When 10 liters of a gas at 1 atm is compressed to 3 liters at constant temperature, what property of the gas changes?

We know that at state 1, we can describe the gas at STP by P1V1=n1RT1

We change the volume, so now there is a second state, state 2 that can be described as

P2V2=n2RT2

We are told that temperatures are constant and since no gas is lost then moles don't change. Hence n1T1=n2T2. So rearrange the equations as follows

P1V1/R=n1T1

P2V2/R=n2T2

Set them equal per my explanation above:

P1V1/R=P2V2/R

Now multiply by R on both sides

P1V1=P2V2

We want to find the pressure at state 2

P1V1/V2=P2

(1 atm) (10L) / (3L) = 3.33 atm = P2