Making a solid into a stock solution?

Find the molarity of a diluted stock solution?

• You have 5 mL of standard Erythosin B solution. You pipet 2.00 mL into a 50 mL volumetric flask and fill the flask with DI water. This is your stock solution. From your stock solution you pipet 5 mL into a 100 mL beaker and dilute with DI water. How do you find the Molarity of both the stock solution and the next solution?

• Answer:

  A quick check shows a standard solution has a concentration of 0.1% Another quick checks shows only the sodium salt of Erythosin B is soluble in water and has a molecualr weight of 879.9. Now the trick is to find the molarity of the standard solution before you even dilute it. We have a 5 mL solution. Assuming water has a density of 1 gram/mL thethe weight of solvent is 5 grams. So: 0.1 =X/5 x 100 so X = 0.005 grams Now we can calculate the molarity: .005/879.9/.005 Liters = or 1.14 x 10^-3 moles/liter... Now the first dilution: 1.14 x 10^-3 x 2/50 = 4.55 x 10^-5 moles/liter... The second dilution: 1.14 x 10^-3 x 2/50 x 5/100 = 2.27 x 10^-6 moles/liter