Chemistry: Post your doubts here!

[user]

you right mate... H2 has two hydrogen atoms... number of atoms will be 2*moles... for hydrogen 2*.5... for neon 1*1... for CO2 it will be 3*.5..(as carbon has 3 atoms , 2 of oxygen 1 carbon)

oh ok,....get it now..
JazakAllah khairen...

btw u sure ..right

 

[umarashraf]

han han pakka yaken hai bhae... Application k elaya sb kuch theek thaak aata hai mjhe bhai

 

[user]

han han pakka yaken hai bhae... Application k elaya sb kuch theek thaak aata hai mjhe bhai

application is very easy!
just go through all the pastpapers...more of common sense than learning..

 

[umarashraf]

yar hmein school mein karwai he nahi gai... phy to asaan hai kaafi ... bs telecommunication mein masla hai thora... baqi chem ka to koi haal nahi...

 

[Zsiddiqui]

For question 2, 2 double bonds in he compound were removed, thus 4 mole of hydrogen was required (since 2 hydrogens are added to the chain once you break a double bond). If you notice, CH2 was changed to CH3 thus one more mole os Hydrogen was required. So in total, 5 mole of hydrogen was required.

Question 3, first of all find the Mr of P2O5, which is (31*2)+(16*5)= 142.
Now find the amount of phosphorus present in this amount, which is (31*2)/142= 0.4366...
The question said, only 30% of it dissolves, so 0.4366*30= 13.09, which is almost equal to 13.1, hence answer is B.

Question 4-
Ethanol will not react with NaOH. Since a hydroxyl group is present in both of them, no nucleophilic substitution will happen, so it has to be ethanoic acid.

Question 5-
This question is hell tricky, and I've been trying this for half an hour now, and I don't see how to differentiate between A and D, both of 1 mol of A and D, will require 2 mol of HCN, and both will change the color of potassium dichromate.
I will try to brain storm this question with one of my friends, and she might come up with something.
Till then I'll be grateful if someone can reason out why it should be D, not A, please?
Here's the question:

thank you soo much

 

[Ashique]

thank you soo much

And me and my friend were discussing the question, and here's what she said- a ketone does NOT get oxidized , its the final oxidation product of a secondary alcohol . but primary alcohols when oxidized they give aldehydes right ? these aldehydes can be further oxidized to carboxylic acids , thats why they change the color of the potassium dichromate solution . Ketones don't because they can never undergo oxidation reaction to give a new compound . You tell me what would ketones give on oxidation?

 

[Ashique]

It actually makes sense.

 

[Zsiddiqui]

It actually makes sense.

it does

 

[Zsiddiqui]

10 At a total pressure of 1.0 atm, dinitrogen tetraoxide is 50% dissociated at a temperature of 60 oC, according to the following equation
N2O4 = > 2NO2

What is the value of the equilibrium constant, Kp, for this reaction at 60 oC?

A
1/3atm

B
2/3atm

C
4/3atm

D
2atm

helpppppppppppppppppppppppppppppppppppppppp the correct ans is C how 2 get it

are you sure its C ? and your question is right ?

 

[xxfarhaxx]

Hey,can anyone please explain the question number 13 in this paper??? i posted it before too. thanks.
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_12.pdf

its a mattr of balncin da equation..
5HIO----> 2I2+ 1HIO3+ 2H2o

ANS: B

 

[xxfarhaxx]

Someone help me with this question! How can i find number of isomers?
View attachment 8582

fr any cis trans isomer 2^x (x=no of double bonds)... in ths case it has 3 double bonds thrfr 2^3=8 (Ans C)
hpe i hlpd!!!

 

[xxfarhaxx]

can some1 help me from these, please:
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w09_qp_11.pdf
Q11, Q13,Q23,Q29

11. ass: A- cu2+ ions remain unchanged at the end of the reaction..c reactions 2 n 3..

13- C - SiCl4 + 2H2O----> SiO(s) +HCl(acidic)

 

[user]

Assalamoalaikum wr wb!

need help, plz... ...

