Define the following terms: (i) Mole fraction (ii) Molality (iii) Molarity (iv) Mass percentage

 

(i) Mole fraction (XA):-

It is the ratio of number of moles of one component to the total number of moles present in that solution

Let take a binary solution in which A is solute and B is solvent then

Molar fraction (XA) = number of moles of A/(number of moles of A + Number of moles of B)

If there are there are n number of component  in solution then

Molar fraction of component A =   number of moles of A / total number of moles

Sum of molar fraction of all components will always = 1

ie  NA+ NB+NC …………….= 1  (NA, NB  and NC .. are number of moles of components)

(ii) Molality(m) :-

Molality of any solution is represent the number of moles of solute present in per kg of solvent.

Molality(m) =  number of moles of solute / mass of solvent in Kg

Let take mass of solvent is = 1 kg then

Molality is numerically equal to number of moles of solute

So we can write the definition of molality that  “ molality  is the number of moles of solute present in 1 kg of solvent  “

Example 1.   Find the molality of solution containing 3 moles of  NaCl in 2 liter water ?

Density of water is 1kg/ liter so that    2 liter water = 2 kg of water

 Use above formula

    Molarity = 3/ 2 = 1.5 m

Calculate the molality of a solution containing 164 g of HCl in 753g of H20(water)

Number of moles of HCl = given mass of HCl / molar mass of HCl

Molar mass of HCl =  35.5 + 1  = 36.5 g /mol

Number of moles of HCl   = 164/36.5

                                           =4.5 moles

Mass of solvent in Kg  = 753 g* 1Kg/100 g  =  0.753 Kg

Molality = 4.5/0.753

            = 5.97m

Molarity(M):-

Molarity of any solution  is number of moles of solute per liter of solution

Molarity = number of moles of solute / volume of solution in liter

When volume of solution is 1 liter

Then molarity is numerically equal to number of moles of solute

So we can write a another definition of molarity that  “molarity is equal to number of moles of solute present in 1 liter of solution”

Mass percentage :-

(i) Mass percentage (w/w): The mass percentage of a component A of a solution is defined as mass of component A present in 100 unit mass of the solution. 

Mass % of a component A = mass of component A *100/Total mass of the solution

For example  let consider  a solution which has 15% salt in water by mass,  it has 15 g of glucose is dissolved in 85 g of water  in a 100 g solution.

(ii) Volume percentage (v/v): The volume percentage of a component A is defined as volume of component A present in 100 unit volume of the solution.

 Volume % of a component A = volume of component A *100/Total volume of the solution

For example, 15% methanol solution in water means that 15 mL of

Methanol is present in 100  ml of solution , and water will 100 – 15 = 85 ml

(iii) Mass by volume percentage (w/v): ): The mass by volume  percentage of a component A is defined as mass of  component A in gram present  100 ml volume of the solution.

Mass  by volume % of a component A = mass of component A in gram  *100/Total total volume  of the solution in ml .