Chemistry: Post your doubts here!

[geek101]

i dont get why we use excess air when converting SO2 to SO3 in the contact process......i know it shifts the equilibirium shifts to the right but i dont get why...??????

According to the Le Chateilier's (dunno how to spell it ), so according to the principle if any change in the factors which affect the equilibrium is altered than the reaction behaves in a way which opposes the change and tries to attain eq. Sooo if you add excess O2 the concentration of the reactants increases and so the equilibrium shifts to the right to produce more products so they reaction will come to eq. again, hence increasing the conc. of the products so SO3 increases!

 

[smzimran]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_qp_1.pdf
Can anyone explain 1,4,22,30 and 39 from this paper plz ??

Aoa,
Q1:
Mr (TiO2) = 47.9 + 16 + 16 = 79.9
Mr (FeTiO2) = 55.8 + 47.9 + 3(16) = 151.7
FeTiO3 ---> TiO2 + FeO
molar ratio b/w FeTiO3 and TiO2 is 1:1
so
151.7 g of ore gives 79.9 g of TiO2
1 g of ore gives (79.9 / 151.7 g) of TiO2
19 tonnes of ore will give
= 19 * (79.9 / 151.7 g)
= 10 tonnes
So, A is the answer!


Q4:
The hydrogencarbonate anion is (HCO3)-1
Total electrons = 1 of H + 6 of C + 3(8) of O + 1 extra electron due to -1 charge
= 1 + 6 + 24 + 1
= 32
So, C is the answer!


Q22:
B and C have two similar groups (methyl) attached to one side of carbon having double bond, that means there cannot be cis-trans isomers for these
A will form cis-trans isomers if 1 H is substituted on the R.H.S but it will not have a chiral centre
D already has cis-trans isomers, and it will have a chiral centre at the RED carbo atom if one of the BLUE hydrogen is replaced by halogen!
CH3CH=CHCH2CH3
So, D is correct!

 

[00tanveer]

Assalamualaikom

A garden fertiliser is said to have a phosphorus content of 30.0% ‘P
2O5 soluble in water’.

What is the percentage by mass of phosphorus in the fertiliser?

A
6.55% B 13.1 % C 26.2% D 30.0%
Can anyone please answer this with an explanation? Thank you.

 

[smzimran]

W.salam,
Mr(P) = 31
Mr(P2O5) = 2(31) + 5(16) = 142
In 100 g of fertiliser, there is 30.0% P2O5
That means mass of fertiliser is
= (100/30) * Mr(P2O5)
= (100/30) * 142
= 473.3 g

% of phosphorus in fertiliser is
= (2*31 / 473.3) * 100 [(2*31) because there are 2 atoms of P in one molecule of P2O5 ]
= 13.1 %
So, A is the answer!

 

[00tanveer]

But the mark scheme of this paper( O/N 03) says the answer is B...

 

[smzimran]

But the mark scheme of this paper( O/N 03) says the answer is B...

Oh yes, a little mistake by me, check i corrected the first part!

 

[00tanveer]

Thanks bro

 

[Student12]

Aoa,
Q1:
Mr (TiO2) = 47.9 + 16 + 16 = 79.9
Mr (FeTiO2) = 55.8 + 47.9 + 3(16) = 151.7
FeTiO3 ---> TiO2 + FeO
molar ratio b/w FeTiO3 and TiO2 is 1:1
so
151.7 g of ore gives 79.9 g of TiO2
1 g of ore gives (79.9 / 151.7 g) of TiO2
19 tonnes of ore will give
= 19 * (79.9 / 151.7 g)
= 10 tonnes
So, A is the answer!


Q4:
The hydrogencarbonate anion is (HCO3)-1
Total electrons = 1 of H + 6 of C + 3(8) of O + 1 extra electron due to -1 charge
= 1 + 6 + 24 + 1
= 32
So, C is the answer!


Q22:
B and C have two similar groups (methyl) attached to one side of carbon having double bond, that means there cannot be cis-trans isomers for these
A will form cis-trans isomers if 1 H is substituted on the R.H.S but it will not have a chiral centre
D already has cis-trans isomers, and it will have a chiral centre at the RED carbo atom if one of the BLUE hydrogen is replaced by halogen!
CH3CH=CHCH2CH3
So, D is correct!

Thankk you How about Q30 and 39 ?

 

[xhizors]

AsSalamalikum wr wb, i have some question from w11 qp 11 Please help, May Allah(swt) help you and make your way easy for Distinctions Ameen.
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w11_qp_11.pdf
Q4
Q6
Q2
Q1
Q8 why ? HSO3- is donating H means proton donor means acid
Q10
Q14
Q15
Q18
Q16
Q20
Q3
Q9
Q22
Q23

Anyone Plz ?????

