How many grams of solute are in a certain volume of a solution of known molarity?

1. Identify the unknown, including units. You want to find grams of FeCl₃·6 H₂O.
2. Choose a starting point. You have two pieces of information: 20.0 mL of solution and 1.5 M FeCl₃. Your unknown is not a fraction, so your starting point shouldn't be a fraction either. 1.5 M FeCl₃ means 1 mol FeCl₃/1 L solution- it should be used as a conversion factor but not as a starting point. Your strategy is

   20.0 mL solution

   g FeCl3·6 H2O

3. List the connecting conversion factors. Build a set of conversion factor 'stepping stones' to connect the starting point and the unknown. For example, you know (or can easily get) the molecular weight of the iron(III) chloride hexahydrate, so

   20.0 mL solution

   mol g FeCl3·6 H2O

   molecular weight g FeCl3·6 H2O

   g FeCl3·6 H2O

   You know you need to use the molarity of FeCl₃ to connect a volume of solution with moles of solute, so

   20.0 mL solution		L solution	molarity of FeCl3	mol FeCl3
   				mol FeCl3·6 H2O
   			molecular weight FeCl3·6 H2O
   				g FeCl3·6 H2O

4. Multiply the starting measurement by conversion factors. I'll let you try this.
5. Check the result: does the answer make sense? Don't forget this step!

Author: Fred Senese senese@antoine.frostburg.edu