In everyday life, pH is a measure or range that is usually used to describe the level of neutrality or lack of neutrality in a household item. In natural science, pH is a unit of measurement for ions in solution. If you're taking a science or chemistry class, you may need to know how to calculate pH by concentration. Calculate pH using the pH equation: pH = -log[H3O+].
Step
Method 1 of 3: Understanding pH
Step 1. Know what pH means
pH is the concentration of hydrogen ions in a solution. Solutions with a high concentration of hydrogen ions are acidic. Solutions with low concentrations of hydrogen ions are basic, also known as alkalis. Hydrogen ions, also known as hydronium, are written briefly as H+ or H3O+.
- Know the pH range. The pH range is 1-14. The lower the number, the more acidic the solution. The higher the number, the more alkaline the solution. For example, orange juice has a pH of 2 because it is quite acidic. In contrast, bleach has a pH of 12 because it is quite alkaline. Numbers in the middle of that range are usually neutral, such as water, with a pH of 7.
- One pH level has a difference of 10x. For example, when comparing pH 7 to pH 6, pH 6 is ten times more acidic than pH 7. Thus, pH 6 will be 100 times more acidic than pH 8.
Step 2. Define pH in the equation
The pH range is calculated using the negative logarithm. The negative logarithm is the number of times a number has to be divided. The pH equation can be seen as follows: pH = -log[H3O+].
- The equation can sometimes be seen as pH = -log[H+]. Know that equations that have H3O+ or H+ are actually the same equation.
- It is not necessary to know what the negative log means in order to calculate pH. Most calculators used in high schools and universities have a log button.
Step 3. Understand concentration
Concentration is the number of particles of a compound in a solution. Concentration is generally described in units of molarity. Molarity is moles per unit volume (m/v or M). If you use a solution in the laboratory, the concentration will be written on the bottle. When doing your chemistry homework, concentration will usually be given.
Method 2 of 3: Using Concentration to Calculate pH
Step 1. Remember the pH equation
The pH equation is as follows: pH = -log[H3O+]. Make sure that you know the meaning of each term in the equation. See which tribe symbolizes concentration.
In chemistry, square brackets usually mean "concentration of". Thus, the pH equation can be read as "pH is equal to the negative logarithm of the hydronium ion concentration"
Step 2. Identify the actual concentration
Read your chemical equation. Identify the concentration of the acid or base. Write down the entire equation on paper with the known values in the equation. Always add units to avoid confusion.
For example, if the concentration is 1.05 x 10^5 M, write the pH equation as: pH = -log[1.05 x 10^5 M]
Step 3. Solve the equation
When solving pH equations, you must use a scientific calculator. First, press the “negative” button. This button is usually written as “+/-“. Now, hit the “log” button. Your screen should show “-log”. Now, press the open brackets and enter your concentration. Don't forget to add exponents if needed. Follow with closing brackets. At this point, you should see “-log(1, 05x10^5) . Press the equals button. Your pH value should be 5.
Method 3 of 3: Using pH to Calculate Concentration
Step 1. Identify the unknown values
First, write down the pH equation. Next, identify the values you know by writing them right below your equation. For example, if you know that pH is 10, 1, write 10, 1 on the paper, directly below the pH equation.
Step 2. Rearrange the equation
Rearranging equations requires a solid understanding of algebra. To calculate the concentration using pH, you must understand that the concentration must be separated on one side of the equals sign. Start by moving the pH to one side and shifting the hydronium ion concentration to the other side. Notice that the negative sign on the log is moved along with the hydronium ion, making the equation positive for hydronium on the opposite side. Then, subtract the pH from the left side and add pH as the exponent on the right side.
For example, pH = -log[H3O+] will change to +[H3O+] = log^-pH. Note that the pH value has become the inverse log. Then, you can replace the pH with 10, 1.
Step 3. Solve the equation
When working with inverse logs, the calculation process using the calculator is quite unique. Remember that log is a type of multiplication to the power of 10. To enter your equation, enter the number 10. Then, press the "EXP" exponent button. Enter a negative sign, followed by the value. Press the equals button.
For example, we have a pH value of 10, 1. Enter “10”, then press “EXP”. Now, enter “-/+” to make the value negative. Finally, enter the pH value “10, 1”. Press the equals button. You will get 1e-100. This means our concentration is 1.00 x 10^-100 M
Step 4. Rethink your answer
Does the answer above make sense? If you have a pH of 10.1, you know that the hydronium ion must be very small because 10.1 is an alkaline solution. Thus, the very small concentration value makes sense.
Tips
If calculating pH seems difficult to you, there are many other tools available. Use your textbook and ask your teacher for further help
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