Chemistry: Post your doubts here!

[princess Anu]

can somebody check this for me? please

 

[princess Anu]

have I done this correct? mist importantly, part E?

 

[princess Anu]

are my products fine? ( C and the rest)

 

[Lola_sweet]

can someone please give me advice for organic chemistry?
i understand the concepts but its pastpapers tht kill me :/
any idea wht to do?

 

[DeViL gURl B)]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_42.pdf

question 1 part c.

please if someone can give a detailed explanation of these sort of questions .. i always get confused .. in the solution or hydration or lattice energy questions.
if someone has made notes on these .. please SHARE.
PLEASE.
Thank you.

ANYONE? PLEASE.

 

[Tasneem_m98]

Please help (M/J 2012 P11 number 22)

A sample of ethyl ethanoate is hydrolysed by heating under reflux with aqueous sodium hydroxide. The two organic products of the hydrolysis are separated, purified, and weighed.
Out of the total mass of products obtained, what is the percentage by mass of each product?

A) 32.4% and 67.6%
B) 38.3% and 61.7%
C) 42.3% and 57.7%
D) 50.0% and 50.0%

Thank you

 

[Metanoia]

Please help (M/J 2012 P11 number 22)

A sample of ethyl ethanoate is hydrolysed by heating under reflux with aqueous sodium hydroxide. The two organic products of the hydrolysis are separated, purified, and weighed.
Out of the total mass of products obtained, what is the percentage by mass of each product?

A) 32.4% and 67.6%
B) 38.3% and 61.7%
C) 42.3% and 57.7%
D) 50.0% and 50.0%

Thank you

You have misread the question, it should be ethyl propanoate.

Products are thus CH3CH2OH and CH3CH2COONa.

Using their Mr, % by mass of CH3CH2OH = 46/(46+96) =32.4%

 

[Tasneem_m98]

You have misread the question, it should be ethyl propanoate.

Products are thus CH3CH2OH and CH3CH2COONa.

Using their Mr, % by mass of CH3CH2OH = 46/(46+96) =32.4%

Oh thank you so much ! That makes sense

 

[cashew]

can someone please give me advice for organic chemistry?
i understand the concepts but its pastpapers tht kill me :/
any idea wht to do?

just solve maximum pastpapers, that is the only possible good thing to do. Oh and make sure all of your concepts are clear

 

[cashew]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_42.pdf

question 1 part c.

please if someone can give a detailed explanation of these sort of questions .. i always get confused .. in the solution or hydration or lattice energy questions.
if someone has made notes on these .. please SHARE.
PLEASE.
Thank you.

THIS IS A2, RIGHT?

 

[DeViL gURl B)]

THIS IS A2, RIGHT?

yep.

 

[DeViL gURl B)]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w12_qp_42.pdf
question 3 part a (ii)

 

[shazmina]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w12_qp_42.pdf
question 3 part a (ii)

Catalyst is a substance which speeds up the rate of a chemical reaction & is chemically unchanged at the end of the reaction
Example 1:- The Haber Process
N2 (g) + 3H2 (g) ------Fe (s) ------> 2NH3 (g)

Example 2:- S2O8^2- (aq) + 2I^1- (ap) ---------Fe^2+ ( Aq) --------> I2 (aq) + 2SO4^2-(aq)
The catalyst reduces the activation energy & provides an alternative pathway for the reaction to occur

REMEMBER THE ABOVE 2 EXAMPLES .. and thats all u have to write ..... no need to further explain about the state of the catalyst as it clearly shown in the balanced equation .........

 

[shazmina]

http://onlineexamhelp.com/wp-content/uploads/2015/02/9701_w14_qp_53.pdf
Can anyone help me with Question 2, c (ii) ( the last part ) of this paper ...........
thanks in advance

 

[Farhad221]

Asslamoalaikum
I need help with this question

 

[Farhad221]

guys im finding chem aslevel practicals difficult

Use this link http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/
Open files 9701-nos-ps-1.pdf to 9701-nos-ps-27-pdf
These are all the different types of practicals that can be asked to do even if you cant do them reading through them will help clear out what one has to do at the exam.