 

[Romeo10]

Please i need someone to explain to me question no 8 in June 2010 paper 11 .. why it is magnesium??
and also i need the next question
thanks

 

[geek101]

Anyone Plz ?????

Q1. From the table X is a substance with the mp less than zero and bp more than zero so its a liquid >> the options fr X are I2 and Br2. Iodine is a solid at rtp and Br2 is liquid so is Br2. Then Y and Z both have mpoints and bps above 0 so they are solids, BUT they have low densities >> from the options B is the only one with 2 metals with a low density. Na and Mg (1st and 2nd group so low density), tadaa the answer is B

Q2. this is simple calculation. write out the equation of the reaction. Using mole ratios or proportions find out the mass of bromine. the key here is to write the right equation, which is not hard...just balance it correctly.

Q4. Hmm...here the structure given is complicated, so just try to link it to the ones you know from the syllabus. Here ethene has a similar bond structure (just replace the N with a C). We know ethene has angles of approximately 120. So the answer is C.

Q6. First you divide the total enthalpy by 2. to get the enthalpy for one NF3. You get +834. Then to find the enthalpy for one N-F bond you divide by 3 since the compound has 3 N-F bonds. You get +278! C

Q8. here HSO3- changes to SO32- it does so by gaining a proton. see the reactants side only. To make sure look at the top reaction. HSO3- is becoming SO2 <<here it loses a OH- ion to become SO2, which is the property of bases.

Q10. The forward reaction in endothermic. An increase in temp favors the forward reaction and HI is formed, making the solution paler. So D.

excuse me to do only these. sorry. ill definitely post the rest when i have time

 

[00tanveer]

What is the colour of hydrogen bromide gas?

 

[geek101]

What is the colour of hydrogen bromide gas?

its colorless

 

[00tanveer]

The answer is Strontium. Check the mark scheme again.

 

[raamish]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_11.pdf

questions 7, 8,10,21,26,31,29,40,39,34 solutions plzzz

 

[maleeha123]

please someone explain me these questions Q:2,22,27

 

[smzimran]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_qp_1.pdf
Can anyone explain 1,4,22,30 and 39 from this paper plz ??

Q30:
Mr(Ethanol) = 46
Mr(Ethanoic acid) = 60
Mr(Ester) = 88

n(Ethanol) = 30 / 46 = 0.652
n(Ethanoic acid) = 30 / 60 = 0.50

That means ethanoic acid is the limiting reactant, so ester's yield should be checked by ethanoic acid and NOT by ethanol
n(Ester) = 22 / 88 =0.25

yield = (0.25 / 0.50) * 100% = 50%
So, C is the answer!

Q39:
During bromination of propane (C3H8),
the free radical will be .C3H7
It can be of two types:
(i) CH3 - .CH - CH3
(ii) .CH2 - CH2 - CH3

In option 1, one radical of type(i) and another radical of type(ii) are combining, so this is correct
In option 2, two radicals of type(ii) are combining, so this is also correct
However, in option 3, one butyl radical is combining with one ethyl radical, so this cannot be formed in bromination of propane!

This is why B is the correct answer!

 

[smzimran]

please someone explain me these questions Q:2,22,27

Aoa wr wb,
Q2:
In step A, o.s of nitrogen changes from +5 to +3
In step B, it changes from +3 to +2
In step C, it changes from +2 to +1
In step D, it changes from +1 to 0

In steps B,C and D oxidation number decreases by one unit
In step A, oxidation number decreases by 2 units

This is why answer is A.


Q22:
CH3 as an electrophile means CH3+
It has 4 electrons of carbon + 3(1) electrons of hydrogen - 1 electron due to positive charge = 6 electrons

CH3 as free radical means .CH3
It has 4 carbon + 3(1) hydrogen = 7 electrons

CH3 as nucleophile means CH3-
It has 4 carbon + 3(1) hydrogen + 1 due to negative charge = 8 electrons

So, the correct answer is A.
P.s: carbon has 6 electrons in total but i have taken 4 because they are asking only for outer shell(valence) electrons in the question!

Q27:
Because they are talking about ethanoate means you have to look at the acid group to be ethanoic acid
A : propanoic acid (3 carbon)
B : propanoic acid (3 carbon)
C : methanoic acid (1 carbon)
D : ethanoic acid (2 carbon)

That is why D is correct.

 

[maleeha123]

help me out.......... Q:23,29,31,32,33,36,40
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_qp_12.pdf

 

[trgirl]

can anyone help me for question4 a)i and ii) i dont get why the answer is nucleophilic addition???
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_ms_21.pdf