Back To Course

Chemistry 101: General Chemistry14 chapters | 132 lessons | 11 flashcard sets

Are you a student or a teacher?

Start Your Free Trial To Continue Watching

As a member, you'll also get unlimited access to over 75,000 lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed.

Free 5-day trial
Your next lesson will play in
10 seconds

Lesson Transcript

Instructor:
*Amy Meyers*

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn the history of the pH scale, how to describe it and why it is used by scientists. Discover how to calculate the pH of an acid or base solutions given either the hydroxide ion concentration or the hydronium ion concentration. Learn how to start with the pH and calculate the hydroxide and hydronium ion concentrations.

The idea of pH was proposed in 1909 by a Danish chemist named Soren Sorensen to solve an annoying problem chemists were having. Chemists had found that when they were working with acids and the concentration of hydronium ions, [H3O+], the numbers in the calculations involved negative exponents. Sorenson proposed using the negative logarithm of [H3O+] to measure acidity.

Sorenson called the new measurement pH. The p stands for power and the H for hydrogen, so **pH** stands for power of hydrogen. pH is just a value used to express how acidic or basic a solution is. The pH scale runs from 0 (the lowest possible value) to 14 (the highest possible value). Values from 0-6 are acidic, a value of 7 is neutral, and values from 8-14 are basic.

Here's a fun pH experiment you can do at home. Red cabbage is a natural pH indicator. Mix chopped cabbage with boiling water and allow the mixture to sit for 10 minutes to leach out the flavin (a pigment). After 10 minutes, pour out just the water. It should be a purple color. When the solution is purple, the pH is neutral. Divide the solution into different containers and start adding various household liquids to the containers. If the solution turns red, the solution you added was acidic. If it turns yellow/green, then the solution you added was basic.

To calculate the power of pH, the formula is -log [H3O+].

Although this sounds complicated, it really isn't. If you have a hydronium ion concentration [H3O+] of 1 x 10^-7, the pH is 7. -log [H3O+] = -log (1 x 10^-7) = 7

If the [H3O+] is 1 x 10^-1, the pH is 1.

If it is 1 x 10^-13, the pH is 13. See how that works?

The following problem is a bit harder because you are given the molarity of the solution and not the concentrations of the ions.

What is the pH of a 0.00010M solution of HNO3, a strong acid?

Because the problem told you that HNO3 is a strong acid, you know that it dissociates nearly 100%, so you know that the [H3O+] = 1 x 10^-4 M (.0001 = 1 x 10^-4).

pH = -log [H3O+] = -log (1 x 10^-4) = - (-4) = 4

Here's another one. You will need a calculator for this one.

Calculate the pH of a 0.2 M solution of a strong acid.

Strong acid - therefore the [H3O+] is the same as the molarity of the solution, or [H3O+] = 2 x 10^-1

pH = -log [H3O+] = -log (2 x 10^-1) = 0.7

What is the pH of a 0.0136M solution of KOH, a strong base?

This one is a bit harder because you have the hydroxide ion concentration but not the hydronium ion concentration. Because you were told that the KOH is a strong base, you know it dissociates nearly 100% to form 1.36 x 10^-2 [OH-].

If you remember that the Kw, or auto-ionization constant of water, is calculated by the formula [H3O+] [OH-] = Kw = 1 x 10^-14, you can figure out the [H3O+] concentration.

[H3O+] = Kw / [OH-]

[H3O+] = 1 x 10^-14 / 1.36 x 10^-2

[H3O+] = 7.35 x 10^-13

Now that you know [H3O+], you can figure out the pH.

pH = -log [H3O+] = -log (7.35 x 10^-13)

pH = 12.13

Try another one.

Calculate the pH if the [OH-] = 2 x 10^-3 M.

First, figure out the [H3O+] from the formula [H3O+] = Kw / [OH-].

[H3O+] = 1 x 10^-14 / 2 x 10^-3 = 5 x 10^-12

Now that you know [H3O+], you can figure out the pH.

pH = -log [H3O+] = -log (5 x 10^-12) = 11.3

You can use the pH of a substance to calculate the concentrations of the hydronium ions and hydroxide ions.

What are the concentrations of hydronium and hydroxide ions in acid rain that has a pH of 5.05?

Known values:

pH = 5.05

Kw = 1 x 10^-14

[H3O+] = ?

[OH-] = ?

Find the [H3O+] from the equation [H3O+] = 10^-pH.

[H3O+] = 10^-5.05

[H3O+] = 8.9 x 10^-6 M

Use the auto-ionization equation to find [OH-].

[H3O+] [OH-] = Kw = 1 x 10^-14

[OH-] = Kw / [H3O+]

[OH-] = 1 x 10^-14 / 8.9 x 10^-6

[OH-] = 1.1 x 10^-9 M

What is the hydronium ion and hydroxide ion concentration in juice that has a pH of 3.3?

Known values:

pH = 3.3

Kw = 1 x 10^-14

[H3O+] = ?

[OH-] = ?

Find the [H3O+] from the equation [H3O+] = 10^-pH.

[H3O+] = 10^-3.3

[H3O+] = 5 x 10^-4

Use the auto-ionization equation to find [OH-].

