Chapter 12 - Gases and the Kinetic-Molecular Theory - Exercises - STP, Standard Molar Volume, and Gas Densities - Page 441: 37

0.11 mole molecules or 6.62$\times10^{22}$ molecules approximately.

At STP, $\frac{V}{n}$ is always equal to 22.4 L$mol^{-1}$. This gives, Number of moles, n= $\frac{V}{22.4Lmol^{-1}}$= $\frac{2.50L}{22.4Lmol^{-1}}$ $\approx$ 0.11 mole molecules or 0.11$\times$$6.022\times10^{22}$ molecules.