The relative molecular mass of the gas is "39.2 g/mol"39.2 g/mol.
A very important thing happens at STP with ideal gases: 1 mole of any ideal gas occupies "22.4 L"22.4 L - the molar volume of a gas. Since density is defined as mass per unit volume, you can automatically find the mass of 1 mole of gas (notice that I've converted "1 L"1 L to 1 "dm"^3dm3)
rho = m/V => m = rho * V_("molar") = "1.750 g/dm"^3 * "22.4 dm"^3ρ=mV⇒m=ρ⋅Vmolar=1.750 g/dm3⋅22.4 dm3
m = "39.2 g"m=39.2 g
Therefore, the gas' molar mass is
"molar mass" = ("39.2 g")/("1 mole") = "39.2 g/mol"molar mass=39.2 g1 mole=39.2 g/mol
This is pretty close to "Ar"Ar's molar mass, so that could be the identity of the gas.