A balloon is filled with 15.5 g of water. What will be the number of water molecules contained in the balloon?

The correct option is C $5.18\times {10}^{23}$
The molecular formula of water is $H_{2}O$.
Molecular mass of water = $2$ $+$ $16$ = $18u$.
So, gram molecular mass of water = $18g$
Thus, $1$ mole of water has a mass of $18g$ and it contains $6.023\times {10}^{23}$ molecules of water.

So, $15.5g$ of water contains $=\left(\frac{6.023\times {10}^{23}}{18}\right)\times 15.5$
$=5.18\times {10}^{23}$ molecules