(a) $$ \begin{cases} R &= 1.90\ut{\AAA} \\ N_0 &= 6.023 \times 10^{23}\ut{EA/mol} \\ m_{\mathrm{Na}} &= 22.9898\ut{g/mol} \end{cases} $$ $$ \begin{aligned} \Ans &= \rho_{\mathrm{Na}} = ? \\ &= \frac{\mathrm{Mass}}{\mathrm{Volume}} \\ &= \frac{m_{\mathrm{Na}}}{N_0} \cdot \frac{3}{4\pi R^3} \\ &= \frac{22.9898\ut{g/mol}}{6.023 \times 10^{23}\ut{EA/mol}} \cdot \frac{3}{4\pi (1.90\ut{\AAA})^3} \\ &=..