Question

calculate the percentage of oxygen in........ a...
b...

Answer 1

a) 33.3%
b) 66.6%

hope this helps you

Answer 2

1.) HgO

HgO ----> Hg + O

Atomic mass :
Hg= 200, O=16

Adding the masses
200+16=216

**Formula:
The Percentage of required element =mass of the required element/total mass 

the pecentage of oxygen=mass of oxygen / total mass 

=16/216x100%
=7.40%

By no. of atoms = 33.33

2.)CO2

Atomic Mass:
C=6, O=16(8x2)

Adding the masses :
(1x6)+(2x8)=22

**Formula:
The Percentage of required element =mass of the required element/total mass 

the pecentage of oxygen=mass of oxygen / total mass 

16/22*100%=72.727272%
by number of atoms its 2/3*100=66.66666