Chemistry / Unit 3 Chem 0413 worked example, 60 to 90 seconds
Performing a Dilution Calculation
The idea
Performing a dilution calculation means setting the stock solution's molarity times volume equal to the diluted solution's molarity times volume, then solving for whichever quantity is unknown.
In practice
A chemist needs 500 mL of 0.400 M glucose solution, prepared by diluting a 3.00 M stock solution with distilled water. Setting M1V1 equal to M2V2 gives (3.00 M)(V1) = (0.400 M)(0.500 L). Solving for V1 gives about 0.0667 L, or 66.7 mL, of stock solution, which is then measured into a flask and brought up to a total volume of 500 mL by adding water, not by adding 500 mL of water to it.
Where people slip
Adding 500 mL of water to the 66.7 mL of stock, instead of adding water only until the total solution reaches 500 mL, produces a final volume and concentration that are both wrong.
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