Category: Chemical Engineering Math
"Published in Newark, California, USA"
Calculate the voltage of the cell Fe; Fe+2 ║H+; H2 if the iron half cell is at standard conditions but the H+ ion concentration is 0.001 M.
Solution:
The half-cell reaction for Fe is
and it is an oxidation reaction because iron lost two electrons. From Table of Common Standard Reduction Potentials, E0 = 0.447 V. If you reverse the half-cell reaction, the sign of reduction potential will change. Since Fe+2 is at standard condition, then the concentration is 1 M. The concentration of any whole or pure metal in the half-cell reaction is 1 M.
If the concentration of any metal ion is given, then we can calculate the voltage of half-cell reaction by using Nernst Equation as follows
where n is the number of electrons lost or gained in half-cell reaction. Hence, the voltage for Fe is
The half-cell reaction for H is
and it is a reduction reaction because hydrogen gained two electrons. From Table of Common Standard Reduction Potentials, E0 = 0 V. If you reverse the half-cell reaction, the reduction potential is still the same because it is 0 V. If the concentration of H+ is 0.001M, then the voltage for H by Nernst Equation is
Therefore, the voltage of the cell is
Category: Chemical Engineering Math
"Published in Newark, California, USA"
The freezing point constant of toluene is 3.33°C per mole per 1000 g. Calculate the freezing point of a solution prepared by dissolving 0.4 mole of solute in 500 g of toluene. The freezing point of toluene is -95.0°C.
Solution:
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.
From the given problem, the molality of a solute dissolved in toluene 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 a solution is
Note: The good application of freezing point depression is that the ice will keep longer in the container at the lower temperature. Few amount of table salt is added to the crushed ice so that the temperature will be lower than the freezing point of ice which is 0°C. If the temperature is lower than the freezing point, then the ice will not melt in the closed container. The ice cream vendors, food track vendors, and street food vendors are using ice with table salt so that their foods and drinks will keep colder in their container. If you will plan for a picnic and camping, then you can use also ice with table salt especially at the warmer or hotter areas. If the container is left open, then the ice with table salt will melt quite or slowly. Please take note that the ice with table salt will melt for a few days even though the container is closed. Table salt inhibits the melting of ice.
Category: Chemical Engineering Math
"Published in Newark, California, USA"
Calculate the molarity of a solution containing 10.0 grams of sulfuric acid in 500 mL of solution. (MW of H2SO4 is 98.1)
Solution:
The molarity of a solution is defined as the number of moles of a solute per liter of a solution.
In this problem, the weight of solute which is sulfuric acid and the volume of the solution are given.
The number of moles of sulfuric acid is
Therefore, the molarity of sulfuric acid solution is
Category: Chemical Engineering Math
"Published in Vacaville, California, USA"
Acetone is the chemical solvent present in the fingernail polish remover. Its percent composition is 62.0% carbon, 10.4% hydrogen, and 27.5% oxygen. What is the empirical formula for the compound?
Solution:
Basis: 100 grams of sample
Weight of each components:
Weight of carbon is
Weight of hydrogen is
Weight of oxygen is
Moles of each components:
Moles of carbon is
Moles of hydrogen is
Moles of oxygen is
From
the number of moles of each component, we need to divide all of them by
their least number of moles which is oxygen in order to get the number
of atoms in a sample.
Number of carbon in a sample is
Number of hydrogen in a sample is
Number of oxygen in a sample is
Therefore, the empirical formula for acetone is
