Gases: Common Mistakes to Avoid for AP Chemistry (page 2)

By — McGraw-Hill Professional
Updated on Mar 31, 2011

Common Mistakes to Avoid

  1. When using any of the gas laws, be sure you are dealing with gases, not liquids or solids. We've lost track of how many times we've seen people apply gas laws in situations in which no gases were involved.
  2. In any of the gas laws, be sure to express the temperature in kelvin. Failure to do so is a quite common mistake.
  3. Be sure, especially in stoichiometry problems involving gases, that you are calculating the volume, pressure, etc. of the correct gas. You can avoid this mistake by clearly labeling your quantities (moles of O2 instead of just moles).
  4. Make sure your answer is reasonable. Analyze the problem; don't just write a number down from your calculator. Be sure to check your number of significant figures.
  5. If you have a gas at a certain set of volume/temperature/pressure conditions and the conditions change, you will probably use the combined gas equation. If moles of gas are involved, the ideal gas equation will probably be useful.
  6. Make sure your units cancel.
  7. In using the combined gas equation, make sure you group all initial-condition quantities on one side of the equals sign and all final-condition quantities on the other side.
  8. Be sure to use the correct molecular mass for those gases that exist as diatomic molecules—H2, N2, O2, F2, Cl2, and Br2 and I2 vapors.
  9. If the value 22.4 L/mol is to be used, make absolutely sure that it is applied to a gas at STP.
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