What pressure (in atm) will a sample of o2 gas occupy at 100.0 ml, if o2 has a volume of 25.00 ml at 2.0 atm?

Assuming that the O2 gas acts like an ideal gas, we find the following expression to be approximates of the behaviour of this gas:

P V = n R T                   ---> 1

where,

P = pressure exerted by the gas

V = volume occupied

n = number of moles

R = universal gas constant

T = absolute temperature

Further, we assume that the number of moles and the temperature are constant, hence reducing equation 1 into the form:

P V = k                         ---> 2

where k is a constant. Therefore we can equate two states:

P1 V1 = P2 V2

Since P1, V1 and V2 are given and we are to look for P2:

25 mL * 2 atm = 100 mL * P2

P2 = 0.5 atm

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.