How to find the concentration of original solution

Question

If you used 16 mL of 0.05 M HCl to titrate 20 mL of Ca(OH)₂, what is the concentration of hydroxide ions in the original solution?

Answer 1

First, write the balanced equation so that you can see the mole relationship between the Ca(OH)2 and the HCl.

Ca(OH)2 + 2HCl ==> 2H2O + CaCl2

Next, find the moles of HCl that were used:

16 ml HCl x 1 L / 1000 mls x 0.05 mol/L = 0.0008 mols HCl

Next, using the mole ratio in the balanced eq., find the moles of OH^- present:

0.0008 mols HCl x 2 mols OH^- / 2 mols HCl = 0.0008 mols OH^-

Finally, convert moles of OH^- into concentration using the volume of 20 mls:

0.0008 mols OH^- / 20 mls x 1000 mls / L = 0.04 mols/L = 0.04 M