# Chemistry: Post your doubts here!

#### blymphocytes

For Q8 C2O4 has an overall charge of 2- since Oxygen will give - 8 charge you will have to +6 to make the overall charge to 2-,therefore each atom of C will have +3 charge and its oxidation as the the oxidation state changes from +3 to +4.
For Q18 you'll have to make equation of decomposition for each nitrate and use the total mass of gas obtained of No2 and O2 which is 108g and solve by mass ratios.
Thank you

Could you plz explain about the mass ratios and how you are calculating themm

#### Shemyaa

Hello everyone!
could anyone please explain to me how the answers are begin obtained in the following qns.
qn6.-A
qn9.-B (is there a shorter method except trial and error)
qn28.-C
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Okay let me help,
Question 1 :
When you're completely combusting an alkene you would get CO2 and H20.
If you take this example: C2H4+3O2=2CO2+2H20
As you can see, we had ethene(2 carbons) so we had to write (2)CO2 to balance the eqn, if you get the pattern i'm saying now you can solve it easily,
we have 400 dm3 of CO2 formed, therefore, since v=24n, then n=16.66... and since they said 2 moles of the alkenes this means (2CnH2n) so 2n=16 since we balance the eqn, therefore using math, 2n=16, n=8. so you know now its 2(C8H16)

Question 2-
There's no need to guess or try, using oxidation balancing method
MnO4-2 > MnO4-, the oxidation no difference is 1e since in MnO4-2 it's +6, and in MnO4- it's +7
MnO4-2 > MnO2 , the oxidtion number difference is 2e since in MnO2 it's +4 so cross multiplying to make the umber of electrons equal,
2MnO4-2> 2MnO4- and Mn04-2> MnO2
Adding these 2 half-equations together you'll get 2MnO4-2 +MnO4-2 +H2O> 2MnO4- +MnO2+ OH-
so totally you have 3MnO4-2 + H20 > 2MnO4- +MnO2 +OH- (did not balance the Hydrogen yet )
so its B .. revise chp 6 to make sure I'm right.

Question 3-
KMnO4- is an oxidising agent, and ethanol is a primary alcohol.
Heating a primary alcohol under reflux *for an hour* will mean I'm producing a carboxylic acid since I'm not distilling the aldehyde off and allowing it to further oxidise,
so this means it'll produce C2H4O2 OR CH3COOH
now let's find the number of moles of ethanol used, since n=M/Mr then n =2.3/46= 0.05,
since the yield is 60% this means that only 60% of 0.05 moles fo carboxylic acid is produced,
therefore, 0.6*0.05= 0.03.
To find the mass of ethanoic acid produced, we say Mr*n=Mass so 60*0.03 =1.8 g

#### Shemyaa

Hy
Could anyone explain how to get answers to the following
8-A
18-D
39-B

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Question 1, I think someone answered this so i'll skip it,

Question 2, This question is very easy if you remember an easy concept, total mass of reactant=total mass of products.
Since my reactant is 3g, and my gases are 1.53g , then the solid formed as a result is 1.47g
When we're heating a nitrate we get and oxide salt, NO2 and H2O,
Again let's take an example,
Ca(No3)2 = CaO+ NO2+H2O
This means that if it's a group two element, the no. of moles of the anhydrous nitrate salt= oxide salt.
Now it becomes v.easy if you know some good math,
so since we don't know what the element is, i'll donate it's Mr as X so
3/x+28+96= 1.47/x+16
so cross multiplying, 3(x+16)= 1.47(x+124), you'll get X os 87.7 so the element that has a Mr = 87.7 is strontium.

Question 3 ,
This is section B I assume so,
The last option (3) is wrong because in fact, as you go down so as the carbon chain becomes longer or as it becomes heavier, it becomes less volatile. Volatility is basically how easy it evaporates,
The fact that there are C-F and C-Cl bonds which are really strong because of diff in electronegativity and the fact that this molecule can have hydrogen bonds between its molecules means that they are not easily evaporated, so option 3 is wrong.
Option 1 is 100% right, C-F bond is VERY STRONG because fluorine is the most electronegative element, so their bond is much more difficult to break compared to C-Cl bond, and also since we're producing free radicals that means it has to be by homolytic fission.
Option 2 is very obvious, if it's wrong then why did countries do a contract regards stopping the use of CFC's,

#### Fillthegame

Which mass of urea, CO(NH2)2, contains the same mass of nitrogen as 101.1g of potassium nitrate?

