Adding water prior to a titration?

Question

When doing a titration, and when we add water prior to the titration, how does it not affect the calculations for molarity?

Answer 1

It does effect calculations for molarity.  However, titrations determine mass quantities, not concentrations.  Concentrations are determined using the initial volume of analyte added.

For example, suppose you have an acid solution of unknown concentration.  You add 10 mL of this acid to a flask containing 100 mL of water.

Also suppose the true concentration of the unknown is 0.1 moles per liter.  When you dilute the 10 mL with 100 mL of water, you now have 110 mL of solution, but the same mass of acid.  So, the concentration in the flask is now 0.009090909... moles per liter.

Now, in the original 10 mL, you had

     0.1 mole/liter X 0.010 Liter = 0.001 moles of acid

After adding to the 100 mL of water in the flask, you still have

    0.00909... mol/liter X 0.110 Liter = 0.001 moles of acid

When you titrate this acid with a base, 0.001 moles of hydroxide will be required to completely react with the 0.001 moles of acid in the flask.

The calculations for molarity do not use the total volume in teh flask.  It uses the volume of unknown you added, the volume of titrant, and the concentration of the titrant.

The calculation determines X moles of titrant for y mL of unknown; and knowing the reaction between the titrant and the unknown, as well as teh concentration of the titrant, the moles of unknown in "y mL" is calculated.  Concentration of the unknown is then calculated as:

Moles per Liter of unknown = (moles of unknown) / (y mL) * (1 liter / 1000 mL)