Molality Measurement of Concentration of Solutions (Session 20) Ciencias de la Tierra II

Number of moles in a compound = grams Molality is represented with the letter m (non Cap) and the solutions are known as Molal. The Molality is defined as the number of moles of solute dissolved in a kilogram of dissolvent. m = solute moles Kg of dissolvent A solution of 3 m of KOH has 3 moles of KOH dissolved in a kilogram of dissolvent. Number of moles in a compound = grams Molecular Weight Ciencias de la Tierra II

Ciencias de la Tierra II Activity 1 Prepare a watery solution of 1 m of calcium bromide. 1 mol of CaBr2 = 200 g of CaBr2 Furthermore 200 g of CaBr2 are dissolved in exactly 1 kilogram of water. Ciencias de la Tierra II

Ciencias de la Tierra II Activity 2   Prepare the following solutions: A solution of sodium hydroxide 0.5 m A solution of sulfuric acid 1.4 m A glucose solution 0.8 m Ciencias de la Tierra II

Elevation of the boiling point Tb -Tb0 = Kb * m Where: bf is the mole constant for the elevation of the boiling point of a pure dissolvent. m is molality. Ciencias de la Tierra II

Decrease in the freezing point Tf0 -Tf = Kf * m Where: Kf is the constant mole of the decrease of the boiling point of a pure dissolvent. m is molality. Ciencias de la Tierra II

Ciencias de la Tierra II Molal constants of elevation of boiling point (Ke) and decrease of the freezing point (Kc) Dissolvent Normal boiling point (°C) Ke (°C/m) Normal freezing point Kc (°C/m) Water 100.00 0.52 0.00 1.86 Benzene 80.10 2.53 5.50 5.12 carbon tetrachloride 76.80 5.02 -22.30 29.80 Ethanol 78.40 1.22 -114.60 1.99 Chloroform 61.20 3.63 -63.50 4.68 Brown, T. et al. (1991) Ciencias de la Tierra II

Ciencias de la Tierra II Problems Calculate the freezing point and the boiling point of a solution of 100 g of ethylene antifreeze, C2H6O2 in 900 g of H2O. Calculate the freezing point of a solution that has 600 g of CHCl3 and 42 g of eucalyptol, C10H18O , a fragrant substance that is found in eucalyptus leafs. Order the following watery solutions in increasing order of their boiling points: Glycerin 0.03m KBr 0.02m Benzoic acid HC7H5O2 0.03m Ciencias de la Tierra II

Ciencias de la Tierra II Bibliography Brown, T. et al. (1991) Chemistry: The Central Science. 5th ed. United States of America: Prentice Hall. Burns, R. (1996) Chemistry Fundaments. 2nd. ed. United States of America : Prentice Hall. Chang, R. (1992). Chemistry. 4th ed. United States of America : McGraw Hill. Ciencias de la Tierra II