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# When calculating moles for diatomic molecules, which mass do we use?

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1. E.g if a question asked how many moles in 64g of oxygen, do we do 64/(16+16) or just do 64/16.

Same for hydrogen, do we divide by 1 or 2.

For your answer could you also tell me the rule for which this applies?

I have a question if you want to try:

using the ionic equation for the displacement of bromide ions by chlorine, what mass of chlorine is required to produce 1 tonne of bromine.

When calculating moles i didnt know whether to use the diatomic mass.....the answer is 0.4 tonnes btw.

Thank You
2. (Original post by THESTRESS)
E.g if a question asked how many moles in 64g of oxygen, do we do 64/(16+16) or just do 64/16.

Same for hydrogen, do we divide by 1 or 2.

For your answer could you also tell me the rule for which this applies?

I have a question if you want to try:

using the ionic equation for the displacement of bromide ions by chlorine, what mass of chlorine is required to produce 1 tonne of bromine.

When calculating moles i didnt know whether to use the diatomic mass.....the answer is 0.4 tonnes btw.

Thank You
If you're calculating the number of moles, use the diatomic mass because you're finding it in its elemental state. For oxygen in ethanol though you would use 16 as the mass.

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