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Titration of a Strong Acid or a Strong Base

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  • 0:02 Titration
  • 2:40 Titration Curves
  • 4:11 Concentration of Unknown
  • 6:49 pH During Titration
  • 10:56 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Discover what titration is and how to calculate the concentration of an acid or base that has been titrated to equivalence. Learn the meaning of titrant, standard solution and equivalence point. Study titration curves and learn how to determine pH during any point of a titration between a strong acid and strong base.

Titration

Scientist 1: Hey, do you know the concentration of this strong acid solution X?
Scientist 2: No. But I know how we can figure it out!
Scientist 1: How?
Scientist 2: By strategically mixing it with a strong base.
Scientist 1: How could that possibly tell us anything useful about the concentration of X? Performing a reaction could change the properties of the substance!
Scientist 2: Don't worry, my friend! Pay attention, and I will tell you.

Let's think about the reactions between strong acids and strong bases. These reactions are often referred to as neutralization reactions. Any guesses why? Well, when you add strong acids to strong bases, the hydrogen ions from the acid react with the hydroxide ions from the base to create a neutral product: water!

H+ + OH- --> H2O

Acids and bases react in a 1:1 ratio. When you add base to acid (or vice versa), once the pH is neutral (7), you know that you have equal parts acid and base present. If you keep track of how much solution you've added to reach neutral pH, you can determine the concentration of the unknown! This process is called titration. Titration is when a solution of known properties (called the titrant) is used to analyze properties of an unknown solution. Titrations are often used in acid-base chemistry to determine the concentration of an unknown solution.

In a titration, like the one proposed by our cartoon scientists, we take a sample of our strong acid of unknown concentration. We then add some strong base, just a little at a time, until the concentration of the hydrogen ions equals the concentration of hydroxide ions. This point, known as the equivalence point, is when the amount of hydrogen ions and hydroxide ions are equal and the pH = 7. We find this point either by monitoring the pH with a probe or adding indicator solution, a special chemical that changes colors within a specific pH range.

Ideally, the equivalence point occurs when titration stops. The point at which titration stops is called the endpoint. Since we know how much titrant we added, we can infer the concentration of the unknown solution. We'll look at this more in depth later.

Titration Curves

Data collected during titration can be used to create a titration curve, or a graph of pH versus volume of titrant added. Titration curves start with the pH of the unknown solution and end with the pH of the titrant. All titration curves have similar characteristics; to me, they all look like slides. Like many slides, titration curves begin with a slope that is gentle and gradual initially, dramatic in the middle, then gentle at the end. The equivalence point occurs in the middle of the dramatic slope.

Titration Curve
graph showing titration curve

A titration in which a strong acid is titrated using a strong base will have a titration curve that looks like Graph 1 above. A slide modeled after this curve would certainly be an intense, if not scary, ride. Initially, the pH of the solution is low, changing very gradually until suddenly, there is a sharp increase in pH. As previously noted, the equivalence point is located in the middle of this slope. For strong acid-strong base titrations, the equivalence point is seven. If titration continues beyond the endpoint, pH values continue to rise to that of the pure base.

Can you picture what a titration curve would look like when a strong base is titrated with a strong acid? It should be the opposite of the titration curve we just looked at. Initially the pH is high, changing very gradually until there is a sudden, sharp decrease in pH. As titration continues, pH decrease becomes more gradual.

Determining the Concentration of an Unknown Substance from Titration

The way you determine the concentration of the unknown substance from the titration data is to use the equation: (cH^+)(VH^+) = (cOH^-)(VOH^-).

The concentration of hydrogen ions times the volume of hydrogen ions equals the concentration of hydroxide ions times the volume of hydroxide ions. Make sure concentration is in terms of molarity, or mol/L, and volume is in terms of liters.

  1. Gather the information. You need to know the concentration and volume of acid/base that you added and the amount of the unknown substance you started with.
  2. Set up your equation and fill in all the known values.
  3. Do the calculations.

Here's an example: a 40 mL HCl solution is titrated with 24.64 mL of a 0.55 M KOH solution. What is the concentration of the HCl solution?

Step One: Gather the Information

The volume of the hydrogen ions is 40 mL. We convert that to liters by dividing by 1000.

  • VH^+= 40 mL = 40 mL * 1 L / 1000 mL = 0.04 L
    • cOH^- = 0.55 mol/L
  • VOH^- = 24.64 mL = 24.64 mL * 1 L/1000 mL = 0.02464 L
    • cH^+ = ?

Step Two: Set up the Equation and Fill in the Known Values

Here's our equation again: (cH^+)(VH^+) = (cOH^-)(VOH^-). We can rearrange to solve for concentration of hydrogen ions by dividing both sides by the volume of hydrogen ions: cH^+= (cOH^-)(VOH^-) / VH^+.

Now we plug in our values: cH^+ =(0.55 mol/L)(0.02464 L) / 0.040 L.

Step Three: Do the Calculations

Liters will cancel out:

  • cH^+ =(0.55 mol/L)(0.02464 L) / 0.040 L.
    • cH^+ = 0.3388 mol/L

The concentration of the HCl solution is 0.3388 mol/L.

Determining pH During Titration

Titrations between strong acids and bases will always result in a pH of 7 at the equivalence point. How cool is that? This phenomenon occurs because strong acids and strong bases dissociate 100% in solution. Also, the conjugate acids and bases produced by strong acids and strong bases do not affect pH.

If you need to determine the pH during titration at points other than the initial or endpoint/equivalence point, then you will need to do some math! Determining the pH at any given point of a titration for a strong acid and strong base is easy. To determine pH, we'll need the concentration and volume of the substance being titrated and we'll need the concentration and volume of the titrant added at the interval of interest. We'll do a little stoichiometry to figure out the pH. Our process is best summarized though in three easy steps:

  1. Determine the moles of each reactant present.
  2. Determine which reactant is in excess.
  3. Calculate pH based on the concentration of the excess reactant.

Imagine we are titrating a 10 mL sample of 2 M HCl with 0.5 M KOH and want to know the pH after we have added 20 mL of KOH. We know that HCl and KOH react in a 1:1 ratio to produce H2O and KCl.

HCl + KOH --> H2O + KCl

Neither of these products influences pH, so we don't have to consider their presence in our stoichiometric calculations.

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