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How to Calculate the Initial Concentration of Acetone

The part of chemistry that deals with the relative concentrations of chemicals in reactions is called stoichiometry. Working stoichiometry problems is good practice to help you learn basic chemistry. Acetone is one possible example. If you have a reaction involving acetone where you know the amount of product present at equilibrium, the amount of acetone remaining at equilibrium and the equation for the reaction, you can work backward to figure out how much acetone must have originally been present.

Instructions

- 1
  Write down the chemical equation for the reaction that takes place. The acid-catalyzed bromination of acetone, for example, is as follows:
  
  C3H6O + Br2 ---> C3H5BrO + HBr
- 2
  Determine the ratio of molecules of acetone to molecules of organic product. In the acid-catalyzed bromination of acetone, for example, it takes one molecule of acetone to make one molecule of bromoacetone, so the ratio is 1:1.
- 3
  Multiply the number of moles of product present at equilibrium by the ratio. If there are 100 millimoles of bromoacetone at equilibrium, for example, you would multiply by 1 to obtain 100 millimoles of acetone required to make them.
- 4
  Add the number of moles of acetone remaining at equilibrium to your result from the first step. If there are 50 millimoles of acetone remaining at equilibrium, for example, you would add this to 100 to obtain 150 millimoles originally present.
- 5
  Divide the final number of moles by the total volume of the starting solution (as measured in liters) to obtain the starting concentration of acetone. If you started out with a 500 ml solution, for example, you would convert 500 ml to liters by dividing by 1000 to obtain 0.5, then divide the number of moles (0.15) of acetone by this amount to obtain an initial concentration of 0.3 moles per liter.

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