A 15.00 mL sample of HCl solution of unknown concentration was titrated with .185 M Ca(OH)2. It took 23.9 mL of the base to reach the equivalence point of the titration. What is the concentration of the acid?
`Ca(OH)_2+2HCl rarr CaCl_2+2H_2O`
`Ca(OH)_2:HCl = 1:2`
Amount of `Ca(OH)_2` consumed `= 0.185/1000xx23.9 = 0.00442`
Amount of HCl moles reacted `= 2xx0.00442`
Let us say the concentration of the HCl solution is M.
`M/1000xx15 = 2xx0.00442`
`M = 0.5893`
So the concentration of the unknown HCl solution is 0.5893M.
At the equivalence point, the moles of H+ must equal the moles of OH-.
"A" is for acid and "B" is for base:
Solve for the molarity of H+.
`"M_A" = 0.2947`
Notice that the mole ratio is different between HCl and Ca(OH)2. You must double the moles of H+ to account for this unbalanced ratio.
`2(0.2947) = 0.590"M"`
The initial concentration of HCl is 0.590 M. Note that there needs to be three sig figs.