A.) 22g B.) 30g C.) 44g D.) 60g

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#### Kanekii

View attachment 64773Any one can explain? answer is marked!
CuCo3 + 2HCl = CuCl2 +Co2 + H20
n of Hcl=2x25x10^-3
n of Hcl=0.05mol
2mol:24dm^3
0.05mol:x
x=0.6 dm^3
Since Hcl was the limiting reactant so we used it to find the volume of gas.

#### mygjjk

ans is D because they are asking for standard enthalpy change of reaction. the definition of s.e.c of reaction is "the enthalpy change when the amount of reactants shown in the equation react to give products under standard conditions" since there are 6 F's and 6 H's in the reactants, therefore 6 moles of HF are produced don't be confused with the other standard enthalpy changes. the other standard enthalpy changes have a definition along the lines of "when one mole of _____ react". but since its enthalpy change of reaction, there is nothing involving "one mole of ____". i hope that makes sense lol at least that's how i understand it.

• blymphocytes and Khitan

#### mygjjk

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ans is B , can someone pls explain this?
First u have to combine the half equations that form FeC2O4. So,
Fe2+ + C2O4 2- > Fe3+ + 2CO2 + 3e-
Now balance this equation with the MnO4- equation. ur going to have to find a number to multiply with so that the number of electrons are the same so u can cancel them out.
5 x (Fe2+ + C2O4 2- > Fe3+ + 2CO2 + 3e-)
3 x (MnO4- + 8H+ + 5e- >Mn2+ + 4H2O)

This gives:
3MnO4- + 5Fe2+ + 5C2O4 2- + 24H+ > 3Mn2+ + 5Fe3+ + 10CO2

To find how many moles of FeC2O4 that react with 1 mole of potassium manganate, simply 5/3 = 1.67 • taliaaa

#### taliaaa

Which statement about a 3p orbital is correct?
A It can hold a maximum of 6 electrons.
B It has the highest energy of the orbitals with principal quantum number 3.
C It is at a higher energy level than a 3s orbital but has the same shape.
D It is occupied by one electron in an isolated phosphorus atom.
ans is D
I'm not really sure why the ans is not A, can someone pls explain, thankyou

#### Kanekii

Which statement about a 3p orbital is correct?
A It can hold a maximum of 6 electrons.
B It has the highest energy of the orbitals with principal quantum number 3.
C It is at a higher energy level than a 3s orbital but has the same shape.
D It is occupied by one electron in an isolated phosphorus atom.
ans is D
I'm not really sure why the ans is not A, can someone pls explain, thankyou
A P Subshell has 3 orbitals and each orbital can hold a maximum of 2 electrons. If it were a 3P subshell then the maximum number of electrons that it could hold would be 6. Therefore statement A is incorrect. Statement D is correct as Phosphorus has 3 electrons in its 3P subshell, Each 3P orbital holds one electron.

• blymphocytes and taliaaa

#### Kanekii

View attachment 64779 can someone help with this? answer is B
In order for atoms to be in the same plane the bond angle must be 120 degrees
The central carbon atom in 1 and 2 has 3 B.P so the bond angle is 120 while in 3 the central carbon will also make a bond with hydrogen so it will have 4 B.P hence the bond angle will be 109.5 not 120.

#### iman iqbal

In order for atoms to be in the same plane the bond angle must be 120 degrees
The central carbon atom in 1 and 2 has 3 B.P so the bond angle is 120 while in 3 the central carbon will also make a bond with hydrogen so it will have 4 B.P hence the bond angle will be 109.5 not 120.
thank you

• Kanekii

#### mygjjk

In order for atoms to be in the same plane the bond angle must be 120 degrees
The central carbon atom in 1 and 2 has 3 B.P so the bond angle is 120 while in 3 the central carbon will also make a bond with hydrogen so it will have 4 B.P hence the bond angle will be 109.5 not 120.
sorry if this is a dumb question but what do u mean by B.P?