Question

How many molecules are present in $10g$ of $H_{2}O$?

Answer 1

$1$ mole of $H_{2}O$ has $18$ g of mass

Thus $10$ g of $H_{2}O$ contain$=\frac{1}{18}\times 10moles$

$1$ mole contains $6.022\times {10}^{23}$ molecules

Thus no. of molecules in $10$ gof $H_{2}O$ is $\frac{10}{18}\times 6.022\times {10}^{23}$ molecules $=3.34\times {10}^{23}$ molecules of $H_{2}O$