You have a solution of 0.15M Ba(OH)2 and a solution of H2SO4 whose concentration you want to determine. You start with 25 ml H2SO4 in the Erlenmeyer and titrate in the Ba(OH)2. At the end of the titration, you find that you have used 20.4 ml Ba(OH)2 and an additional 2.2 ml H2SO4 because you hadto back-titrate.

How many moles of OH- are in 20.4 ml of a 0.15 Ba(OH)2 solution?

mols Ba(OH)2 = M x L = ?

mols OH^- = twice that of Ba(OH)2

To determine the number of moles of OH- in the 20.4 ml Ba(OH)2 solution, we first need to calculate the number of moles of Ba(OH)2 present in that volume.

Step 1: Calculate the number of moles of Ba(OH)2
First, convert the volume of the Ba(OH)2 solution from milliliters (ml) to liters (L).
20.4 ml ÷ 1000 = 0.0204 L

Next, use the molarity (M) of the Ba(OH)2 solution to calculate the number of moles.
Molarity (M) = moles/Liters
0.15 M = moles/0.0204 L

Rearranging the equation to solve for moles:
moles = Molarity × Volume
moles = 0.15 × 0.0204
moles = 0.00306 moles

So, there are 0.00306 moles of Ba(OH)2 in 20.4 ml of the 0.15 M Ba(OH)2 solution.

Step 2: Determine the number of moles of OH-
Since each mole of Ba(OH)2 contains 2 moles of OH-, we can multiply the number of moles of Ba(OH)2 by 2 to get the moles of OH-.
moles of OH- = 0.00306 moles × 2
moles of OH- = 0.00612 moles

Therefore, there are 0.00612 moles of OH- in 20.4 ml of the 0.15 M Ba(OH)2 solution.