Question

how many molecules of oxygen are present in 5 ml of water

Answer 1

At standard conditions of Temperature and Pressure :
1 mole of O₂ = 6.022 $\times$ 10²³ = 22.4 L
Thus, 22.4 L of O₂ contains 6.022 $\times$ 10²³ molecules
Let, 5 mL of O₂ contains x molecules
So x can be calculated using unitary method.
x = $$\begin{gathered} \frac{5\times 6.022\times {10}^{23}}{22.4} \\ =1.344\times {10}^{23} \end{gathered}$$
molecules

Thus 5 mL of O₂ contain 1.344 $\times$10²³ molecules