Question

The density of copper is $ 9 \times 10^3\, kg/m^3 $ and its atomic mass is $ 63.5\, u $ . Each copper atom provides one free electron. Estimate the number of free electrons per cubic metre in copper,

• A. $ 10^{19} $
• B. $ 10^{23} $
• C. $ 10^{25} $
• D. $ 10^{29} $

Answer 1

Answer: (D)

Density of copper,
$\rho =9\times 10^{3} kg/m^{3}$
$=9\times 10^{6} g/m^{3}$
Avogadro number, $N_{A}=6.023\times10^{23}$
Mass of $1$ mole of copper atom, $M=63.5\,g$
Thus, number of free electrons per volume is
$n=\frac{N_{A}}{M} \rho=\frac{6.023\times10^{23}}{63.5}\times9\times10^{6}$
$=8.5 \times 10^{28}\,m^{-3}$