Silicon has a density of 2.23g/cm3 and atomic mass of 28.09g/mol calculate the number of atom in 1cm3 of silicon

The density of silicon is given as $2.23\ \frac{\text{g}}{\text{cm}^3}$.
Mass = Density $\times$ Volume

$$\text{Mass} = 2.23\ \frac{\text{g}}{\text{cm}^3} \times 1\text{ cm}^3 = 2.23\text{ g}$$

The atomic mass of silicon is $28.09\ \frac{\text{g}}{\text{mol}}$.
Number of moles = $\frac{\text{Mass}}{\text{Atomic Mass}}$

$$\text{Number of moles} = \frac{2.23\text{ g}}{28.09\ \frac{\text{g}}{\text{mol}}} \approx 0.07938\text{ mol}$$

Use Avogadro's number, which is $6.022 \times 10^{23}\ \frac{\text{atoms}}{\text{mol}}$.
Number of atoms = Number of moles $\times$ Avogadro's number

$$\text{Number of atoms} = 0.07938\text{ mol} \times 6.022 \times 10^{23}\ \frac{\text{atoms}}{\text{mol}}$$ $$\text{Number of atoms} \approx 4.781 \times 10^{22}\text{ atoms}$$

The number of atoms in $1\text{ cm}^3$ of silicon is:

$$4.781 \times 10^{22}\text{ atoms}$$