Chemistry!

[MikeCottTrials]

Could someone with chemistry skills explain to me how to find the number of atoms in a certain amount of element.

Lets say, for demonstration purposes, how do I find the number of atoms in 0.3g of oxygen?

I understand that we need the molar mass, and we need to do something with avagadros number... but I'm at a loss when it comes to putting this into action

[MikeCottTrials]

I may well have just realised it' quite easy. But I may not have at the same time

[Luke Dunstan]

Hope this helps, although I doubt it will.

[monkeyseemonkeydo]

1.12875x10²² atoms (In 0.3g of O₂)?

Been a long time since I did chemistry...

[Luke Dunstan]

1.12875x10²² atoms (In 0.3g of O₂)?

Been a long time since I did chemistry...

Or maybe 5.64375x10²¹ if the atomic mass of Oxygen is 32 (because it's diatomic) rather than 16.

Edit: That can't be right... anyone?!

[monkeyseemonkeydo]

Lol. I forgot that you get 2 moles for an O₂ molecule so I'm back to my original answer...

Edit: in an effort to allow someone to shoot me down my thinking is:

Atomic weight of Oxygen = 16g therefore 16g = 1 mole.

0.3g = (0.3/16)*1 = 0.01875 moles.

0.01875 x 6.02x10²³ = 1.12875x10²² atoms.

[Jon Alty]

What you need to do is to work out the number of moles of the substance you have, and then multiply the number of moles by Avagadros constant - as avagadros constant states that 1 mole of ANY substance will contain 6.0221415 × 10^23 atoms.

Oxygen is a slightly wierd example, as its a gas, so working out the number of moles is different than for solids, so you would need the volume and not the weight. But lets presume that this oxygen is in liquid form, then you would do;

0.3/16 = 0.01875 moles

0.01875*6.0221415 × 10^23 = 1.12915153 × 10^22

If you want to work out for another solid, then replace the 0.3 with the amount u have (in g) and replace the 16 with the atomic mass of that element (as 16 is the Mr of oxygen) and then times by avagadros constant (6.0221415 × 10^23).

For gases, to work out the moles, do the volume, in dm^3 (which is essentially litres), and divide it by 24, as 1mole of any gas occupies 24dm^3 . And then multiply by Avagadros constant.

P.S - You know the ^ symbol means 'To the power of' ?

Btw, i used google as a calculator so im not actually sure on the calculations, but Avagadros constant is correct .

Hope I helped mate, and dont quote me cos i may be wrong, but i hope not aha.

Edited December 17, 2010 by Jon Alty

[monkeyseemonkeydo]

Damn I'm good. Just missing a few decimal places on Avogadro's constant. Last time I did Moles must've been... 10 years ago! Christ I'm old.

[dirt jumper jake]

is this like relative atomic mass? or am i wrong?

[MikeCottTrials]

Thanks guys! I've got a few questions I need to get through over christmas so I've no doubt this will help!

[MikeCottTrials]

Ok then guys, christmas has ended and work has recommenced.

Thanks to the responses I'm feeling quite confident with these questions now, however one question I do have is about the example question.

Rather than just working out the answer for oxygen, would I be correct to divide by 32 instead of 16 if it where o2 and not just one oxygen?

Also if I wanted to do this for a compound like water, would I just add all the atomic weights together and divide by the number of moles?

[Tomm]

Rather than just working out the answer for oxygen, would I be correct to divide by 32 instead of 16 if it where o2 and not just one oxygen?

Depends by what method you're using.

If you have a known mass of oxygen, then it doesn't matter whether a molecule is bonded to another, it won't affect the mass. So in this case you would divide by 16.

However, if you're working it out using the ideal gas equation from the volume of oxygen, you'd be absolutely correct in saying that each O2 molecule has a relative mass of 32 (I.e. divide by 32).