Lead (II) nitrate is a poisonous substance which has been used in the manufacture of special explosives and as

Question

Lead (II) nitrate is a poisonous substance which has been used in the manufacture of special explosives and as a sensitizer in photography. Calculate the mass of lead in 139 g of Pb(NO3)2.

Answer 1

Molar mass of Pb(NO3)2 is: 331.18 g/mol
Molar mass of Pb is: 207.2 g/mol
Hence it is a simple ratio to find the mass of lead in 139 g of Pb(NO3)2:
139 g*(207.2/331.2) = 87.0 g
Notice I only take 3 significant figures, since your given data 139g only has 3 significant figures. The last gidit 0 after decimal is important to indicate the error is less than 0.05 g.