1. Use the Ideal Gas Law:

The Ideal Gas Law states: PV = nRT

* P = Pressure (atm)

* V = Volume (L)

* n = Number of moles

* R = Ideal gas constant (0.0821 L·atm/mol·K)

* T = Temperature (K)

2. STP Conditions:

* Standard Temperature and Pressure (STP) is defined as 0°C (273.15 K) and 1 atm.

3. Solve for Moles (n):

* Rearrange the Ideal Gas Law to solve for n: n = PV / RT

* Plug in the values: n = (1 atm)(9.1 L) / (0.0821 L·atm/mol·K)(273.15 K)

* Calculate: n ≈ 0.406 moles of oxygen

4. Convert Moles to Molecules:

* Use Avogadro's number (6.022 x 10^23 molecules/mol) to convert moles to molecules:

* 0.406 mol O2 x (6.022 x 10^23 molecules/mol) ≈ 2.44 x 10^23 molecules of oxygen

Therefore, there are approximately 2.44 x 10^23 oxygen molecules in 9.1 L of gas at STP.