Chemistry: Post your doubts here!

[smartangel]

4 Sulfur dioxide, SO2, is added to wines to prevent oxidation of ethanol by air. To determine the
amount of SO2, a sample of wine is titrated with iodine, I2. In this reaction, one mole of SO2 is
oxidised by one mole of I2.
What is the change in oxidation number of sulfur in this reaction?
A +2 to +4 B +2 to +6 C +4 to +5 D +4 to +6
ans D. whats the equation for this rxn?

 

[TheMan123]

Yea i understood that part.. but i'm still confused. Why did you take 4 for NO2? & the 2 for XO.
We don't take the values of the balancing part for the Molar mass isn't it ?

Sorry I was gone for awhile, where was I...
Normally we don't but you have to understand the maths going on here...
OK, alternative method. Assuming we have 2 moles of the nitrate

2X(NO3)2 --> 2XO + 4NO2 + O2
Masses will be:
(248+2x)g-->(32+2x)g +(216 )g
x is 1 mole
Real masses are:
5g ............ --> 1.71g +[ 3.29g ]

the ratios of the reactants and products are the same but the amounts are different
therefore:
ratio of nitrate mass to oxide mass will be the same in both situations
(248+2x)/ (32+2x)=5/1.71
cross multiply
424.08 + 3.42x =160 + 10x
264.08 = 6.58x
x= 40.1 grams per mole
x=calcium

 

[smartangel]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q7

 

[nerdybookworm]

ok sorry i posted the wrong link..now here help........paper 1 m/j 03 Q 2,4,8,10,16,20,27,28,30 & 39......


http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s03_qp_1.pdf

 

[Student12]

Sorry I was gone for awhile, where was I...
Normally we don't but you have to understand the maths going on here...
OK, alternative method. Assuming we have 2 moles of the nitrate

2X(NO3)2 --> 2XO + 4NO2 + O2
Masses will be:
(248+2x)g-->(32+2x)g +(216 )g
x is 1 mole
Real masses are:
5g ............ --> 1.71g +[ 3.29g ]

the ratios of the reactants and products are the same but the amounts are different
therefore:
ratio of nitrate mass to oxide mass will be the same in both situations
(248+2x)/ (32+2x)=5/1.71
cross multiply
424.08 + 3.42x =160 + 10x
264.08 = 6.58x
x= 40.1 grams per mole
x=calcium

Thanks

 

[user]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q7

Aoa wr wb!

1 has no lone pairs and 4 atome attached, so angle is 109.5

2 has no lone pairs and 3 atoms attached, so angle is 120

3 has 2 lone pairs and 2 atoms attached..so angle is 107

so ans. is 3 => 1 => 2

therefore C

 

[smartangel]

Aoa wr wb!

1 has no lone pairs and 4 atome attached, so angle is 109.5

2 has no lone pairs and 3 atoms attached, so angle is 120

3 has 2 lone pairs and 2 atoms attached..so angle is 107

so ans. is 3 => 1 => 2

therefore C

thanks..what abt 4 Sulfur dioxide, SO2, is added to wines to prevent oxidation of ethanol by air. To determine the
amount of SO2, a sample of wine is titrated with iodine, I2. In this reaction, one mole of SO2 is
oxidised by one mole of I2.
What is the change in oxidation number of sulfur in this reaction?
A +2 to +4 B +2 to +6 C +4 to +5 D +4 to +6
ans D. whats the equation for this rxn?

 

[Muhammad Asif]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q7

the answer woud be C.....in 1 there are four electron cloud regions around central atom C here with no lone pair thus the angle is 109.5°....in 2 there are three electron clouds(bonds)with no lone pair thus the angle is 120.....in 3 there are 2 electron clouds on central atom O with two lone pairs thus angle is reduced to 104.5 thus answer is 3-----1-----2

 

[user]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w11_qp_13.pdf

QUESTION - ANSWER
15 - B
23 - A
25 - C
36 - A

Someone plz explain ?

Aoa wr wb!

Q:15
Mr of CaCO3 = 100.1
Mr of CO2 = 44
CaCO3 àCaO + CO2
So 1 mol of CaCO3 will give 1 mol CO2
We have a total of 1200 million tonnes (=1200 x 10^6 x 10^3 g) of CaCO3
No of moles of CaCO3 = 1200 x 10^6 x 10^3 /100.1
This is equal to no of moles of CO2 = mass of CO2/44
Mass of CO2 = no. of moles x 44 = 527 x 10^6 x 10^3 g = 527 million tonnes
so the ans. is B

 

[Ashique]

HELPP !!

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w11_qp_13.pdf

QUESTION - ANSWER
15 - B
23 - A
25 - C
36 - A

Someone plz explain ?

15-

Question 23
You need to know the general formulas of the organic compounds listeed-
A- alcohols- CnH2n+1OH, hydrogen to carbon ratio- 4:1
B- Aldehydes- CnH2n+1CHO. hydrogen to carbon ratio- 2:1
C carboxylic acid, CnH2n+2COOH. hydrogen to carbon ratio- 2:1
D- Halogenoalkanes, CnH2n+1X, hydrogen to carbon ratio- 3:1

Greatest ratio shown by alcohols. Hence answer is A.

