a) mH2SO4=2*1u+32u+4*16u=98 u
2u---x%
98u---100%
x=2,04%H
32u---x%
98u---100%
x=32,65%S
100%-(2,04%+32,65%)=65,31%O
b) mHNO3=1u+14u+3*16u=63 u
1u---x%
63u---100%
x=1,59%H
14u---x%
63u---100%
x=22,22%N
100%-(1,59%+22,22%)=76,19%O
c) mCaCO3=40u+12u+3*16u=100 u
40u---x%
100u---100%
x=40%Ca
12u---x%
100u---100%
x=12%C
100%-(40%+12%)=48%O
d) mFe2S3=2*56u+3*32u=208 u
112u---x%
208u---100%
x=53,85%Fe
100%-53,85%=46,15%S