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How to Find Molar Concentration of Ions





The solution is a homogenous mixture of solute in solvent. The solute is that component which is found in small amount compare to other component that is solvent. 
Solvent is found in high concentration in a solution. The nature of a solution depends upon the solute and solvent. 
It also depends upon the concentration of solution and chemical nature of solute and solvent. The concentration of a solution represents the amount of solute present in a certain amount of solvent. 
There are several ways to represent the concentration of a solution such as molarity,normality,ppm,formality,percentage composition etc. 
Let’s discuss one of the ways to express the concentrations of solutions quantitatively which is known as molarity of the solution. 
It is one of the most widely used quantitative relations to express the concentration of solution. 
It can be defined as the number of moles of solute present in one liter of solution. The mathematical relation of molarity with the number of moles of solute can be written as given below;
Molarity (M) = Number of moles of solute (n) / Volume of solution in liters (V)
Hence, one molar solution will contain 1.00 mol of solute in one liter of the solution. 

Let’s take an example of molarity. Find the molarity of a solution which is made by dissolving 23.4 g of sodium sulphate (Na2SO4) to form 125 mL of solution. 
The molar mass of sodium sulphate is 142 gm; hence we can calculate the moles of sodium sulphate would be 23.4 gm / 142 gm/mol  = 0.165 mol of Na2SO4. The volume of solution is 125 ml or 0.125 L; hence the molarity of the solution would be;
Molarity = 0.175 mol / 0.125 = 1.32 M

We can calculate the concentration of free ions found in the solution of ionic compounds.
The use of mole ratio is the best way to calculate the concentration of ions in the solution. 
We know that anions are negatively charged ions while cations are positively charged ions
The dissociation equation for a certain compound can be written as given below;
xA -->   yB + zC
Here A stands for the substance is being dissolved before while B and C are ionic forms in the equation. x,y, and z represents the coefficients of ions involve in the reaction. 
To calculate the concentration of ion with the help of mole ratio; let’s start with same reaction; the concentration of solute is [A], concentration of 1st ion is [B] or y/x [A]. 
The concentration of 2nd ion is [C] or z/x [A]. For example; calculate the ion concentration of 0.050 mol/L solution of KCl. 
KCl (s) --> K+ (aq) + Cl- (aq)
The concentration of KCl is 0.10 mol/L, we have to calculate the concentration of K+ and Cl- ion. 
[K+] = 1/1 x (0.10 mol/ L ) = 0.10 mol /L
[Cl-] = 1/1 x (0.10 mol/L) = 0.10 mol/L

Find the concentration of ions formed in the solution of 0.010 mol/L of aluminium sulphate.
We know that the decomposition of aluminium sulphate results the formation of 2 aluminium ions and 3 sulphate ions. 
Al2(SO4)3 (s) --> 2Al3+(aq) + SO42- (aq)
[Al3+]= 2/1 x (0.010 mol/L) = 0.020 mol/L
[SO42-] = 3/1 x (0.010 mol/L) = 0.030 mol/L