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# Acids and Bases - Neutralizing a Base with an Acid

## Worked Chemistry Problems

By Todd Helmenstine

You can't always see when a base is neutralized, so you need to know how to calculate it.

Steve McAlister, Getty Images
Question

What volume of 0.075 M HCl is required to neutralize 100 ml of 0.01 M Ca(OH)2 solution?

Solution

HCl is a strong acid and will dissociate completely in water to H+ and Cl-. For every mole of HCl, there will be one mole of H+. Since the concentration of HCl is 0.075 M, the concentration of H+ will be 0.075 M.

Ca(OH)2 is a strong base and will dissociate completely in water to Ca2+ and OH-. For every mole of Ca(OH)2 there will be two moles of OH-. The concentration of Ca(OH)2 is 0.01 M so [OH-] will be 0.02 M.

To the solution will be neutralized when the number of moles of H+ equals the number of moles of OH-.

Step 1: Calculate the number of moles of OH-.

Molarity = moles/volume

moles = Molarity x Volume

moles OH- = 0.02 M/100 ml
moles OH- = 0.02 M/0.1 L
moles OH- = 0.002 moles

Step 2: Calculate the Volume of HCl needed

Molarity = moles/volume

Volume = moles/Molarity

Volume = moles H+/0.075 M

moles H+ = moles OH-

Volume = 0.002 moles/0.075 M
Volume = 0.0267 L
Volume = 26.7 ml of HCl

26.7 ml of 0.075 M HCl is needed to neutralize 100 ml of 0.01 M Ca(OH)2 solution.