# Chemistry: Post your doubts here!

#### Azzbentolo

Ethanal, CH3CHO, can be reduced using an aqueous methanolic solution of NaBH4, as the reducing agent.
This is a nucleophilic addition reaction.
What could be the first step of this mechanism?
A) attack of an H+ ion at the carbon atom of the carbonyl group
B) attack of an H+ ion at the oxygen atom of the carbonyl group
C) attack of an H- ion at the carbon atom of the carbonyl group
D) attack of an H- ion at the oxygen atom of the carbonyl group

Can someone explain why the ans is C?

#### KashishV

Hey guys, A level electrolysis question up ahead-

the question is on the right and the mark scheme on the left. According to my calculations, 1519.685 C of charge was used and therefore time = 1519.685/0.5 which is also what they said. My answer is, however, t=3039 seconds and NOT 5034 seconds. So is the mark scheme wrong in this case? or is there something that I am overlooking?

#### Riolande

I know this was asked like 6 years ago but i dont understand the explanations so im hoping this time round i can get it

Ques: Two moles of compound P were placed in a vessel. The vessel was heated and compound P
was partly decomposed to produce Q and R. A dynamic equilibrium between chemicals P, Q and
R was established.
At equilibrium x moles of R were present and the total number of moles present was (2 +2/x ).
What is the equation for this equilibrium reaction? [Key: --> is the equilibrium sign]
A P --> 2Q + R
B 2P --> 2Q + R
C 2P --> Q + R
D 2P --> Q + 2R

#### yRommmyyyy

pls guys post predictions under here

#### Dania_4083

NOVEMBER 2015 P11 QUESTION 16

X is the oxide of a Period 3 element. X reacts with water to give an acidic solution.
A solution is prepared by reacting 0.100g of X with excess water. This solution was neutralised
by exactly 25.0cm3 of 0.100 moldm–3 sodium hydroxide solution.

What could be the identity of X?
A A l2O3
B MgO
C P 4O10
D SO3

Can anyone explain why answer is D?

#### Dania_4083

W17 qp11 question 17

71.0 g of chlorine, Cl 2, react with an excess of sodium hydroxide solution at a particular temperature. The reaction produces exactly 35.5 g of product X.
What is product X?
A H 2O
B NaCl
C NaCl O
D NaCl O3

Can someone explain this

#### kanekishh

Which of the following solids contain more than one type of chemical bond?
1)brass( an alloy of copper and zinc)
2)graphite
3)ice

#### kanekishh

Q8) Use of the Data Booklet is relevant to this question.

The approximate percentage composition of the atmosphere on four different planets is given in
the table below.

The density of a gas may be defined as the mass of 1 dm3
of the gas measured at s.t.p.

Which mixture of gases has the greatest density?

planet major gases / % by number of molecules
A Jupiter H2 89.8, He 10.2
B Neptune H2 80.0, He 19.0, CH4 1.0
C Saturn H2 96.3, He 3.25, CH4 0.45
D Uranus H2 82.5, He 15.2, CH4 2.3

#### KashishV

Q8) Use of the Data Booklet is relevant to this question.

The approximate percentage composition of the atmosphere on four different planets is given in
the table below.

The density of a gas may be defined as the mass of 1 dm3
of the gas measured at s.t.p.

Which mixture of gases has the greatest density?

planet major gases / % by number of molecules
A Jupiter H2 89.8, He 10.2
B Neptune H2 80.0, He 19.0, CH4 1.0
C Saturn H2 96.3, He 3.25, CH4 0.45
D Uranus H2 82.5, He 15.2, CH4 2.3
The question has been previously explained- https://xtremepape.rs/threads/chemistry-post-your-doubts-here.9859/post-827046
all the best

#### huda kazmi

hi i wanted to ask something relating to electronegativity. In the syllabus it says understand, in simple terms, the concept of electronegativity and apply it to explain the properties of molecules such as bond polarity (see also Section 3.3(c)), the dipole moments of molecules (3.3(d)) and the behaviour of oxides with water. How can i apply the concept to the behaviour of oxides?

#### sudinshrestha

Y is an organic compund. Y gives ppt aqueous silver nitrate. All of this ppt dissolves when concentrated ammonia is added. What is possible identity of Y ?
1- bromopropane
2-chlororethane
3-iodo-2-methylpropane
B

#### Rayyan_48

A few past paper questions but first
I need to know what's the method to find out bonds between elements in a molecule or elements
Also how do we solve questions in which we are giving some properties of elements like boiling point and conductivity and we have to find the elements cuz we don't know the boiling point of the elements so how? AN EXAMPLE OF THIS TYPE OF QUESTIONS

Now the past paper questions start please answer them all with explanations
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#### KateCake

Hi.Could someone explain me how to complete the table I am not quite sure ?9701/s19/52/qu2(d)(ii)

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Anyone?

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#### Mazeeen mohd

Guys can anyone explain me what's the difference between pi and sigma bonds?

#### Kiara_S

Re: All Chemistry help here!! Stuck somewhere? Ask here!

tonnes of CHEMISTRY WORKSHHETS
http://www.chemactive.com/a_level_chemi ... tions.html
So I wanted to ask a question from periodicity... It says in the book that the oxidation number increases across a period because the elements in period 3 till phosphorus use all their electrons in bonding with chlorine. And their oxidation numbers are positive as chlorine has the highest electronegatavity...
I didn't get a word of this! What does this mean??