 

[Farhad221]

guys im finding chem aslevel practicals difficult

try this website out too
http://www.labskills.co.uk/a-level-labskills-trial/chemistry/index.html

 

[Xaptor16]

33) Valence shell electron pair repulsion theory should be used to answer this question. Which species are trigonal planar?
1 BH3
2 CH3+
3 PH3
the answer is B (1 and 2 only are correct)
can someone please explain how to use the Valence shell electron pair repulsion theory to answer this? i don't get it!
Solids W, X, Y and Z are compounds of two different Group II metals. Some of their applications are described below.
Compound W is used as a refractory lining material in kilns.
Compound X is used as a building material. It can also be heated in a kiln to form compound Y. When Y is hydrated, it forms compound Z which is used agriculturally to treat soils. Which statements about these compounds are correct?
1 More acid is neutralized by 1 g of W than by 1 g of X.
2 The metallic element in W reacts with water more quickly than the metallic element in Y.
3 Adding Z to a soil decreases the pH of the soil.
the answer is D (1 only is correct)
and here too please, i get that W is probably MgO, X is probably CaCO3, Y may be CaO and Z most likely Ca(OH)2. i know that 2 and 3 are wrong, but please explain why 1 is correct, thanks in advance!

 

[Metanoia]

33) Valence shell electron pair repulsion theory should be used to answer this question. Which species are trigonal planar?
1 BH3
2 CH3+
3 PH3
the answer is B (1 and 2 only are correct)
can someone please explain how to use the Valence shell electron pair repulsion theory to answer this? i don't get it!
Solids W, X, Y and Z are compounds of two different Group II metals. Some of their applications are described below.
Compound W is used as a refractory lining material in kilns.
Compound X is used as a building material. It can also be heated in a kiln to form compound Y. When Y is hydrated, it forms compound Z which is used agriculturally to treat soils. Which statements about these compounds are correct?
1 More acid is neutralized by 1 g of W than by 1 g of X.
2 The metallic element in W reacts with water more quickly than the metallic element in Y.
3 Adding Z to a soil decreases the pH of the soil.
the answer is D (1 only is correct)
and here too please, i get that W is probably MgO, X is probably CaCO3, Y may be CaO and Z most likely Ca(OH)2. i know that 2 and 3 are wrong, but please explain why 1 is correct, thanks in advance!

VSEPR in simple terms states hat LP-LP repulsion> BP-LP repulsion > BP-BP repulsion.
And such repulsion will to an extent, determine molecular shapes.
All you have to do is follow the usual method of figuring out the number of lone pairs and bond pairs in a molecule to determine the shape.

1. 3 BP, 0 LP
2. 3 BP , 0 LP
3. 3 BP, 1 LP (trigonal pyramidal)


1g of MgO = 1/40 moles of MgO
1g of CaCO3 = 1/100 moles of CaCO3

Using HCl as example
MgO + 2HCl --> MgCl2 + H2O (HCl neutralised = 1/20 mol)
CaCO3 + 2HCl --> CaCl2 + CO2 + H2O (HCl neutralised = 1/50 mol)

 

[Xaptor16]

VSEPR in simple terms states hat LP-LP repulsion> BP-LP repulsion > BP-BP repulsion.
And such repulsion will to an extent, determine molecular shapes.
All you have to do is follow the usual method of figuring out the number of lone pairs and bond pairs in a molecule to determine the shape.

1. 3 BP, 0 LP
2. 3 BP , 0 LP
3. 3 BP, 1 LP (trigonal pyramidal)


1g of MgO = 1/40 moles of MgO
1g of CaCO3 = 1/100 moles of CaCO3

Using HCl as example
MgO + 2HCl --> MgCl2 + H2O (HCl neutralised = 1/20 mol)
CaCO3 + 2HCl --> CaCl2 + CO2 + H2O (HCl neutralised = 1/50 mol)

Thank you so much! really helped me!