P.S. I'm reposting them

 

[mominzahid]

Be isn't really a typical group 2 element. Like I said before, Be2+ is smaller and has a higher charge density than the other group 2 elements and its charge density is similar to that of Al's. The rest of the group 2 elements are bigger than Al and don't really have a high charge density.

Let me try to give a better explanation:

Al is a small atom with a 3+ charge. All of these group 2 elements have a 2+ charge. For the atoms to have the same electronegativity, you would want the atom to be smaller to make the net effect the same. Be is the only atom which is smaller, so the effect of charge density on both Al and Be is similar.

Thank you so much that helped.

 

[KurayamiKimmi]

Q2) Write down all of the data you have.

Mr = 14x + 16y
Mass = 0.23 g
Moles = 0.23 / (14x + 16y) - (1)
Moles = 0.005 (since they said it occupies 120 cm^3) - (2)

Equate (1) with (2):

0.07x + 0.08y = 0.23
7x + 8y = 23

And then try each option and see if they add up to 23. In B, 7(1) + 8(2) = 23 so it's right.

There's probably a few other methods of solving this question, but this one worked for me.

12) NaCl is completely ionic, SiCl4 is completely covalent. AlCl3 has a lot of covalent character and isn't that much ionic and MgCl2 is the opposite of that. Since they asked which one shows just some covalent character, MgCl2 should be right.

16) "The enthalpy of formation of HCl and HI is represented by the equations:


H2 + Cl2 --> 2HCl

and

H2 + I2 --> 2HI

The enthalpy change of these reactions can be found by looking at the bonds broken and formed in terms of their energies (remembering that bond breaking is endothermic and bond formation is exothermic):

ΔH(rxn) = ΔH(reactant bonds) - ΔH(product bonds)

So if the HI bond energy is much smaller than the HCl bond energy (option C) then the HI reaction will be more endothermic...

... whereas if the bond energy of I2 were smaller than the bond energy of Cl2 (option D) the reaction forming HI would be more exothermic.

And as we know that the reaction forming HI is more endothermic, the correct answer is C"

18) "A solid nitrate fertiliser reacts with an alkali to produce a gas which turns damp pH paper blue."

Read the bolded parts. The anion must be nitrate and the cation must be NH4+.

So you have NH4NO3 whose empirical formula is N2H4O3.

26) For it to be chiral, the central C atom MUST be bonded to 4 different groups. One of them will be OH, the minium the rest will have to be is:

- CH3
- CH3CH2
- CH3CH2CH2

In total, that's 6 + the central C atom which is 7.

36)
1 is right, the more down you go group 7, the more readily dissociation occurs.
2 this only happens with Flourine I think.
3 is wrong because only the halide ions are good reducing agents.

Thank you so much !!! ^_^

 

[ultraviolet]

Oct/Nov 2010 Q: 3, 7, 9 and 10 please :|
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_11.pdf

SOMEONE HELP ME!!!!!!!!!!!

 

[7amed]

A household bleach contains sodium chlorate(I), NaClO, as its active ingredient. The
concentration of NaClO in the bleach can be determined by reacting a known amount with
aqueous hydrogen peroxide, H2O2.
NaClO(aq) + H2O2(aq) → NaCl(aq) + O2(g) + H2O(l)
When 25.0 cm3 of bleach is treated with an excess of aqueous H2O2, 0.0350mol of oxygen gas is
given off.
What is the concentration of NaClO in the bleach?
A 8.75 × 10

moldm

0.700moldm

C 0.875moldm

D 1.40moldm'
ANSWER IS D
Anyone please

 

[samwickz]

are you sure its C ? and your question is right ?

ya deffa
http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s06_qp_1.pdf
Q no 10

my ans is A wbt u?

but d marking scheme ans IS C

 

[saudha]

Use of the Data Booklet is relevant to this question.
Which element is likely to have an electronegativity similar to that of aluminium?
A barium
B beryllium
C magnesium
D strontium


why is it B....how?

 

[saudha]

Concentrated sulfuric acid can behave both as a strong acid and as an oxidising agent.
With which compound does concentrated sulfuric acid react in this way?
A ethanol
B magnesium carbonate
C propanenitrile
D sodium bromide
why D why not A???