This is a measure of the relative amount of a substance in a mixture. The % composition by mass is simply given by
For an example of a mass composition computation see problem 4 from August 31 topics.
Chem 116
Fall 1998
Lecture Notes
September 21, 1998
Ionic Compounds and Solubility
Ionic Compounds and Solubility
Ionic Compounds - compounds held together through the electrostatic (or coulombic) attraction of positive ions to negative ions. In the solid phase, ionic compounds exist as an extended crystal lattice with ions distributed such that positive ions are adjacent to negative ions and vice versa.
The charge of monatomic (single atom) ions depends on the position of the atom in the periodic table. Representative group metals, present in columns with labels ending with the letter A (IA, IIA, IIIA, etc.), will form cations with a charge equal to the column number (+1 for IA, +2 for IIA, +3 for IIIA, etc.). Nonmetals will form anions with a negative charge equal to 8 minus the column number (-1 for VIIA, -2 for VIA, -3 for VA, etc.). The objective for both representative metals and nonmetals is to form an ion that has the same electron configuration as a noble gas (often referred to as a stable octet corresponding to eight valence electrons). This is a relatively stable electron configuration as evidence by the low reactivity of noble gases.
As for transition metals, many element form ions of different charges depending on the situation. For example, both Fe2+ and Fe3+ are found in nature. However, most transition metal form at least one ion of +2 charge, a notable exception being silver which only forms an ion of +1 charge, Ag+.
A number of ions are found in nature that are polyatomic. That is, they contain more than one atom. The atoms in these ions are covalently bound to each other like a molecule. However, unlike typical molecules these beasts have an ionic charge. You might call them molecular ions. Examples of polyatomic ions include sulfate (SO42-), nitrate (NO3-), and carbonate (CO32-). Click here for a list of common polyatomic ions.
Solubility - Many ionic compounds are soluble in water (or aqueous solutions). The solubility depends on the nature of the ions present. See your text (pp. 48 - 49) for solubility rules.
This is a measure of the relative amount of a substance in a mixture. The % composition by mass is simply given by
For an example of a mass composition computation see problem 4 from August 31 topics.
Chromatography is a technique used to separate components of a mixture. There are a variety of chromatographic techniques, but the theory is similar for each. In each type, there is a stationary phase and a mobile phase. Mixtures to be analyzed are introduced onto a stationary phase. A mobile phase then sweeps over the stationary phase. Components from the mixture may move into the mobile phase and be carried along with it. This is where the separation occurs because the rate with which a component moves depends on it's relative affinities for the stationary phase and the mobile phase. Compounds that have a greater affinity for the mobile phase will move faster (or farther) whereas compounds that have a greater affinity for the stationary phase will move slower (or not as far). A table of basic chromatographic techniques and their differences is shown below.
Basic Chromatographic Techniques
Type Stationary Phase / Polarity* Mobile Phase / Polarity* Use Paper Chromatography Paper Liquid Simple and Quick Analysis Thin-Layer Chromatography (TLC) Silica Gel or Alumina / Polar Liquid / Nonpolar Simple and Quick Analysis Column Chromatography Silica Gel or Alumina / Polar Liquid / Nonpolar Separate and Isolate Large Quantities Gas Chromatography (GC) Liquid Adsorbed on Solid Support Column / Polar Gas / Nonpolar Quantitative and Qualititative Analysis of Volatile Mixtures High Performance Liquid Chromatography Solid Liquid Quantitative and Qualititative Analysis of Non-volatile Mixtures
* Note that phase polarities are typical. Other materials may be used to reverse the polarities. Reverse-phase chromatography refers to the technique in which the stationary phase in nonpolar and the mobile phase is polar.
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