Chemistry: Post your doubts here!

[drowning-in-studies]

6 A white powder is known to be a mixture of magnesium oxide and aluminium oxide.
100 cm3 of 2 mol dm–3 NaOH(aq) is just sufficient to cause the aluminium oxide in x grams of the
mixture to dissolve.
The reaction occurring is Al 2O3 + 2OH– + 3H2O → 2Al (OH)4-
800 cm3 of 2 mol dm–3 HCl (aq) is just sufficient to cause all of the oxide in x grams of the mixture
to dissolve.
The reactions occurring are Al 2O3 + 6H+ → 2Al3+ + 3H2O
and MgO + 2H+ → Mg2+ + H2O.
How many moles of each oxide are present in x grams of the mixture?

aluminium magnesium
oxide oxide
A 0.05 0.25
B 0.05 0.50
C 0.10 0.25
D 0.10 0.50
How do you do this?
Ans D

 

[madridtas_CR7]

I am taking both AS level and A2, my major problems are with the practical papers. I am retaking chemistry AS level this year, along with the A2. Last year I scored 2 A's in my theory papers and a U grade on my practical paper. Have you got any suggestions on how I study both P3 and P5?

Work Hard and Believe in your seolf

 

[drowning-in-studies]

26 High-energy radiation in the stratosphere produces free-radicals from chlorofluoroalkanes,
commonly known as CFCs.
Which free-radical is most likely to result from the irradiation of CHFCl CF2Cl ?
A CHFClC*FCl
B *CHClCF2Cl
C *CHFCF2Cl
D *CFClCF2Cl
Ans is C. I don't understand the theory Why isn't it B or D?

 

[Cadence]

26 High-energy radiation in the stratosphere produces free-radicals from chlorofluoroalkanes,
commonly known as CFCs.
Which free-radical is most likely to result from the irradiation of CHFCl CF2Cl ?
A CHFClC*FCl
B *CHClCF2Cl
C *CHFCF2Cl
D *CFClCF2Cl
Ans is C. I don't understand the theory Why isn't it B or D?

Someone solved this ques tho
u can check the previous page
The ans is C because C-Cl bond is broken.

 

[Cadence]

6 A white powder is known to be a mixture of magnesium oxide and aluminium oxide.
100 cm3 of 2 mol dm–3 NaOH(aq) is just sufficient to cause the aluminium oxide in x grams of the
mixture to dissolve.
The reaction occurring is Al 2O3 + 2OH– + 3H2O → 2Al (OH)4-
800 cm3 of 2 mol dm–3 HCl (aq) is just sufficient to cause all of the oxide in x grams of the mixture
to dissolve.
The reactions occurring are Al 2O3 + 6H+ → 2Al3+ + 3H2O
and MgO + 2H+ → Mg2+ + H2O.
How many moles of each oxide are present in x grams of the mixture?

aluminium magnesium
oxide oxide
A 0.05 0.25
B 0.05 0.50
C 0.10 0.25
D 0.10 0.50
How do you do this?
Ans D

First u know n(NaOH)=0.2mol, so n(Al2O3) will be 0.1mol, The total n(HCL)=1.6mol
since n(Al2O3)=0.1mol, n(HCL) used to dissolve AL2O3 is 0.6mol, n(HCL) left to dissolve Mgo is 1.6-0.6=1mol, so n(MgO) will be0.5mol

 

[Cadence]

did u searched it on that channel?

yea i did but i can't find any paper for the year 2014
btw thx for the notes below

 

[Cadence]

10.When solid ammonium chloride dissociates at a certain temperature in a 0.500 dm3 container,
ammonia and hydrogen chloride are formed.
NH4Cl (s) NH3(g) + HCl (g)
The initial amount of ammonium chloride was 1.00 mol, and when the system had reached
equilibrium there was 0.300 mol of ammonium chloride.
What is the numerical value of Kc for this reaction under these conditions?
A 0.490 B 1.63 C 1.96 D 3.27
ans is C But how do we get that?

did u get ur ans?

 

[drowning-in-studies]

did u get ur ans?

I keep getting D??

 

[HELLNO]

I keep getting D??

NH4Cl is in a solid state! So whenever you want to find kc you only include gaseous and liquid states (Since its concentration) Its like a rule
The 1 mol of NH4Cl decreased to 0.3 (-0.7)
So each of the products increase from 0 to 0.7 moles
Convert that to concentration
0.7/0.5= 1.4 mol/dm3
Apply the formula of Kc
Kc= 1.4*1.4= 1.96 which is C
Get it?

 

[drowning-in-studies]

NH4Cl is in a solid state! So whenever you want to find kc you only include gaseous and liquid states (Since its concentration) Its like a rule
The 1 mol of NH4Cl decreased to 0.3 (-0.7)
So each of the products increase from 0 to 0.7 moles
Convert that to concentration
0.7/0.5= 1.4 mol/dm3
Apply the formula of Kc
Kc= 1.4*1.4= 1.96 which is C
Get it?

OOHH! Thanks

 

[soumayya]

cananyone helpme with the drawing in 1(b) ?? and 2(d) plzz??

http://onlineexamhelp.com/wp-content/uploads/2016/03/9701_s15_qp_52.pdf

 

[Thought blocker]

ughh!! paper y, y u do no rahem :'(

 

[June Phang]

But how do you decide when to add and subtract? Is it in reference to where the arrows are pointing? What am i missing out?
like, -75+92 or -75-92?

When you change the direction

 

[June Phang]

Q.25 anyone please reply soonn

What is the ans tho

 

[drowning-in-studies]

6 The standard enthalpy change for the reaction
2NF3(g) → 2N(g) + 6F(g) is ΔH
o = +1668 kJ
What is the bond energy of the N–F bond?
A –556 kJ mol–1
B –278 kJ mol–1
C +278 kJ mol–1
D +556 kJ mol–1
Ans is C...How??

 

[Thought blocker]

6 The standard enthalpy change for the reaction
2NF3(g) → 2N(g) + 6F(g) is ΔH
o = +1668 kJ
What is the bond energy of the N–F bond?
A –556 kJ mol–1
B –278 kJ mol–1
C +278 kJ mol–1
D +556 kJ mol–1
Ans is C...How??

delth H / 6
thus 1668/6 = C

 

[drowning-in-studies]

delth H / 6
thus 1668/6 = C

Im really sorry... I just realised I've asked this question before and that you replied
16 days and i forgot
But thank you loads for being so helpful

 

[Thought blocker]

Im really sorry... I just realised I've asked this question before and that you replied
16 days and i forgot
But thank you loads for being so helpful

Lol, happens. Anytime.

 

[drowning-in-studies]

Q 39 and Q 40??

 

[Thought blocker]

Q 39 and Q 40??

39)1 and 2

40) This is easiest last question, i ever saw xD Sorry to say like this, but just u have to manipulate its structure and match with the given compound in question. All three is having that formula after oxidation. Try once, if u dont get ask me. Bored in making that one in 39 xD