Some Chemistry Help

This year I'm crashing higher chemistry (I know I'm crazy)
I've got a practice Nab tomorrow and was looking for some help on something that's pretty simple.
I know how to work out the gram formula mass of a compound, adding up all the elements, but if the question said for example methane, I would have no idea what its formula was. Is there an easy way to work out all the formulas?
Thanks

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Reply 1

kiwi-93

i think the formula for methane is given in your data book? at least i think it was in the int 2 data book. failing that, i'd recommend learning unit 2 of the int 2 course...

Reply 2

Meteorshower

Ok you need to know that alkanes have a general formula. This is C(n)H(2n+2) where n is the number in the series. Since methane is the first (something you'll just have to know) then the formula is CH4. Any other questions i'll be happy to help

Reply 3

Slumpy

If I were you I'd also learn the order of the first 8 of most homologous series.
It's fairly standard, for the alkanes, it goes:
Methane
Ethane
Propane
Butane
Pentane
Hexane
Septane
Octane

With the prefix telling you the number of carbons, so you can work out the formula as meteorshower said.

Reply 4

Carlo08

My way of remembering them was:

Monkeys: Eat: Piss: Butties : Please : Help
meth eth prop but pent hex

So so example propane is 3rd so has 3 carbons in a straight chain..... is an alkane so no double bonds so its C3H8

Reply 5

Holdo24

I've just checked and it's not in the higher data book. Thank you for the help! Sorry if i make no sense. Not done chemistry before. Is there just a formula for alkanes?

Reply 6

Carlo08

The general formula for alkanes are : 2n + 2.... and if you remember the series:

Meth
Eth
Prop
But
Pent
Hex

You can work it out easily.... so anything with But in it has 4 carbons in a chain... if its an alkane then its 2n + 2 ..... so C4H10

Reply 7

Meteorshower

Holdo24

I've just checked and it's not in the higher data book. Thank you for the help! Sorry if i make no sense. Not done chemistry before. Is there just a formula for alkanes?

There's more on this in the second unit about these homologous series, but yes you just have to remember the formulae.

Alkanes - CnH(n+2)
Alkenes - CnH2n
Alkynes - CnH(2n-2)

Reply 8

nessiehibs

Monkeys Eat Peanuts But Peanuts Have Hard Outsides

Reply 9

Holdo24

Thanks everyone :smile:

(Monkeys Eat Peanuts But Peanuts Have Hard Outsides... what does hard stand for? :s-smilie:

)

Reply 10

Meteorshower

The first letter of each of those correspond with the first letter of the homologous series

Reply 11

namedeprived

Hard stand for heptane(C7H16) and outsides stand for octane(C8H18).

Reply 12

Holdo24

Methane
Ethane
Propane
Butane
Pentane
Hexane
Septane
Octane

?

Reply 13

namedeprived

Holdo24

Methane
Ethane
Propane
Butane
Pentane
Hexane
Septane
Octane

?

Septane should be heptane but apart from that it's all right.

Reply 14

Holdo24

Ah ok i see.
Someone said that further up the thread and I was confused. Thank you

Reply 15

asdfghjkl123456789

sorry- edited message. doesn't matter.

Reply 16

asdfghjkl123456789

sorry, i saw it after i posted.

Reply 17

username165864

Holdo24

Thanks everyone :smile:

(Monkeys Eat Peanuts But Peanuts Have Hard Outsides... what does hard stand for? :s-smilie:

)

Heptane.

Reply 18

Holdo24

So what are the alkenes and alkynes? :s-smilie:

Reply 19

Meteorshower

Holdo24

So what are the alkenes and alkynes? :s-smilie:

Alkenes are like alkanes but there is a carbon-carbon double bond somewhere, same with alkynes except a triple bond. This isn't in the first unit though so I wouldn't worry too much yet.