How do I calculate g of solute in a given volume of solution using the percent concentration?

If I have a solution that is 5%(w/v) of an alcohol in water, what mass of alcohol is present in 1.0 L of this solution?
Kathleen 10/08/99

The percentage tells you that there are 5 grams of alcohol in every 100 mL of solution. You have 1.0 L, which is 10 100 mL portions, so you have 50 grams of alcohol.

You can also treat this as a simple unit conversion problem and solve it with the five step plan for converting units:

1. Identify the unknown, including units. You want to find grams of alcohol.
2. Choose a starting point. You have two pieces of information: the volume of the solution, and its concentration. "g alcohol" is not a fraction, so your starting point shouldn't be a fraction either. 5% (w/v) alcohol means 5 g alcohol per 100 mL of solution- it should be used as a conversion factor but not as a starting point. Your strategy is

   1.0 L solution

   g alcohol

3. List the connecting conversion factors. Conversion factor are stepping stones that connect the starting point and the unknown. The concentration connects a volume of solution with a mass of alcohol:

   1.0 L solution

   mL solution

   100 mL solution = 5 g alcohol

   g alcohol

4. Multiply the starting measurement by conversion factors.

   1.0 L solution

   (

   1000 mL solution 1 L solution

   )

   (

   5 g alcohol 100 mL solution

   )

   = 50 g alcohol

5. Check the result: does the answer make sense? Don't forget this step!

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/g-solute-from-percent-concentrations.shtml