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ellis182:
Guys just a really easy question i need clarification with..

How many moles of O-atoms in 25.7g of CaSO4?

I know moles = mass/molar mass

so..

25.7/(40.8 + 32.07 + 4x16) = .188

Is this the answer or do i multiply .188 by 4 because there is 4 oxygen atoms in CaSO4?

Also..

How many molecules in 13.72g oh hydrazine? N2H4? i dont even know where to start!

If anyone could help me at all it would be much appreciated!

AWK:
1 OK

--- Quote ---How many molecules in 13.72g oh hydrazine? N2H4? i dont even know where to start!

If anyone could help me at all it would be much appreciated!
--- End quote ---
Calculate moles of hydrazine as in the first problem, then multiply by Avogadro number

Pirt:
Hey ellis you are right about how to do it.

heres how i worked it out, hope this helps.

the weight on 1mole of CaSO4 is 136.9g.

therefore if you work out how much mass is needed as a ratio

1st part ratio between mass you have and mass for one mole is:

25.7 / 136.9 = 0.188

Ca       : S      : O
40.8    : 32.1  : 64  <------ mass in g for 1 mole

Divide mass for 1 mole by the ratio above (0.188)

7.67    : 6     :  12  <-------- mass in g for 0.188 moles

now you have the mass of O in your compound which is 12g so you can do

12/16 = 0.75 moles   [mass / RMM = no moles]

Hope this helps

Prit