# I have a chemistry question here...

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phillippeengelposted 5 years ago

I know that this is not a homework forum, but I want to help a student who has difficulties, which I could not solve some, in this particular subject. It would be great if anyone can help.

Question: 3.9g of an element X reacts with oxygen gas to form 4.7g of an oxide X2O (X under-script O). The relative atomic mass(Ar) of X is_______?

Thank you once again.

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EmpressFelicityposted 5 years ago in reply to this

The relative atomic mass of the element is 39 (it's potassium, FWIW).

3.9 g of X reacts with 0.8 g of oxygen (since 4.7 - 3.9 = 0.8)

Get rid of the decimal points by multiplying by 10 on each side.

This means that 39 g of X reacts with 8 g of oxygen.

Handily enough, the atomic mass of oxygen is 16, which is 8 x 2.

Therefore to react with 16 g (one mole) of oxygen, you need 78 g of X.

For every one mole of oxygen you're using two moles of X so one mole of X weighs 39 g.

The relative atomic mass of X is 39.

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phillippeengelposted 5 years ago in reply to this

Wow, thank you so much EmpressFelicity!

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EmpressFelicityposted 5 years ago

You're very welcome

Perhaps I should dredge up all my old school and uni notes and write some chemistry hubs lol.

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Pearldiverposted 5 years ago in reply to this

No.... Don't write Hubs on this!!

It's Far More Interesting Learning your equations in the forums!

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