[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

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!

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