Answer
$N = 7.2 \times 10^{24}$
Work Step by Step
Use the formula mass, density and volume to find the mass of the sample
$m = \rho V$
Where
$\rho = 9.0 g/cm^3$
$V = 4.0 \times 10^{-5}m^{-3}$
$V = 40.0 cm^3$
$M = 60g/mol$
So
$m = (9.0 g/cm^3) (40.0 cm^3)$
$m = 360 g$
Now find the number of moles, n
$n = \frac{m}{M}$
$n = \frac{360 g}{60g/mol}$
$n = 6.0 mol$
Now we can find the conduction electrons
$N = n N_A$
The n is 6.0 but the atom is bivalent so 6.0 x 2 = 12 mol
$N = (12.0 mol) (6.022 \times 10^{23}/mol)$
$N = 7.2 \times 10^{24}$
