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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 $"O"_2$ .