Stoichiometry: Common Mistakes to Avoid for AP Chemistry
Measurements include initial and final masses, and initial and final volumes. Calculations may include the difference between the initial and final values. Using the formula mass and the mass in grams, moles may be calculated. Moles may also be calculated from the volume of a solution and its molarity.
Once the moles have been calculated (they are never measured), the experiment will be based on further calculations using these moles.
Common Mistakes to Avoid
- Avogadro's number is 6.022 × 1023 (not 10–23).
- Be sure to know the difference between molecules and moles.
- In empirical formula problems, be sure to get the lowest ratio of whole numbers.
- In stoichiometry problems, be sure to use the balanced chemical equation.
- The stoichiometric ratio comes from the balanced chemical equation.
- When in doubt, convert to moles.
- In limiting-reactant problems, don't consider just the number of grams or even moles to determine the limiting reactant—use the mol/coefficient ratio.
- The limiting reactant is a reactant, a chemical species to the left of the reactant arrow.
- Use the balanced chemical equation.
- Percent yield is actual yield of a substance divided by the theoretical yield of the same substance multiplied by 100%.
- Molarity is moles of solute per liter of solution, not solvent.
- Be careful when using Avogadro's number—use it when you need, or have the number of atoms, ions, or molecules.
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