A sphere of lead of mass 10 g has net charge `-25 xx 10^(-9)C`.
(i) Find the number of excess electrons on the sphere.
(ii) How many excess electrons are per lead atom ? Atomic number of lead is 82 and its atomic mass is 207 g/mol.

Explanation

(a) The charge of an electron `=1.6xx10^(-19)` C
So, the number of excess electrons `=(-2.5xx10^(-9)C)/(-1.6xx10^(-19)C)=1.56xx10^(10)`
(b) Atomic numb of lead is 82
10 g of lead will have `(10g)/(207" g"//"mol")xx6.02xx10^(23)` atoms/mol`=2.91xx10^(22)` atoms
`:." "` The number of excess electrons per atom `=(1.56xx10^(10))/(2.91xx10^(22))=5.36xx10^(-13)`