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There are a number of ways to express concentration of a solution. This includes molarity. Molarity is expressed as the number of moles of solute per volume of the solution. The mass of NaOH needed for the solution is calculated as follows:

 

2 M = amount in moles of solute / 2.5 L

amount in moles of solute = 5 mol NaOH

amount in grams = 5 x 40 = 200 grams NaOH

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Answer:

                 Mass = 200 g

Solution:

             Molarity is the amount of solute dissolved per unit volume of solution. It is expressed as,

                         Molarity  =  Moles / Volume of Solution    ----- (1)

Data Given;

                  Molarity  =  2.0 mol.L⁻¹

                  Volume  =  2.5 L

                   M.Mass  =  40.0 g.mol⁻¹

First calculate Moles as,

                         Moles  =  Molarity × Volume

Putting Values,

                         Moles  =  2.0 mol.L⁻¹ × 2.5 L

                         Moles  = 5 mol

Secondly, calculate Mass using following formula,

                         Moles  =  Mass ÷ M.Mass

Solving for Mass,

                         Mass  =  Moles × M.Mass

Putting Values,

                         Mass  =  5 mol × 40.0 g.mol⁻¹

                         Mass =  200 g

Solution Preparation:

                                    Add 200 g of NaOH in a Volumetric Flask and add distilled water upto the mark of 2.5 L. Shake it and you have prepared a 2.5 L of 2.0 M solution of NaOH.

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