A gaseous metallic chloride MClx consists of 20.22% of M by mass. The formula of the chloride is [M = 27, Cl = 35.5]

The given percentage of M in the gaseous metallic chloride MCl_x is 20.22%. We need to determine the formula of the chloride. First, we can assume that we have 100g of the compound. This would mean that there are 20.22g of M and 100 - 20.22 = 79.78g of Cl in the compound. Next, we need to determine the ratio of M to Cl in the compound. We can do this by dividing the mass of each element by its atomic weight and then dividing by the smaller result to get the simplest ratio. For M, (20.22 / 27) gives a result of 0.748 and for Cl, (79.78 / 35.5) gives a result of 2.25. Dividing both by 0.748 gives a ratio of approximately 1:3. This means that the formula for the compound is MCl3, which is option (C).