Quantites in Chemistry - relative atomic mass
 

There are 4 questions that i am unable to answer and they seem to revolve around the same idea. So basically I am missing out one fundamental idea, any help mostly appreciated.

1 Cobalt Oxide (CoO) contains 78.6% by mass of Cobalt. Calculate the relative atomic mass of Cobalt.

2 Find the relative atomic mass of Nickel if 3.370g nickel was obtained by reduction of 4.286g of the oxide (NiO).

3 4.150g tungsten was burned in chlorine and 8.950g tungsten chloride (WCl6) was formed. Find the relative atomic mass of tungsten.

4 If 3.72g of element X exactly reacts wih 4.8g of oxygen to form a compound whos molecular formula is shown, from other experiment, to be X4O10, what is the relative atomic mass of X?

relative atomic mass
 

1. the ratio is as follows:
A(Co)/%Co=A(O)/%O => A(Co)=%Co*A(O)/%O
I calculated 58.76 if no mistakes
2. the same calculation
3. you must take into account that are 6 chlorine atoms in the molecule (you must divide by 6)
4. is a little more complex, but you have to observe that it is a combination reaction, i.e. all X react with all oxygen and the oxide is 3.72+4.8 = 8.52 g of X4O10. finally you must divide X by 4 and O by 10.
Good luck