a) mK2CO3=2*39u+12u+3*16u=138u
78u---x%
138u---100%
x=56,52%K
12u---x%
138u---100%
x=8,7%C
100%-(56,52%+8,7%)=34,78%O
b) mLi2S=2*7u+32u=46u
14u---x%
46u---100%
x=30,43%Li
100%-30,43%=69,57%S
c) mHNO2=1u+14u+2*16u=47u
1u---x%
47u---100%
x=2,13%H
14u---x%
47u---100%
x=29,79%N
100%-(2,13%+29,79%)=68,08%O