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 strong bases completely dissociate, so the reaction yields a solution with a neutral pH (pH = 7). Because of the complete dissociation of 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.
Key Takeaways: Acid-Base Neutralization
- Solving a chemistry problem where a strong acid neutralizes a strong base is straightforward because both the acid and the base completely dissociate.
- In contrast, neutralization involving a weak acid and/or a weak base requires that you know and use the dissociation constant.
- Neutralization occurs at the point where the number of moles of H+ equals the number of moles of OH-.
Review of Neutralization Reaction
Neutralization relies on dissociation of an acid and a base. Dissociation is where the acid or base breaks into its component ions. The ions participating in a neutralization reaction are the H+ from the acid and the OH- from the base. The general form of the reaction is:
acid + base → salt + water
AH + B → A + BH
As an example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), it produces table salt or sodium chloride (NaCl) and water:
HCl + NaOH → NaCl + H2O
Neutralization requires equal amounts of H+ and OH-. So, knowing the volume and concentration of either the acid or base lets you find the volume and concentration of its partner in the reaction.
Solving an Acid-Base Neutralization Problem
What volume of 0.075 M HCl is required to neutralize 100 milliliters of 0.01 M Ca(OH)2 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 milliliters
- moles OH- = 0.02 M/0.1 liters
- moles OH- = 0.002 moles
- Step 2: Calculate the Volume of HCl needed
- Molarity = moles/volume
- Volume = moles/Molarity
- Volume = moles H+/0.075 Molarity
- moles H+ = moles OH-
- Volume = 0.002 moles/0.075 Molarity
- Volume = 0.0267 Liters
- Volume = 26.7 milliliters of HCl
Performing the Calculation
26.7 milliliters of 0.075 M HCl is needed to neutralize 100 milliliters of 0.01 Molarity Ca(OH)2 solution.
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!
Sources
- Skoog, D.A; West, D.M.; Holler, J.F.; Crouch, S.R. (2004). Fundamentals of Analytical Chemistry (8th ed.). Thomson Brooks/Cole. ISBN 0-03-035523-0.
- Snoeyink, V.L.; Jenkins, D. (1980). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. New York: Wiley. ISBN 0-471-51185-4.
- Trummal, Aleksander; Lipping, Lauri; Kaljurand, Ivari; Koppel, Ilmar A.; Leito, Ivo (2016). "Acidity of Strong Acids in Water and Dimethyl Sulfoxide". The Journal of Physical Chemistry A. 120 (20): 3663–3669. doi:10.1021/acs.jpca.6b02253
- Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). New York: Houghton Mifflin Company.
