when it does form , bond forming is exxothermic and so it is at an higher temperature
3 After 20 minutes, the sulfuric acid has produced more gas than the ethanoic acid.
This is because the reaction between ethanoic acid and magnesium is in equilibrium which means the reaction never reaches completion.Moreover , the stoichichemtry of the equilibrium shows us that more reactants are present than products (reactants =3m products =2m)

how do u knw its in equilibrium?

 

[smzimran]

17 Which statement explains the observation that magnesium hydroxide dissolves in aqueous ammonium chloride, but not in aqueous sodium chloride?

AThe ionic radius of the NH4+ ion is similar to that of Mg2+ but not that of Na+.
BNH4Cl dissociates less fully than NaCl.
CThe ions Na+ and Mg2+ are isoelectronic (have the same number of electrons).
DThe ion NH4+ acts as an acid.


Ans: D

Can someone please explainn? thanks in advance

Very simple
Mg(OH)2 is a base and it will react with an acid
It will not react with NaCl as it is not an acid
It reacts with NH4Cl because it acts as an acid because NH4+ ion donates a proton (Bronsted Lowry acid)

 

[KurayamiKimmi]

yield = Actual yield/measured yield x100 right?
we know there is 30 g of ethanoic acid
so Measured yield = 30(98)/60 x100 = 45 , closest to 50 so answer is C

do you mean theoretical yied by measured yield ?=O

 

[geek101]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w11_qp_12.pdf Q4 someoneeee! pretty please!