A sample of gas (26.1 g) initially at 3.00 atm was compressed from 21.00 L to 6.00 L at constant temperature. After the compression, the gas pressure was __________ atm.

P1V1 = P2V2

To find the final gas pressure after compression, we can use Boyle's Law, which states that the pressure and volume of a gas are inversely proportional when the temperature is kept constant.

Boyle's Law equation is expressed as:
P₁V₁ = P₂V₂

Where:
P₁ = initial pressure of the gas
V₁ = initial volume of the gas
P₂ = final pressure of the gas (what we want to find)
V₂ = final volume of the gas

Given:
Initial pressure, P₁ = 3.00 atm
Initial volume, V₁ = 21.00 L
Final volume, V₂ = 6.00 L

Substituting the given values into the Boyle's Law equation, we have:
P₁V₁ = P₂V₂

(3.00 atm)(21.00 L) = P₂(6.00 L)

Now, let's solve for P₂:
P₂ = (3.00 atm)(21.00 L) / (6.00 L)

P₂ = 10.50 atm

Therefore, the gas pressure after compression is 10.50 atm.