A mixture of 25% O₂, 35% N₂ and 40% CO₂ by mass exerts a total pressure of 700mm Hg. The partial pressures of these gases follows the order

$\mathrm{Partial} \mathrm{pressure}=\mathrm{mole} \mathrm{fraction}\times \mathrm{total} \mathrm{pressure}$

$$\begin{gathered} \mathrm{For} {\mathrm{O}}_2, \mathrm{n}=\frac{25}{32}=0.78 \\ \mathrm{For} {\mathrm{N}}_2, \mathrm{n}=\frac{35}{28}=1.25 \\ \mathrm{For} {\mathrm{CO}}_2, \mathrm{n}=\frac{40}{44}=0.91 \end{gathered}$$

More the number of moles, more the mole fraction, which implies more the value of partial pressure.

So, the correct order is: ${\mathrm{P}}_{{\mathrm{N}}_2}>{\mathrm{P}}_{{\mathrm{CO}}_2}>{\mathrm{P}}_{{\mathrm{O}}_2}$