Calculate the mass of:
$(a)$ $1$ atom of carbon
$(b)$ $0.02$ mole of sodium
$(c)$ $1$ molecule of water.

$(a)$ We know that,$1$ mole of carbon atoms $= 12 \text{ g}$.
Also,$1$ mole of carbon atoms $= 6.022 \times 10^{23}$ atoms.
Therefore,the mass of $1$ atom of carbon $= \frac{12 \text{ g}}{6.022 \times 10^{23}} \approx 1.99 \times 10^{-23} \text{ g}$.
$(b)$ We know that,$1$ mole of sodium atoms $= 23 \text{ g}$.
Therefore,the mass of $0.02$ mole of sodium $= 0.02 \times 23 \text{ g} = 0.46 \text{ g}$.
$(c)$ The molar mass of $H_{2}O = (2 \times 1) + 16 = 18 \text{ g/mol}$.
Since $1$ mole of $H_{2}O$ contains $6.022 \times 10^{23}$ molecules,the mass of $1$ molecule of $H_{2}O = \frac{18 \text{ g}}{6.022 \times 10^{23}} \approx 2.99 \times 10^{-23} \text{ g}$.