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# Moles Problem watch

1. Hi, don't quite understand how this works for these questions.

1.How many atoms are in 0.25 moles of O2 at STP.

I got the answer as 0.25 x Av constant which is 1.5x10^23, but the answer is 3.01x10^23 which is half of Av constant not 1/4 which doesnt seem to make much sense to me.

2. How many litres are in 56 grams of N2 at STP.

1 Mole of a gas has 22.4L and here there are 4 moles so won't it be 89.6, but the answer is 44.8L. Once again, don't understand how they got this answer.

Really Appreciate the help!
2. (Original post by ChemBoy1)
Hi, don't quite understand how this works for these questions.

1.How many atoms are in 0.25 moles of O2 at STP.

I got the answer as 0.25 x Av constant which is 1.5x10^23, but the answer is 3.01x10^23 which is half of Av constant not 1/4 which doesnt seem to make much sense to me.

2. How many litres are in 56 grams of N2 at STP.

1 Mole of a gas has 22.4L and here there are 4 moles so won't it be 89.6, but the answer is 44.8L. Once again, don't understand how they got this answer.

Really Appreciate the help!
For question 1:
There are two atoms of oxygen in a molecule.
This means there are 0.5 moles of oxygen atoms.
Hence 0.5*6.02x10^23=3.01x10^23
3. Hi, sorry didn't quite understand that 0.5 moles part, could you explain that please. Thanks!
(Original post by danielwinstanley)
For question 1:
There are two atoms of oxygen in a molecule.
This means there are 0.5 moles of oxygen atoms.
Hence 0.5*6.02x10^23=3.01x10^23
4. (Original post by ChemBoy1)
Hi, sorry didn't quite understand that 0.5 moles part, could you explain that please. Thanks!
Sure..

so there are 0.25 moles of oxygen molecules
A mole is just an amount of substance
So 2*0.25 = 0.5
so there are 0.5 moles of oxygen atoms
5. For question 2:

If we use Number of moles = mass/molar mass
Molar mass = 14+14
you get 56/24.0 = 2.00 moles

So there are 2 moles.
6. Thanks, that really helps!
(Original post by danielwinstanley)
For question 2:

If we use Number of moles = mass/molar mass
Molar mass = 14+14
you get 56/24.0 = 2.00 moles

So there are 2 moles.

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