Question 25-
When ethanol refluxed with potassium dichormate, you'll get a carboxylic acid. Molar ratio remails 1:1. So moles of ethanol react was 2.76/46= 0.06 mol,
Since molar ratio is 1:1, 0.06 mol of ethanoic acid will also form, so mass of ethanoic acid formed= 0.01*61=3.66g
The yeild of product was 75%, so 75% of 3.66= 2.75, almost equal to 2,70, hence answer is C.

 

[nerdybookworm]

15-

Question 23
You need to know the general formulas of the organic compounds listeed-
A- alcohols- CnH2n+1OH, hydrogen to carbon ratio- 4:1
B- Aldehydes- CnH2n+1CHO. hydrogen to carbon ratio- 2:1
C carboxylic acid, CnH2n+2COOH. hydrogen to carbon ratio- 2:1
D- Halogenoalkanes, CnH2n+1X, hydrogen to carbon ratio- 3:1

Greatest ratio shown by alcohols. Hence answer is A.

Question 25-
When ethanol refluxed with potassium dichormate, you'll get a carboxylic acid. Molar ratio remails 1:1. So moles of ethanol react was 2.76/46= 0.06 mol,
Since molar ratio is 1:1, 0.06 mol of ethanoic acid will also form, so mass of ethanoic acid formed= 0.01*61=3.66g
The yeild of product was 75%, so 75% of 3.66= 2.75, almost equal to 2,70, hence answer is C.

can yu help me too..i posted my link a bit up..scroll up n chk n ans..plzz

 

[user]

Assalamoalaikum wr wb!

need help, plz... ...

P.S. I'm reposting them

 

[Casablanca]

How many isomers with the formula C5H10 have the structures that involve pi bonding?
A- 3
B- 4
C- 5
D- 6

shouldn't the answer be 2? But there is no option 2, so it can't be. What is the answer and why?

---

The products obtained by cracking an alkane, X, are methane, ethene, and propene.

The mole fraction of ethene in the products is 0.5.

What is X?

A- C6H14
B- C8H18
C- C9H20
D- C11H24

How do you calculate this?

 

[Goku]

Can you please explain to me why the Answer is C ? How do you go on about such questions? Please and thankyou!!
the propellent used in the solid rocket booster is a mixture of Al and compound X..Compound X contains Cl in an oxidation state of +7
which of the following could be compound X
A) NH4Cl b) NH4ClO3 C) NH4ClO4 D) N2H5Cl

 

[smartangel]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q 28 please explain!

 

[smartangel]

Sulfur dioxide, SO2, is added to wines to prevent oxidation of ethanol by air. To determine the
amount of SO2, a sample of wine is titrated with iodine, I2. In this reaction, one mole of SO2 is
oxidised by one mole of I2.
What is the change in oxidation number of sulfur in this reaction?
A +2 to +4 B +2 to +6 C +4 to +5 D +4 to +6
ans D. whats the equation for this rxn?

 

[angelgirl:)]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s10_qp_11.pdf
question no 40....plz help..

 

[smartangel]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q29..but-1-ene , but-2-ene, cis,trans. that makes it 4 isomers? why is the ans 3? Are they considering cis trans as one? if so,then why?

 

[Casablanca]

Can you please explain to me why the Answer is C ? How do you go on about such questions? Please and thankyou!!
the propellent used in the solid rocket booster is a mixture of Al and compound X..Compound X contains Cl in an oxidation state of +7
which of the following could be compound X
A) NH4Cl b) NH4ClO3 C) NH4ClO4 D) N2H5Cl

Oxidation no. of N is -3.
Oxidation no. of H is +1.
Oxidation no. of O is -2.
Sum of oxidation no.s in a compound must equal zero.

When you calculate it, C is the only option.

Because, look:

NH4ClO4 -> -3+4+x-8=0
-> 1+x-8=0
-> -7+x=0
->x=7

 

[Student12]

15-

Question 23
You need to know the general formulas of the organic compounds listeed-
A- alcohols- CnH2n+1OH, hydrogen to carbon ratio- 4:1
B- Aldehydes- CnH2n+1CHO. hydrogen to carbon ratio- 2:1
C carboxylic acid, CnH2n+2COOH. hydrogen to carbon ratio- 2:1
D- Halogenoalkanes, CnH2n+1X, hydrogen to carbon ratio- 3:1

Greatest ratio shown by alcohols. Hence answer is A.

Question 25-
When ethanol refluxed with potassium dichormate, you'll get a carboxylic acid. Molar ratio remails 1:1. So moles of ethanol react was 2.76/46= 0.06 mol,
Since molar ratio is 1:1, 0.06 mol of ethanoic acid will also form, so mass of ethanoic acid formed= 0.01*61=3.66g
The yeild of product was 75%, so 75% of 3.66= 2.75, almost equal to 2,70, hence answer is C.

Alright thanks
Though thought of making corrections..
- General formula of carboxylic acid is CnH2n +1 COOH
- the second sentence of Q25 is o.o6*61=3.66