Well, we would need to find the number of moles of ammonia #(NH_3)# from the ideal gas law, and then we can find the mass.
The ideal gas formula states that
#PV=nRT#
#P# is pressure in atms (for this case)
#V# is the volume in liters (for this case)
#n# is the moles of the substance
#R# is the ideal gas constant (varies, depending on other factors)
#T# is temperature in Kelvins.
Now, we need to do some conversions.
#20^@C = 293.15K#
#n=(PV)/(RT)# (since we need to find moles)
Since pressure is in atms, volume in liters, we use #R=0.082057 L
\ atm \ mol^-1 \ K^-1#
(Source: https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Gases/Gas_Laws/The_Ideal_Gas_Law)
Now, we can setup the equation.
#n=(2.55atm*3.00L)/(0.082057 \ L \ atm \ mol^-1 \ K^-1*293.15K)#
Also here, it is important to cancel units as well.
#n=(2.55cancel(atm)*3.00cancelL)/(0.082057 \ cancelL cancel(atm) \ mol^-1 \ cancel(K^-1)*293.15cancelK)#
#n=7.65/(24.06mol^-1)#
#n~~0.32mol#
So, there exists #0.32# moles of ammonia #(NH_3)#. To find the mass, we need to find the molar mass of ammonia and multiply it by the moles.
#"mass"="molar mass"*"moles"#
Ammonia has a molar mass of #17.031g"/"mol#.
(Source: https://socratic.org/questions/what-is-the-molar-mass-of-ammonia-nh3)
#:."mass"= 17.031g"/"cancel(mol)*0.32cancel(mol)#
#"mass"=5.45g#
