Molecules, Moles, and Composition Analysis
Chapter 2: Section 10 Homework
Textbook assignment: Read Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 2: Section 10.
- 2.10 Determining Compound Formulas. A common method of chemical analysis allows us to use the principle of constant composition to go from the mass of the compound to its formula by looking at an intermediate amount: the percentage of the mass of a sample that is a given element. The proportions of the masses tell us something indirectly about the proportions of the atoms in the molecule. The percentage composition can't tell us directly, because atoms of different elements have different masses. We have to convert percentage composition to molar masses, then we can use the molar masses to determine the empirical formula of the compound. This still doesn't tell us the molecular formula, but it does give us the proportions of the elements in the compound.
||Formulae or Notation
|Mass to Moles Conversion
|grams/mole is the molar mass, equal to the atomic weight in grams listed on the periodic table
||molar mass = grams/mole of element|
|Mass percent of element in compound
||The percent masses for all elements in the compound should add up to 100%.
Read the following weblecture before chat: Atoms, Molecules, and the Mole
Videos for Chapter 2
Use the simulation below to build molecules.
- Click on the "Single" option.
- Build the water molecule by moving the atoms up into the central design area. Click on the 3D option to see your molecule in 3D. Drag it in the 3D display to examine its shape. Click on the "bond" option to show the bonds between atoms. How many bonds are there between each H and the O atom? Close the 3D display and drag the water molecule to its position in the "collection".
- Click on the right arrow in the tray area to get a new set of atoms. Build the oxygen and hydrogen gas molecules. Examine the bonds and shape of your molecules in 3D.
- Click on the right arrow in the tray area to get a new set of atoms. Build the carbon dioxide and nitrogen gas molecules. Examine the bonds and shape of your molecules in 3D.
- Build at least one more collection.
- Click on the "Multiple" option.
- Build the required molecules for your collection. Refill the supply pans if necessary with the yellow arrow, lower left.
- With the "Playground" option, see how many different molecules you can build with the atoms supplied. When you have a legitimate molecule, the display will name it and allow you to view it in 3D. What are the names of the atoms you've created? Which ones have double bonds? Do any have triple bonds?
Be sure that you work through all the examples and understand them, and read the problem-solving tips! The more foundation work you lay now, the less trouble you will have later on.
If you've never seen Coulomb's Law before (p. 78), don't be confused by the format here; it is really a simple relationship that says the force between two charges is equal to the product of the charges (q1*q2) divided by the square of the distance between them, and multiplied by a constant that depends on the units we use to measure the charge and distance: Fe= kQq/r2. It's similar to the inverse square law that governs gravity and for the same reason: we have a point source for the force.
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chapter.
Develop your skills with distillation and recrystallization techniques. Working with your teacher and teammates, design a lab to extract a target component from a solution or mixture.
References: Illustrated Guide to Home Chemistry Experiments [Required text], or Home School Chemistry Kit Manual which comes with the Home Scientist Chemistry Kit CK101 set and is available online at The Home Scientist.
- IGHCE 6.1/HSCKM I-4: Differential Solubility: Separate Sugar and Sand
- IGHCE 6.2 Distillation: Purify Ethanol
- IGHCE 6.3/HSCKM I-1: Recrystallization: Purify Copper Sulfate
- IGHCE 6.4/HSCKM I-3: Solvent Extraction: Isolate iodine present in Lugol’s solution
You may find some of the basic lab skills described in Lab Techniques 1: weighing samples, filtering solutions, and cleaning equipment and Lab Techniques 2: Reading thermometers, handling hot equipment, using the Q test useful as well.
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