Modifying an Existing Solution

Make Stronger Make Weaker

It is advantageous to be able to modify a solution to a stronger or weaker concentration.  The  reason for this may be to recycle solutions to updated formulas or to fix incorrectly measured solutions.  For a description of solution preparation see Making a Percent Solution.

To make a solution STRONGER:

Example: to modify a 10% solution to be a 12% solution.

Measure the volume of the existing solution.

Example: 50.0 ml

• Calculate how much material is present in the solution using the following equation.

(grams in existing solution) = (% of existing solution) * (ml of existing solution)
Example: (5.00 g) = (0.10 g/ml) * (50.0 ml)
Note:  The % is expressed as a decimal, such as 10% = 0.10

Calculate how much material to add using the following equation.

(grams to add) =
[(% of desired solution) * (ml of existing solution) * (1.10)] - (grams in existing solution)
Example: (1.60 g) = [(0.12 g/ml) * (50.0 ml) * (1.10)] - (5.00 g)
Note:  The 1.10 factor is used to create a larger volume thus compensating for any error caused by any increase in volume due to the material added.  This factor should be fine in most instances.  If the amount of material added is very substantial then this factor may be increased throughout this procedure.  This factor may also be increased to produce a larger volume of the desired solution.
 

Weigh out material.  (see section on Weighing Out Dry Chemicals)

 

Add material into the solution.

 

Mix and wait for all material to be completely dissolved (warming if necessary.)

 

Add H₂O to bring the volume to 1.10 times the original volume and mix.

Example:  Makes 55.0 ml final 12% solution for the factor of 1.10

The concentration of the solution is now that desired with a volume of 10% more.

To make a solution WEAKER:

Example: to modify a 10% solution to be an 8% solution.

Measure the volume of the existing solution.

Example:  50.0 ml
 

Calculate how much material is present in the solution using the following equation.

(grams in existing solution) = (% of existing solution) * (ml of existing solution)
Example: (5.00 g) = (0.10 g/ml) * (50.0 ml)
Note:  The % is expressed as a decimal, such as 10% = 0.10
 

Calculate how much H₂O to add using the following equation.

(ml H₂O to add) =
[(grams in existing solution) / (% of desired solution)] - (ml of existing solution)
Example:  (12.5 ml) = [(5.00 g) / (0.08 g/ml)] - (50.0 ml)
Note:  Since no solid material is being added,  there is no offset in volume.
 

Measure out the volume of H₂O to add.

 

Add H₂O to the solution.

The concentration of the solution is now that desired.
 

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