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# Calculating Mr from density??

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1. Attachment 527339527341So I've worked out what the precipitate is Fe(OH)3 but I don't get how to work out the gas is oxygen using a calculation?
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Just quoting in Puddles the Monkey so she can move the thread if needed
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(Original post by Puddles the Monkey)
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3. Anyone pls
4. You know that the density is 1.333x10^-3 grams for every cm^3.
The mass can then be calculated by multiplying this by 24000 because 1 mol of gas occupies 24000cm3.
This gives you 31.992g in 1 mol, or in other words 31.992g.mol^-1. This is the same units as Mr so it is very likely given the context that the gas is O2. (sorry dunno how to do subscripts etc lol)
5. Okay, so you need to find the Mr to prove that the gas released is oxygen. You have the density which is 1.333*10-3, density is the mass over the volume right okay

the gas is released at room temperature and pressure which means that number of moles= V of gas / 24000

number of moles= mass/Mr , okay so mass= number of moles*Mr

Substitute these two equations into the density formula, so denisty= (mass) number of moles*Mr / (volume) number of moles*24000, number of moles cancel each other so you're left with Mr/2400=density ................... so Mr/2400= 1.333*10-3 which equals 32 which equals the Mr of oxygen
6. AqsaMx Heya, I've popped this in Chemistry for you

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