We first want to convert 20 mL of HCl into moles of HCl. Do that by converting 20 mL to L and then multiplying by the molarity: 0.02L*0.10M= 0.002 mol HCl. Now we can take that value and divide it by the 10 mL of Ca(OH)2 because molarity is mol/L: 0.002 mol HCl/0.010L= 0.2M, this is the concentration of [OH]. From the equation Ca(OH)2 <--> Ca^2+ + 2OH^- we can see that OH and Ca have a 2:1 ratio meaning that for every mole of Ca made, 2 moles of OH are made. This tells us that our [Ca] concentration is half of the [OH] concentration or, 0.1 M.
I commented 6 months ago