The gas mixture consisting of helium and fluorine has a molar mass of 21 grams / mol

The gas mixture consisting of helium and fluorine has a molar mass of 21 grams / mol, determine the volume of helium at n. y in a mixture weighing 4, 2 grams.

The molar mass of a mixture of two gases is calculated by the formula M = M1 * f1 + M2 * f2, where M1, M2 are the molar masses of the components of the mixture, and f1 and f2 are their volume fractions.
Let the volume fraction of helium = x, then f fluorine = 1 – x. Substitute the values into the formula:
21 = 4x + 36 – 38x, whence x = 0.44
Then the volume fraction of helium is 44%. Find the volume of a mixture weighing 4.2 g by the formula V = Vm * m / M = 22.4 l / mol * 4.2 g / 21 g / mol = 4.48 l
Then the volume of helium is 4.48l * 44% = 1.97l
Answer: V helium = 1.97 l