The number of moles of solute required to make a solution of molarity 1 is 1 mole per liter. To make a solution of manganese dioxide with a molarity of 2.1 M will require 2.1 moles of the solute per liter of the solution.
The molar mass of manganese (IV) oxide or manganese dioxide is 86.9368 g / mole. 2.1 mole of manganese dioxide weighs 2.1*86.9368 = 182.56 g.
As we need to make 5.6 liters of the 2.1 M solution the amount of manganese dioxide required is 1022.37
Therefore we get the mass of manganese dioxide required to make 5.6 liters of a 2.1 M solution as 1022.37 g
the amount of moles = volume(ml)*molarity*10⁻³
MnO₂1mol → 86.9368 g
MnO₂11.76mol → 86.9368*11.76g ~ 1022.38g