1. The Ideal Gas Law

The Ideal Gas Law is expressed as:

PV = nRT

where:

* P = pressure (in atmospheres, atm)

* V = volume (in liters, L)

* n = number of moles

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

* T = temperature (in Kelvin, K)

2. Rearrange the Formula to Solve for 'n'

We want to find the number of moles (n), so rearrange the formula:

n = PV / RT

3. Plug in the Values

* P = 3.5 atm

* V = 20 L

* R = 0.0821 L·atm/mol·K

* T = 280 K

4. Calculate

n = (3.5 atm * 20 L) / (0.0821 L·atm/mol·K * 280 K)

n ≈ 3.04 moles

Therefore, approximately 3.04 moles of an ideal gas will occupy a volume of 20 liters at 3.5 atm and 280 K.