$12 đvC = 1,9926.10^{-23}(g)$
$\Rightarrow 1 đvC=\dfrac{1,9926.10^{-23}}{12}=1,6605.10^{-24}(g)$
a,
$91 đvC= 91.1,6505.10^{-24}=1,511055.10^{-22}(g)$
b,
Nguyên tử canxi nặng 40 đvC.
$\Rightarrow m_{Ca}=40.1,6605.10^{-24}=6,642.10^{-23}(g)$
Nguyên tử chì nặng 207 đvC.
$\Rightarrow m_{Pb}=207.1,6605.10^{-24}=3,437235.10^{-22}(g)$