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Chemistry: Post your doubts here!

Metanoia

Metanoia

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Gehad Mohamed said:
Now make sense
Click to expand...
Maybe a final point...

The reason why one would want to reflux alcohol with dichromate in the first place is that they wish to obtain acid instead of aldehyde.

So whatever "yield of product" mentioned later, the product should be understood to be acid.

Even if some aldehyde is indeed formed, its not included in the mass of product as it is not desired in the first place.

That might also be a reason why the yield is less than 100%, cause some became aldehyde instead of acid.
 
FarahMJ

FarahMJ

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Help with this please, ans is C
0451e-f6a6a0be-97a2-465a-98c9-9db2c87c924b.png
 
Gehad Mohamed

Gehad Mohamed

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Metanoia said:
Maybe a final point...

The reason why one would want to reflux alcohol with dichromate in the first place is that they wish to obtain acid instead of aldehyde.

So whatever "yield of product" mentioned later, the product should be understood to be acid.

Even if some aldehyde is indeed formed, its not included in the mass of product as it is not desired in the first place.

That might also be a reason why the yield is less than 100%, cause some became aldehyde instead of acid.
Click to expand...
Thanks alot :)
 
Gehad Mohamed

Gehad Mohamed

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Hassan Ali Abid said:
Exactly thats what i was thinking but did u notice the wording of question??? -_-

2.76 g of ethanol were mixed with an excess of aqueous acidified potassium dichromate(VI). The
reaction mixture was then boiled under reflux for one hour. The organic product was then
collected by distillation.
The yield of product was 75.0%.
What mass of product was collected?

There you see we have to consider the Acid formed not the Alcohol and who cares if we get acid or an Alcohol as a distillate.
Click to expand...
Thanks for your help :)
 
Hadi Murtaza

Hadi Murtaza

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Hadi Murtaza said:
3 : 1 mole ratio of O₂ , so only 30 cm³ of oxygen react wid 10 cm³ of CS₂. Oxygen left is 20 cm³, CO₂ and SO₂ formed has volume 10cm³ n 20 cm³ respectively from mole ratio. Total volume of gas at end of reaction = 20 cm³ of unreacted O₂ + 10 cm³ CO₂ and 20 cm³ SO₂ :)
Click to expand...
CO₂ n SO₂ r both acidic, so they both react completely wid NaOH, n all dat is left is da 20 cm³ of unreacted O₂ :)
 
Metanoia

Metanoia

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sweet.sugar said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s05_qp_1.pdf
can somebody plz explain some questions from m/j/5
Q1-C
Q2-B
Q13-B
Q14-C (but D is also correct the sum of first two ionization energies for ca is greater)
Q38-A
ANY KIND OF HELP WOULD BE APPRECIATED :)
Click to expand...

Q1.

CxHy + O2 --> CO2 + H2O

from info, 10 cm3 of CxHy reacts with 50 cm3 of O2 to produce 30 cm3 of CO2

1CxHy + 5O2 --> 3CO2 + ?H2O

Comparing both sides of equation,

balancing carbon : x =3
balancing oxygen : 10 = 6 + ? , therefore ? = 4

1CxHy + 5O2 --> 3CO2 + 4H2O

Finally, balancing H: y = 8.

Q2.
From 1st reaction , 1 mole of NaN3 produces 1 mole of Na and 1.5 mole of N2.

The 1 mole of Na produced in 1st reaction will be used in the 2nd reaction, producing 0.1 mol of N2.

So total N2 produced = 1.5 (from 1st reaction) + 0.1 (from 2nd reaction) = 1.6 moles

Q13.

Be and Al has what we call a diagonal relationship.

Be2+ has a smaller charge than Al3+, but Be2+ has a smaller radius than Al3+. This causes their charge density (charge/radius) to be similar which in turns allows them to have similar chemical properties.

This also applies to Li and Mg and other pairings..

Google "diagonal relationship" for more details.
http://chemistry.tutorvista.com/inorganic-chemistry/s-block-elements.html


Q14. Unlikely that Ca's IE is higher than Mg, as the electrons of Ca are in a further shell.
Sum of Mg 1st 2 IE = 736 + 1450
Sum of Ca 1st 2 IE = 590 + 1150

Q38.

C2H4 + H2 --> C2H6
C2H4 --> polyethene (polymerisation)
C2H4 + cold KMno4 --> HOCHCHOH (forming diol)
 
Metanoia

Metanoia

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IGCSE13 said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_12.pdf Q10 ans B , Q18 ans b , Q20 ans C , Q25 ans D and Q38 ans A Please help if u can
Click to expand...

Q10. Graphite is made up of sheets of hexagonal rings, the angle among the carbon atoms are 120 degrees.

Q18.
A. (Wrong) Br2 is non-polar molecule (ID-ID intermolecular attractions), should be soluble in non-polar solvents.
B. (Correct) Br2 molecule mass is 80 x 2 = 160, which is heavier than air (Mr approximately 30).
C. (Wrong) Br2 vapourise easily (low boiling point) due to ID-ID intermolecular attraction.
D. (Wrong) Gaseous bromine is reddish brown.

Q20. Questions like these are quite time consuming if many atoms are involved. Might be more sensible to spend time on other questions first during the actual exam.

Picture 9.png

Q25. First 3 reactions require heating. The 4th one is reaction of 2,4-DNPH with carbonyl, which can be carried out at room temperature.

38.

I seemed to have 2 and 3 as the answer. The answer provided was A?
 
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