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ABS - Titration Lab Results & pH (Lesson)

Titration Lab Results & pH

Introduction

The pH scale helps us measure the concentration of acids and bases. More acidic solutions have a low pH and more basic solutions have a higher pH. But how is pH measured? How is it calculated? Did you know that you could measure pOH? In this lesson we will learn how pH is related to hydrogen ion concentration. We will also learn about other calculations that will help us communicate the various acidity and basicity of solutions.

Titration Lab Results

In this segment, the students discuss the data from their titration lab. Our host explains the importance of titration in real world applications and discusses auto ionization of water and the calculation of pH.

Download the note taking guide for Chemistry Matters Unit 7 Segment I. Links to an external site.

Download the key to the questions to consider for Chemistry Matters Unit 7 Segment I. Links to an external site.

Example One: Calculate the pH of a HF solution with a concentration of 1.23 x 10-3M.  Solution: pH = -log (1.23 x 10-3M) = 2.91

Example Two: Calculate the pH of 1.86 x 1012M H2SO4 solution. Solution: pH = -log (1.86 x 1012M) = 12.3

A Closer Look at pH

How to Calculate pH when Given the Molarity of an Acid

Step 1: Determine if the acid is monoprotic, diprotic, or triprotic. This will help you determine the concentration of the H+. If the acid is monoprotic, or has 1 hydrogen in the front of the formula, the concentration of the H+ is the same as the concentration of the acid.

  • Example: 2.5 M HCl has a concentration of H+ = 2.5 M

If the acid is diprotic, or has 2 hydrogens in the front of the formula, the concentration of the H+ is two times the concentration of the acid.

  • Example: 2.5 M H2SO4 has a concentration of H+ = 2 x (2.5 M) = 5.0 M

If the acid is triprotic, or has 3 hydrogens in the front of the formula, the concentration of the H+ is three times the concentration of the acid.

  • Example: 2.5 M H3PO4 has a concentration of H+ = 3 x (2.5 M) = 7.5 M

Once you have determined the concentration of H+ (also denoted as [H+]), you can move to step 2.

Step 2: Using the formula for pH (pH = -log [H+]), plug in your value for [H+].

Step 3: Using your scientific calculator, calculate the value for pH.

Determine the pH of a 0.010 M HNO3 Solution

Step 1: Because there is only 1 H at the beginning of the formula of this acid, it is monoprotic. Therefore, [H+] = the concentration of the acid.

  • [H+] = 0.010 M or 1.0 x 10-2 M

Step 2: pH = -log [H+] = -log [1.0 x 10-2]

Step 3:

  • pH= -(-2.0)
  • pH = 2.0

Note: Scientific calculators all work differently. You need to learn how your calculator works, and always use the same calculator for all assignments, quizzes, and tests. Try this problem until you determine the order of entry that gives you the correct answer. Notice that you are taking the negative log. When you type log 1.0 EE -2 into your calculator and hit equals, you will see -2.0 displayed. Multiply this answer by -1 to calculate the pH.

Calculate the pH of a solution of 0.0025 M H2SO4

Step 1: Determine if the base contributes 1 or 2 hydroxide ions to the solution. Bases that contribute 1 hydroxide ion are called monobasic. Bases that contribute 2 hydroxide ions are called dibasic. This will help you determine the concentration of the OH-, which will, in turn, help us to calculate the [H+] needed to calculate pH. If the base contributes one OH- ion, the concentration of the OH- is the same as the concentration of the base.

  • Example: 1.3 M NaOH has a concentration of OH- = 1.3 M

If the base is dibasic, the concentration of the OH- is two times the concentration of the base.

  • Example: 1.3 M Ca(OH)2 has a concentration of OH- = 2 x (1.3 M) = 2.6 M

Once you have determined the concentration of OH- (also denoted as [OH-]), you can move to step 2.

At this point, there are two different paths that you can choose to solve your problem. Both are correct, and each works equally well. The first path was demonstrated in the video. The second path takes advantage of a formula you have not yet been introduced to.

Path 1:

Step 2: Use the [OH-] and Kw to solve for [H+].

  • [H+] [OH-] = Kw = 1.0 x 10-14
  • [H+] = 1.0 x 10-14 / [OH-]

Step 3: Using the formula for pH (pH = -log [H+]), plug in your value for [H+].

Step 4: Using your scientific calculator, calculate the value for pH.

Path 2:

It is important to note here that not all acids and bases completely ionize or dissociate in water. Those that do are called strong acids and strong bases. Notice the use of the word strong here. Often times, students think that the term strong refers to the pH of the acid or base. That is not true. Strong refers to how much the acid or base dissociates in water. The above methods for working pH problems will only work for strong acids and strong bases. The acid and base must 100% dissociate so that the concentration of the H+or OH- is equal to (or a multiple of) the concentration of the original acid or base. Acids and bases that do not ionize or dissociate completely are called weak acids and bases. To calculate their pH, you need to know something called the dissociation constant (Ka or Kb) for that acid or base. We will not work problems involving weak acids and bases in this class. So, it is safe to assume that the above methods work for all of the problems that you will encounter in this class.

Step 2: Using the formula for pOH (pOH = -log [OH-]), plug in your value for [OH-].

  • pOH = -log [OH-]

Step 3: Using your scientific calculator, calculate the value for pOH.

Step 4: Using the formula pH + pOH = 14.0, solve for pH. This formula is derived from the relationship [H+] [OH-] = Kw = 1.0 x 10-14. You do not need to be worried with how it was derived, but you may find it useful.

  • pH = 14.0 - pOH

Calculate the pH of a 0.0010 M NaOH Solution

Step 1: NaOH has only one hydroxide ion in its formula. Therefore, the [OH-] = the concentration of the base.

  • [OH-] = 0.0010 M or 1.0 x 10-3 M

Path 1:

Step 2: Use the [OH-] and Kw to solve for [H+].

  • [H+] [OH-] = Kw = 1.0 x 10-14
  • [H+] = 1.0 x 10-14 / [OH-]
  • [H+] = 1.0 x 10-14 / 1.0 x 10-3 M
  • [H+] = 1.0 x 10-11 M

Step 3: pH = -log [H+] = -log [1.0 x 10-11]

Step 4: Using your scientific calculator, calculate the value for pH.

  • pH = -log [1.0 x 10-11] = -(-11)
  • pH = 11

Path 2:

Step 2: pOH = -log [OH-] = -log 1.0 x 10-3

Step 3:

  • pOH = -log 1.0 x 10-3 = -(-3)
  • pOH = 3.0

Step 4:

  • pH = 14.0 - pOH
  • pH = 14.0 - 3.0
  • pH = 11

Acids, Bases, and Salts Practice

Acids, Bases, and Salts Extension

The letters "pH" represent the French words "pouvoir hydrogene" which means "hydrogen power". The definition of pH is pH is equal to the negative log (logarithm) of the hydrogen ion concentration of a solution. The logarithm of a number is the power to which 10 must be raised to equal that number. A pH value of less than 7 indicates a(n) acidic solution. A pH value of 7 indicates a neutral solution. A pH value of more than 7 indicates a(n) basic solution. The greater the [H+] concentration, the lower the pH. If a solution is neutral the [H+] concentration equals the [OH-] concentration.

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