1. Understand STP

STP stands for Standard Temperature and Pressure. It's defined as:

* Temperature: 0°C (273.15 K)

* Pressure: 1 atm

2. Use the Ideal Gas Law

The ideal gas law relates pressure (P), volume (V), number of moles (n), the ideal gas constant (R), and temperature (T):

PV = nRT

3. Rearrange the equation to solve for moles (n)

n = PV / RT

4. Plug in the values

* P: 1 atm

* V: 135 L

* R: 0.0821 L⋅atm/mol⋅K (Ideal gas constant)

* T: 273.15 K

5. Calculate

n = (1 atm * 135 L) / (0.0821 L⋅atm/mol⋅K * 273.15 K)

n ≈ 6.00 moles

Therefore, there are approximately 6.00 moles of oxygen gas in 135 L at STP.