The equation of the perfect gas is

#PV=nRT#

#P# is the pressure that we want to calculate, #V# is the volume in #"m"^3#, #n# are the mols, #R# is the constant of perfect gas #8.314472 ("m"^3"Pa")/("mol" "K")#, #T# is the temperature in #"K"#.

First of all we need to transform our quantity in the proper units:

#V=11.2 "L"=11.2*10^-3 "m"^3#

#T=30^\circ"C"=30+273.15 "K"=303.15"K"#.

The pressure is

#P=(nRT)/V=(0.25 cancel"mol"*8.314472 (cancel("m"^3)"Pa")/(cancel"mol" cancel"K")*303.15cancel"K")/(11.2*10^-3 cancel("m"^3))#

#=(0.25* 8.314472*303.15)/(11.2)*10^3"Pa"#

#=56261.88" Pa"#, or, if we want in a more familiar unit, we can convert it in atm using the conversion #1"atm"=101325"Pa"# then

#56261.88" Pa"= 56261.88/101325" atm"#

#=0.555" atm"#.