a) mCl2O7=2*35,5u+7*16u=183u
71u---x%
183u---100%
x=38,8%Cl
100%-38,8%=61,2%O
b) mHCl=1u+35,5u=36,5u
1u---x%
36,5u---100%
x=2,74%H
100%-2,74%=97,26%Cl
c) mCu(OH)2=63,5u+2*16u+2*1u=97,5u
63,5u---x%
97,5u---100%
x=65,13%Cu
32u---x%
97,5u---100%
x=32,82%O
100%-(65,13%+32,82%)=2,05%H