MOLALITY OF A SOLUTION

 QUESTION

A sugar syrup of weight 214.2g contains 34.2g of sugar. Calculate the molal concentration (Molality) 

ANSWER

Weight of sugar syrup = 214.2 g

Weight of sugar =34.2g

Molecular mass of sugar = 342  ( 12 *12 + 22*1 +11*16)

a) Number of moles = 34.2/342 =0.1

Weight of water = 214.2-34.2 = 180g

Molal concentration or Molality= (No of moles×1000)/ (Mass of Solvent)

                                                   = ( 0.1 × 1000)  /  180

                                                   = 0.555 = 0.56 mol/ Kg

THE MOLE FRACTION OF THE SOLUTION

 THE MOLE FRACTION OF THE SOLUTION (PROBLEM)

QUESTION

  A Solution contains 25% water, 25% ethanol and 50% acetic acid by mass. Calculate the mole fraction of each compound?

ANSWER

If we have 100 g of solution,

Mass of Water = 25 g                       Molecular mass of water=18g

Mass of Ethanol= 25g                      Molecular mass of Ethanol=46g

Mass of Acetic acid=50g                  Molecular mass of Acetic acid= 60g

No of moles of water 25/18  = 1.39

No of moles of Ethanol= 25/46 = 0.54

No of moles of  Acetic acid = 0.83

Total no of moles = 1.39+0.54+0.83 = 2.76

Mole fraction of water =1.39/2.76 =0.503

Mole fraction of ethanol = 0.54/2.76=0.196

Mole fraction of acetic acid =0.83/2.76= 0.301