The formula for finding the relative density of a gas is: D2 = Mr1 / Mr2, where Mr1 / 2 is the molar mass of gases, or their relative molecular mass.
We are given by condition the value of the density of an unknown gas in the air. It is known that the molar mass of air is 29 g / mol.
We substitute the known values into the formula and find the molar mass of the unknown gas: Mr1 = D2 * Mr2, Mr1 = 0.138 * 29 = 4.002 g / mol.
Found the molar mass of the gas. According to the periodic table, it can be determined that the unknown gas is helium.
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