Answer
$3.441\times10^{-10}\,g$
Work Step by Step
Molar mass of Pb=$207.2\,g/mol$
That is, the mass of $6.022\times10^{23}$ Pb atoms is equal to 207.2 g.
$\implies$ mass of 1 Pb atom=$\frac{207.2\,g}{6.022\times10^{23}}=3.441\times10^{-22}\,g$
Then, mass of $1.00\times10^{12}$ Pb atoms=
$1.00\times10^{12}\times3.441\times10^{-22}\,g$
$=3.441\times10^{-10}\,g$
