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Molar concentration formula

 

Molar concentration is the most convenient method of expressing the concentration of a solute in a solution. Molarity is defined as the number of moles of solute dissolved per liter of solution, i.e., M = mol/L. All mole calculations are applied to determine the amount in moles of the solution, for which it is the molar concentration.

The balanced chemical equation leads to a mole ratio between the acid and the base, which in turn leads to amount of moles of the base. Finally the molar concentration of base is determined by the known volume and amount in moles.

The molar concentration formula is given by,

 

 

Example 1

Determine the molar concentration of NaOH for the reaction between HCl and NaOH.

Solution:

The balanced chemical equation can be framed as,

HCl + NaOH NaCl + H2O

For an acid

n(HCl) = 35.0 / 1000dm3 × 0.250 mol dm-3

          = 8.75 ×× 10-3mol

 

     the mole ratio NaOH : HCl = 1:1.

The amount of NaOH present in moles is 8.75 × 10-3mol.

Now we have the equation,

 

First convert the volume of aqueous NaOH into dm-3

25 cm3 = 25 / 1000dm3

            = 25 × 10-3dm3

Therefore, molar concentration of NaOH

           = 8.75×10−3 / 25.0×10-3

Molar concentration of NaOH = 0.350 mol dm-3

 

Example 2

The concentration of Ca(HCO3)2 is 0.85 gmol/L. Convert this concentration into geq/L.

Solution:

Given :

[C] = 0.85 gmol/ L

[C]eq = [C][MM] / eq.mass

MM = 40.1(2) + 2{1+12+3(16)}

   = 202.2

Number of reference species = 2

Therefore, eq.mass = Ca(HCO3)2 / 2

                  = 202.2 / 2

 

[C]eq = 0.85[202.2] / 202.2/2

= 1.7 geq/L