Acids and Bases - Neutralizing a Base with an Acid

You can't always see when an acid or base is neutralized, so you need to know how to calculate it.
You can't always see when an acid or base is neutralized, so you need to know how to calculate it. WLADIMIR BULGAR / Getty Images

When an acid and a base react with each other, a neutralization reaction occurs, forming a salt and water. The water forms from the combination of the H+ ions from the acid and the OH- ions from the base. Strong acids and bases completely dissociate, so the reaction yields a solution with a neutral pH (pH = 7). Because of the complete dissociation between strong acids and bases, if you're given a concentration of an acid or base, you can determine the volume or quantity of the other chemical required to neutralize it.

This example problem explains how to determine how much acid is needed to neutralize a known volume and concentration of a base:

Acid-Base Neutralization 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.

So, 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

Answer

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

Tips for Performing the Calculation

The most common mistake people make when performing this calculation is not accounting for the number of moles of ions produced when the acid or base dissociates.

It's easy to understand only one mole of hydrogen ions is produced when hydrochloric acid dissociates, yet also easy to forget it's not a 1:1 ratio with the number of moles of hydroxide released by calcium hydroxide (or other bases with divalent or trivalent cations).

The other common mistake is a simple math error. Make sure you convert milliliters of solution to liters when you calculate the molarity of your solution!

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Helmenstine, Todd. "Acids and Bases - Neutralizing a Base with an Acid." ThoughtCo, Mar. 24, 2017, thoughtco.com/neutralizing-a-base-with-acid-609579. Helmenstine, Todd. (2017, March 24). Acids and Bases - Neutralizing a Base with an Acid. Retrieved from https://www.thoughtco.com/neutralizing-a-base-with-acid-609579 Helmenstine, Todd. "Acids and Bases - Neutralizing a Base with an Acid." ThoughtCo. https://www.thoughtco.com/neutralizing-a-base-with-acid-609579 (accessed October 19, 2017).