Barium with a mass of 10.79 g completely reacted with an unknown halogen, the mass of which was 20 g.

Barium with a mass of 10.79 g completely reacted with an unknown halogen, the mass of which was 20 g. Determine which halogen reacted.

So, let’s write the reaction equation:
Ba + Hal2 = BaHal2 (Hal2 – this is how halogen is denoted)
Since the number of substances = 1 mole (look at the coefficients), we make up the proportion:
mass of barium / molar mass of barium = mass of halogen / molar mass of halogen.
m1 / M1 = m2 / X
10.79 / 137 = 20 / X
X = 254
Since it is halogen, its molecule consists of 2 atoms (Hal2)
So 254/2 = 127
We look at the periodic table, atomic mass = 127 is iodine.
Answer: Iodine (I2).