[H3O+] [OH-] = Kw = 1 x 10^-14

[OH-] = Kw / [H3O+]

[OH-] = 1 x 10^-14 / 5 x 10^-4

[OH-] = 2 x 10^-11

pH is a value used to express the acidity or alkalinity (basicity) of a solution. It is measured on a scale of 0-14, with 0-6 being acidic, 8-14 being basic, and a value of 7 representing neutral. You can calculate the pH of an acid or base solution given the hydronium ion concentration by using the formula pH = -log [H3O+] or if given the hydroxide ion concentration by the formula [H3O+] = Kw / [OH-], then using the pH = -log [H3O+] formula.

You can also use the pH of a substance to calculate the concentrations of the hydronium and hydroxide ions by using the formula [H3O+] = 10^-pH.

At the end of this lesson, you'll be able to:

- Tell what pH stands for and explain the scale used to measure it
- Calculate the pH of a solution when given either the hydronium ion concentration or the hydroxide ion concentration
- Solve equations involving the hydronium or hydroxide ion concentration when given the pH

To unlock this lesson you must be a Study.com Member.

Create your account

Are you a student or a teacher?

Already a member? Log In

BackDid you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

You are viewing lesson
Lesson
6 in chapter 10 of the course:

Back To Course

Chemistry 101: General Chemistry14 chapters | 132 lessons | 11 flashcard sets

- Go to Atom

- Go to Gases

- Go to Solutions

- Decomposition and Synthesis Reactions 6:20
- The Arrhenius Definition of Acids and Bases 7:53
- The Bronsted-Lowry and Lewis Definition of Acids and Bases 6:15
- Neutralization and Acid-Base Reactions 5:43
- Dissociation Constant and Autoionization of Water 6:41
- The pH Scale: Calculating the pH of a Solution 9:49
- Precipitation Reactions: Predicting Precipitates and Net Ionic Equations 5:55
- Assigning Oxidation Numbers to Elements in a Chemical Formula 6:50
- Electrochemical Cells and Electrochemistry 4:22
- Cathode and Anode Half-Cell Reactions 9:45
- Balancing Redox Reactions and Identifying Oxidizing and Reducing Agents 6:38
- The Activity Series: Predicting Products of Single Displacement Reactions 6:02
- Writing and Balancing Combustion Reactions 9:19
- Single-Displacement Reaction: Definition & Examples 7:53
- Go to Chemical Reactions

- Go to Kinetics

- Computer Science 336: Network Forensics
- Computer Science 220: Fundamentals of Routing and Switching
- Global Competency Fundamentals & Applications
- Introduction to the Principles of Project Management
- Praxis Elementary Education: Reading & Language Arts - Applied CKT (7902): Study Guide & Practice
- Practical Applications for Marketing
- Practical Applications for HR Management
- Practical Applications for Organizational Behavior
- Analyzing Texts Using Writing Structures
- Understanding Arguments & Reasoning in Texts
- TASC Test Score Information
- What is the TASC Test?
- Praxis Prep Product Comparison
- GED Prep Product Comparison
- CBEST/CSET Prep Product Comparison
- ASVAB Prep Product Comparison
- GACE Prep Product Comparison

- Teaching ELL Students to Read
- Technology Used in Reading Instruction
- Pectoralis Major: Action, Origin & Insertion
- Creating a Learner-Centered Classroom & Environment
- Practical Application: Assessing Soft & Hard Skills for Customer Service
- Business Case Study: Risk Management at Nokia vs. Ericsson
- Network Component Addressing: Types & Significance
- Positive Personal Outcomes of Global Competence
- Quiz & Worksheet - Reading Group Strategies
- Quiz & Worksheet - Integrated Reading Instruction
- Quiz & Worksheet - Writing Numbers in Standard Form
- Quiz & Worksheet - Methods for Teaching ELL Students
- Quiz & Worksheet - Professional Development Ideas for Teachers
- Flashcards - Measurement & Experimental Design
- Flashcards - Stars & Celestial Bodies

- Political Science for Teachers: Professional Development
- Teaching Resources for Children's Books
- Praxis Middle School English Language Arts (5047): Practice & Study Guide
- Developmental World History: High School
- How to Apply for College Grants & Scholarships
- Learning Experiences in Curriculum
- Ohio EOCE - Geometry: Theorems & Proofs
- Quiz & Worksheet - Veterinary Medicine Chemical Hazards
- Quiz & Worksheet - Using Henceforth
- Quiz & Worksheet - Process Improvement Groups
- Quiz & Worksheet - Amy Tan's Mother Tongue
- Quiz & Worksheet - Appeal to Force Fallacy

- The Importance of the Front Desk in a Medical Office
- Into the Wild: Chapter 13 Summary
- Life Cycle of a Frog Lesson Plan
- ELM Test Dates
- How to Read TABE Test Scores
- Is SQL Hard to Learn?
- How to Save Money in College
- Five Senses Activities for Kids
- Experiments with Magnets for Kids
- How to Pass a Job Assessment Test
- 6th Grade South Carolina Science Standards
- Average GRE Scores

- Tech and Engineering - Videos
- Tech and Engineering - Quizzes
- Tech and Engineering - Questions & Answers

Browse by subject