In Bi2S3:
change in oxidation state of S =0−(−2)=2 and
change in oxidation state of Bi=(+5)−(+3)=2 so
Eq. wt of Bi2S3=M6+2×2=10
In Al2(Cr2O7)3:
change in oxidation state of Cr =6×+6−(3)=18
Eq. wt of Al2(Cr2O7)3=M6×3=18
In FeS2:
change in oxidation state of S =4−(−1)=5 and
change in oxidation state of Fe=(+3)−(+2)=1.
So, eq. FeS2=M1+2∗5=11
In Mn(NO3)2:
change in oxidation state of S =6−(+2)=4 and
change in oxidation state of N=(+2)−(+5)=−3 so
Eq. wt of Mn(NO3)2=M|4+2∗−3)|=2