Hoay's Chemistry questions

hoay SG

Ideal gases are those that obeys the laws of gases at all temperature and pressures.

But what are real gases? Any examples

Is H2 a real gas or an ideal gas?

UltimaOnline SG

All gases are real gases. Ideal gases are only a theoretical model. When you use PV=nRT, you should be aware your calculations are based on ideal gas behaviour, and therefore your calculations are not totally accurate. For more accurate calculations, you'll have to consider many other variables in your calculations, and use more complicated formulae that is beyond 'A' levels.

Sky_Blue223 SG

Yep there's the answer.

Basically PV=nRT was drawn up to show the main 5-6 factors that affect gases.

Kahynickel SG

Help needed for the question: [A-level Nov 10/11]

In a calorimetric experiment 1.60 g of a fuel is burnt. 45 % of the energy released is absorbed by 200 g of water whose temperature rises from 18 °C to 66 °C. The specific heat capacity of water is 4.2 J /g /K.

What is the total energy released per gram of fuel burnt?

Solution:

q= mc deltaT

q= 200 x 4.2 x 48 = 40320 J.................energy released

Now from this 40320J 45% is absorbed by water means 18144J

total energy release/ gm = 40320/1.6 = 25200J

But the answer is 56000J!!

Hitman Factory 1 SG

entropy of it is used in gas turbine stage calculations.

Hitman Factory 1 SG

or is it enthalpy

PhdGuy SG

Hi Kahynickel,

you got the concept right that Q=mc DeltaT.

But you misunderstood the question.

Now Q=mc DeltaT = 200 x 4.2 x 48 = 40320 J

This energy is the energy absorbed by the water and it constitutes only 45% of the total energy released.

Total energy released = 100/45 X 40320 J = 89600 J

=> energy released per g = 89600/1.6 = 56000J .

Hitman Factory 1 SG

anybody knows where to get gas turbine worked examples on internet?

PhdGuy SG

Hi Hoay,

The ideal gas equation pV = nRT is a model (obtained from experimental facts) that physicists use to approximate behaviour of real gases at extremely low pressures and high temperatures. In other words, all real gases at such conditions (low pressures and high temperatures) tend to become ideal gases and can reasonably be modelled by the ideal gas equation. Fortunately, as UltimaOnline said, real gases and their corresponding models are not in the H2 syllabus.

Kahynickel SG

PhdGuy you are saying that the total energy released has not been given in question. 40320J is the 45% of the total energy that is absorbed by water. Total energy (T.E) hence is:

45% of T.E = 40320

45/100 T.E = 40320

T.E = 40320 x 100/45 = 89600J

hoay SG

How many isomers with the formula C5H10 have structures that involve π bonding?

A 3 B 4 C 5 D 6

Ans. Pentene 1 Structure H2C=CH-CH2-CH2-CH3

Pent-2-ene Two structures cis and trans

2-methyl-but-2-ene 1structure

So 4 structures are possible but D is the answer.

UltimaOnline SG

How many isomers with the formula C5H10 have structures that involve π bonding?

A 3 B 4 C 5 D 6

Ans. Pentene 1 Structure H2C=CH-CH2-CH2-CH3

Pent-2-ene Two structures cis and trans

2-methyl-but-2-ene 1structure

So 4 structures are possible but D is the answer.

You missed out two other possible terminal alkenes (ie. structural isomers with no geometric isomers).

PhdGuy SG

PhdGuy you are saying that the total energy released has not been given in question. 40320J is the 45% of the total energy that is absorbed by water. Total energy (T.E) hence is:

45% of T.E = 40320

45/100 T.E = 40320

T.E = 40320 x 100/45 = 89600J

That's right.

hoay SG

Thank you very much for the clue. I finally got them

You mean 3-methylbutene CH2=CH-CH(CH3)2

and 2-methylbutene CH3CH2-C(CH3)CH2

I have made different combinations but failed to draw the last. After concentrating on the structural isomers term that you described I got them right.

Is there any short way to get the number of different isomers for a given molecular formula?

hoay SG

For A-level chemistry calculations of all types simple and challenging which book you would recommend. Thank you.

UltimaOnline SG

No, there is no shortcut formula for "structural isomer" questions. You have to draw them all out one by one.

For stereoisomers, the maximum number is 2^n, where n = no. of chiral carbons and alkene double bonds (that are capable of geometric isomerism, ie. terminal alkene double bonds excluded). But beware of meso compounds.

But more challenging questions will combine both structural isomers and stereoisomers, so you've no choice but to work them all out manually.

Regarding your query on chemistry calculation questions, refer to my website for the list of books.

CS Toh's Practice Questions book and MCQ book, as well as the RJC (Chan Kim Seng) book and the HCJC (George Chong) book, will cover the simple to intermediate difficulty levels.

For more challenging questions (particularly on the chemical equilibria, acid-base equilibria, solubility equilibria topics), you can use the various University level books, such as "Chemical Principles" (free online ebook), and "Exploring Chemical Analysis".

More info on all books mentioned are on my website :

http://infinity.usanethosting.com/Tuition/#Books_for_H2_Chemistry

hoay SG

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 the identity of X?

A C6H14 B C8H18 C C9H20 D C11H24

I have no idea what to do next.......... except to form an equation:

1 = x+0.5+y

Please help.

UltimaOnline SG

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 the identity of X?

A C6H14 B C8H18 C C9H20 D C11H24

Start by drawing these 3 smaller hydrocarbon products and piece them together like a jigsaw puzzle. Based on the fact that the longest hydrocarbon obtained is only 3 carbons, obviously the original large alkane must be branched. Based on the fact that 50% of the products are ethene, that means two of the four branches must be ethyl groups. The third branch must be the propyl group. The intersection of the branches, itself generates methane.

hoay SG

Which pair of elements have bonds of the same type between their atoms in the solid state?

A aluminium and phosphorus
B chlorine and argon
C magnesium and silicon
D sulphur and chlorine

D is the answer.

But Chlorine is a gas how we are supposed to consider it in solid state?? Sulphur is a solid having S8 molecules chlorine is a gas Cl2. Please elaborate.

UltimaOnline SG

Which pair of elements have bonds of the same type between their atoms in the solid state?

A aluminium and phosphorus
B chlorine and argon
C magnesium and silicon
D sulphur and chlorine

D is the answer.

But Chlorine is a gas how we are supposed to consider it in solid state?? Sulphur is a solid having S8 molecules chlorine is a gas Cl2. Please elaborate.

A - metallic vs covalent

B - covalent vs van der Waals

C - metallic vs covalent

D - covalent and covalent