Calculate the number of molecules present in 2 L of water provided the density of water is 1 g mL^-3.

Since, molar mass of $H_{2}O$ = 18 g mol^-1
2 L of $H_{2}O$ = 2000 mL
Mass of 2000 mL of water = volume × density
= $2000 \times 1$
$=2000\mathit{g }$
Number of moles = $\frac{\mathrm{Mass} \mathrm{of} \mathrm{the} \mathrm{substance} \mathrm{in} \mathrm{grams} }{\mathrm{Molar} \mathrm{mass}}$
    $\Rightarrow$ $\frac{2000}{18}$
1 mole of $H_{2}O$ = $6.022x10^{23}$ molecules of H₂O
Therefore, no. of molecules of $H_{2}O$ = $\frac{2000}{18}\times 6.023\times 10^{23}$

                                                                $\Rightarrow$ $6.69\times 10^{25}$ molecules of $H_{2}O$
Hence, $6.69\times 10^{25}$is the number of molecules present in 2 L of  water provided the density of water is 1 g mL^-3.