Let the amount of barium sulphate is “x”g

The given reaction is:

According to law of conservation of mass,

Amount of reactants = Amount of products formed

Hence, 20.8g + 14.2g = x + 11.7g

⇒ 35g = x + 11.7g

⇒ x = 23.5g

Thus, the amount of barium sulphate is 23.5g.

The law involved is Law of conservation of mass which states that

the sum of the masses of reactants and products remain unchanged.