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

How many grams of FeCl3· 6 H2O are contained in 20.0 mL of 1.5 M FeCl3?
scott willis

You can treat this as a simple unit conversion problem. Solve it using the five step plan for converting units:
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 weightg 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 weightFeCl3·6 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

General Chemistry Online! How many grams of solute are in a certain volume of a solution of known molarity?