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. $10^{28}$

Answer 1

Answer: (D)

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