Chapter 1 explores the relationship between concentration (as mass percent) and
the amounts of solution, solvent and solute. This is illustrated by the interactive
graphic shown.This lesson deals with the various concentration units used by chemists: mass fraction and percent, mole fraction and percent, molality and molarity. It is primarily concerned with converting between units. As a consequence, it is a valuable resource for dealing with the relationship between mass and moles, mediated by the molar mass (molecular weight or atomic weight), and the relationship between mass and volume, mediated by the density.
What is perhaps even more valuable is that this lesson is essentially an exercise in ratio and proportion, a concept that probably causes more trouble than any other mathematical procedure that students encounter in introductory chemistry.
This lesson is very interactive. There is a minimum of exposition. The student is required to work out problems, with progressive help from the computer.
Chapter 1 explores the relationship between concentration (as mass percent) and
the amounts of solution, solvent and solute. This is illustrated by the interactive
graphic shown.
Chapter 2 starts with the definitions of mole fraction and mole percent. There are guided conversions between mass and mole fractions and percents, and problems where the student must determine the masses required to produce solutions of specified mole fraction or percent. The chapter concludes with a similar introduction of molality, followed by suitable exercises, including making up solutions of salts in water to a specified molality.
Chapter 3 is concerned with volumetry, and introduces molarity, with a similar set of problems. There is a description of the principal volumetric devices: volumetric flasks, burettes, pipets and syringes. The chapter concludes with a set of volume-mole-mole-volume and volume-mole-mole-mass calculations based on typical reactions in solution.
Chapter 4 is concerned with interconversion of mole-based units. It starts with exercises on converting mole fraction to molality and vice-versa. Then there are conversions involving molarity, which necessarily require knowledge of the density of the solution.
In Chapter 5, the computer compares and contrasts the molarity and molality of various solutes in water, from 0.1% to 30% by massto show at what concentrations it is legitimate to assume that molality and molarity are essentially equal.
Chapter 6 is a review quiz, with ten problems selected from the entire lesson. The first two wrong guesses result in hints and a reduced score. A third brings forth the complete solution to the problem, and a score of zero for that question. A percent score is given for the quiz. It is recorded in a dataset, and the student may see the top scores (up to 15) and a histogram of all the scores to date.
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Updated July 6, 2001