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Answer 1 · 2017-07-05T15:22:08

It could be #"O"_2#

We're asked to identify the gas based on some gas measurements and it's mass.

What we can do is find the molar mass #M# of the gas, using the equation

#M = (dRT)/P#

where

#d# is the density of the gas, in #"g/L"#. We're given that #4# #"g"# occupies #2.8# #"L"#, so the density is

#(4color(white)(l)"g")/(2.8color(white)(l)"L") = 1.43# #"g/L"#

#R# is the universal gas constant, equal to #0.082057("L"•"atm")/("mol"•"K")#
#T# is the absolute temperature of the gas, in #"K"#. Standard temperature is defined as #273.15# #"K"#
#P# is the pressure of the gas, in #"atm"#. Standard pressure is usually for these types of problems defined as #1# #"atm"#

Plugging in known values, we have

#M = ((1.43"g"/(cancel("L")))(0.082057(cancel("L")•cancel("atm"))/("mol"•cancel("K")))(273.15cancel("K")))/(1cancel("atm"))#

#= color(red)(32.0# #color(red)("g/mol"#

To me, #sfcolor(red)("oxygen gas"# seems pretty close to this value, so it could be #"O"_2#.