Category: Chemical Engineering Math
"Published in Newark, California, USA"
What is the freezing point of a 30% urea, (NH2)2CO solution in water?
Solution:
Basis: 100 grams of 30% (NH2)2CO
The
freezing point constant is defined as the number of degrees the
freezing point will be lowered per mole of solute per 1000 g or 1 kg of
solvent present. This can be written as
where m is the molality of a solution.
The number of moles of (NH2)2CO is
The weight of water is
Hence, the molality of 30% (NH2)2CO solution in water is
From the Table of Freezing and Boiling Information of Solvents, the freezing point constant of water is 
.
The
freezing point depression is defined as the product of the freezing
point constant of a solvent and the molality of a solution.
Hence, the freezing point depression of a solution is
Therefore, the freezing point of 30% (NH2)2CO solution is
Category: Chemical Engineering Math
"Published in Newark, California, USA"
The vapor pressure of water at 25°C is 23.756 torr. A solution consisting of 18.913 grams of a non-volatile substance in 36 grams of water has a vapor pressure of 20.234 torr. What is the molecular weight of the solute?
Solution:
From the description of the given problem above, it is an application of Raoult's Law because it involves the vapor pressures of a solute and a solvent. From this method, we can calculate the amount of a non-volatile solute as well as its molecular weight.
The formula or working equation for Raoult's Law is
where
Psolution = is the vapor pressure of a solution
Xsolvent = is the mole fraction of a solvent
P°solvent = is the vapor pressure of pure solvent
Hence, the mole fraction of a solvent which is water is
Therefore, the molecular weight of a solute is
Category: Chemical Engineering Math
"Published in Newark, California, USA"
Consider the following reaction at 1600°C:
When 1.05 moles Br2 are placed in a 2L flask, 2.50% of Br2 undergoes dissociation. Calculate Kp for the reaction.
Solution:
Consider the chemical reaction above
The first thing that we need to do is to get the molarity of Br2 as follows
At equilibrium, the amount of remaining Br2 is
At equilibrium, the amount of Br formed is
Hence, the equilibrium constant of the given reaction is
Since all products and reactants for the given reactions are all gases, then we have to use the formula as follows
where ∆n is the difference between the sum of the coefficients of the products and the sum of the coefficients of the reactants.
The universal gas law constant for gmole, atmosphere, liters, and K is
.
The absolute temperature of the reaction is
Therefore, the value of Kp for the given reaction is
