Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107	Fundamentals of Chemistry	Fall 2009
Lecture Notes: 17 September	© R. Paselk 2005
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Moles and Molarity, cont.

Masses and particles Examples, cont.

• How many atoms are there in 1.00g of:
  • ethanol? (CH₃CH₂OH)
  • How many Carbon atoms?
• What is the mass of 3.01 x 10²⁰ atoms of carbon in amu? in g?
• What is the mass of iron in 200.0 mmoles of iron(III) sulfate?

 

Stoichiometry

Determination of Empirical Formulae

Want to determine the ratios, in moles, of elements in an analysis.

• Example: A compound of mercury, oxygen, and nitrogen gave an analysis of 61.80% mercury and 8.63% nitrogen by weight. What is the empirical formula for this compound?

When should you round off? One of the problems in finding the simplest formula is determining how much error is legitimate in rounding off. Ultimately this is a decision determined by the error of the experimental data - how many significant figures do we have. For this course we generally have at least three sig figs, but I promise not to get too subtle, so as a rule of thumb for this class only values of x.33x and x.5xx should be assumed to be not in error, and so must be multiplied to get the correct formula. (e.g. XY_2.331 gives X₃Y₇)

Molecular Formulae

Notice that for molecular compounds that the empirical formula is not necessarily the molecular formula! That is, the actual molecular formula could be a multiple of the simplest formula. Thus, to find molecular formulae we need two kinds of information, the empirical formula (from percentage composition) and the molecular weight (from physical characterization):

• Example: A molecule is found to have an empirical formula of C₃H₅ by elemental analysis and a MW of 85 by freezing point depression. What is its molecular formula?

Solution Concentrations

Solutions: a solution occurs when one chemical is completely dissolved or dispersed in another. We most commonly think of solutions as being liquid, but solid solutions also occur, such as the various metal alloys like steel, brass and bronze.

Note that solutions made with clear solvents are generally clear though they can be any color, such as rubies, sapphires, whiskey, etc.

In a solution the substance present in highest concentration is considered to be the solvent, while components in lesser amounts are considered to be solutes. If you dissolve a sugar cube in water you get a sugar solution, where water is the solvent, and sugar is the solute.

Example:

• What is the solvent in 80 proof rum: 80 proof = 40% alcohol in water, so water is the solvent.
• What is the solvent in 151 proof rum: 151 proof = 75.5% alcohol in water, so alcohol is the solvent.

Concentration Terms

Molarity:

The most commonly used concentration term in chemistry is molarity, M = 1 mole of solute dissolved in 1 L of solvent.

• Two types of situation arise giving two kinds of problems:
  • Making molar solutions
    • Make up a 1.000 L solution of 0.25 M NaCl (note that water is the "default" solvent).
    • What is the concentration of a solution made by dissolving 10.00 g of KI in enough water to make 1.000 L

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© R A Paselk

Last modified 20 September 2009