Help with Higher Chemistry

2

Needing some explanations on a couple of questions. Got my prelim in 2 days and starting to panic so decided to do some past papers.

1. A mixture of magnesium chloride and magnesium sulphate is known to contain 0.6 mul of chloride ions and 0.2 mol of sulphate ions. What is the number of moles of magnesium ions present?

2. Which of the following contains the same number of stoms as 16g of hydrogen.
A 16g methane
B 16g oxygen
C 17g ammonia
D 12g argon

3. C2H6 (g) + 3½O2 (g) --> 2CO2 (g) + 3H2O (l)
When 20cm3 of ethane was sparked with 100cm3 of oxygen, what was the final volume of gases? all volumes wer measured at atmospheric pressure and room temperature.

Any explanations and help would be appreciated. (Crashing higher so no SG knowledge)

Reply 1

Preasure

13

Move to chemistry forum perhaps?

1) Your reagents are MgCl2 and MgSO4, so the number of moles Mg2+ ions = (moles Cl- ions / 2) + moles sulphate ions.

2) Number of moles = Mass / Mr. For hydrogen H2, your Mr is 2.01. Multiplying by avagadro's number would give you the number of atoms but you dont need to in this case - just use Number of moles = Mass / Mr to find which of your options has the same number of moles as H2. Don't forget to multiply atomic mass by 2 for diatomic molecules :wink:

3) 1 moles of a gas occupies 22.4 litres (22400cm3) - use that to convert volumes to moles and vice versa. Work out the number of moles of the C2H6 and multiply by 2 to get the resulting moles of CO2, then convert back to volume.

Reply 2

acas13

8

Holdo24

Needing some explanations on a couple of questions. Got my prelim in 2 days and starting to panic so decided to do some past papers.

3. C2H6 (g) + 3½O2 (g) --> 2CO2 (g) + 3H2O (l)
When 20cm3 of ethane was sparked with 100cm3 of oxygen, what was the final volume of gases? all volumes wer measured at atmospheric pressure and room temperature.

Any explanations and help would be appreciated. (Crashing higher so no SG knowledge)

the above answer to this isnt right, you dont need to convert it into moles at all. its just a case of simple ratios. You can tell that oxgen is in excess as 3.5x20 is less than 100/

1 mole + 3.5 moles---->2moles of gas
20cm3 + 70cm3------> 40cm3

so the answer is 40cm3 of gases

Reply 3

namedeprived

Hmnn for question 2 am i wrong in thinking that none of them would have the same.

16g of H2 -> 8L molecules ->16l atoms. (where L = Avogadro's constant.)

None of the others have that as far as i can see?

Edit: Actually, because I'm a geek, I looked back in my past papers (I'm also doing Higher Chem) and found the source of my confusion. You meant to say 16g of hgelium atoms if it's the question I'm looking at.

That would mean that Helium has 4L atoms. Only one of the options has this.

Spoiler

Reply 4

acas13

8

yeah i thought that too... but then doubted myself so i didnt say anything!

Reply 5

sjames06

acas13

the above answer to this isnt right, you dont need to convert it into moles at all. its just a case of simple ratios. You can tell that oxgen is in excess as 3.5x20 is less than 100/

1 mole + 3.5 moles---->2moles of gas
20cm3 + 70cm3------> 40cm3

so the answer is 40cm3 of gases

Ahh, been tricked :p:

It's not the nicest of questions.

C2H6 (g) + 3½O2 (g) --> 2CO2 (g) + 3H2O (l)

Yes, you can do the mol ratios which would, as you have said, give you
20cm3 + 70cm3------> 40cm3

But you had 100cm3 of oxygen to start with, so the composition of the gas at the end would be the 40cm3 of CO2, and the remainder of the oxygen (100-70), so 30cm3 with the total volume being 70cm3 comprising of 40cm3 of CO2 and 30cm3 of unreacted oxygen.

Hope it makes sense! :eek3:

Reply 6

acas13

8

Wow, you actually just sorted quite a bit of the problem i had with multiple choice, i always wondered why i got those wrong! never thought about the excess :smile:

Thankyou x

Reply 7

sjames06

acas13

Wow, you actually just sorted quite a bit of the problem i had with multiple choice, i always wondered why i got those wrong! never thought about the excess :smile:

Thankyou x

No problem, glad to be of help :biggrin:

Reply 8

Swayum

18

In the future, please post in the Academic Help forums :smile:

.

Reply 9

Hard work

8

Original post by sjames06

Ahh, been tricked :p:

It's not the nicest of questions.

C2H6 (g) + 3½O2 (g) --> 2CO2 (g) + 3H2O (l)

Yes, you can do the mol ratios which would, as you have said, give you
20cm3 + 70cm3------> 40cm3

But you had 100cm3 of oxygen to start with, so the composition of the gas at the end would be the 40cm3 of CO2, and the remainder of the oxygen (100-70), so 30cm3 with the total volume being 70cm3 comprising of 40cm3 of CO2 and 30cm3 of unreacted oxygen.

Hope it makes sense! :eek3:

where did u get 20? 70? 40? where are these ratios coming from????

Reply 10

Pigster

20

Original post by Hard work

where did u get 20? 70? 40? where are these ratios coming from????

I'm fairly certain that seven years later, you're unlikely to get a reply.

Reply 11

eveloisallan

hey can someone help me out with this question:
A positively charged particle with electron arrangement 2,8 could be
A a neon atom
B a flouride ion
C a sodium atom
D an aluminium ion

Thanks!! higher chemistry is messing with my brain :/

Reply 12

odin123

2

8 years on and the answer is still incorrect....C2H6 (g) 3½O2 (g) --> 2CO2 (g) 3H2O (I)
The ratio of Ethene to CO2 H20 is 1:3:2 .
You had 20cm3 of ethene to start so you end up with (2 x 20) = 40cm3 of CO2 and (3 x 20)=60cm3 of H20 , 100cm3 in total.....
BUT you had 100cm3 of O2 to start with . The ratio of ethene to O2 is 1:3.5 so if you reacted 20cm3 of ethene you have to also react (20 x 3.5) = 70cm3 of O2 ------> O2 is in excess, 100cm3 to start with you used 70 so how much is left at the end?
so 40 60 30=130cm3

(edited 8 years ago)

Reply 13

odin123

2

Original post by eveloisallan

hey can someone help me out with this question:
A positively charged particle with electron arrangement 2,8 could be
A a neon atom
B a flouride ion
C a sodium atom
D an aluminium ion

Thanks!! higher chemistry is messing with my brain :/

Am i late?
it says positively charged particle so it has to be an ion, leaves only B & D.
positively charged means it has to lose electrons, Fluorine gains 1e to gain a negative charge [2,8]-
Aluminium loses 3e to gain a +3 charge [2.8]+
which one is the positive ?

Reply 14

HenrikWong

8

Original post by eveloisallan

hey can someone help me out with this question:
A positively charged particle with electron arrangement 2,8 could be
A a neon atom
B a flouride ion
C a sodium atom
D an aluminium ion

Thanks!! higher chemistry is messing with my brain :/

It is aluminium ion because aluminium originally with an electron configuration 2.8.3. Therefore losing three electrons forming aluminium 3+ ion will have the electron configuration 2.8.

A - Neon atom 2.8 (not positive charged)
B - Floride ion 2.8 (negative charged)
C - Sodium atom 2.8.1 (No charge)
D - Aluminium ion 2.8 (3+)

Therefore, It is D.