PH is a scale that measures the acidity or basicity of a solution or compound. Scientifically, pH measures the ions present in a chemical solution. If you are attending a science or chemistry class, you need to know how to calculate pH based on the molar concentration of the chemical solution in question. To calculate the pH, the following equation is used: pH = -log [H3O+].
Steps
Method 1 of 3: pH Basics
Step 1. Understand what pH really is
The pH represents the concentration of hydrogen ions within a solution. A solution that has a high concentration of hydrogen ions is acidic, while a solution that has a low concentration of hydrogen ions is basic, also referred to as alkaline. The hydrogen ion is also known by the name of Hydronium and is characterized by the symbol H + or H30 +.
- Understand the pH measurement scale. This scale ranges from 1 to 14, where the lowest number indicates an acid solution and the highest number is basic. For example, orange juice has a pH of 2 due to its acidity. On the other hand, bleach has a pH of 12, which means it is very basic. The numbers in between on the scale represent neutral solutions, such as water that has a pH of 7.
- Each pH level differs from the next or from the previous one by a factor of 10. For example, comparing a pH 7 to a pH 6, the latter is ten times more acidic than the former. As a result, pH 6 is 100 times more acidic than pH 8
Step 2. Define the pH using an equation
The pH measurement scale is defined by a negative logarithm. The negative logarithm of a number simply indicates its divisor into base 10. The pH equation is as follows: pH = -log [H3O +].
- Sometimes the pH equation can be represented as follows: pH = -log [H +]. Both forms represent the same equation.
- To calculate the pH, however, it is not necessary to fully understand the meaning of negative logarithm. Most calculators that are used in middle and high schools are capable of calculating the logarithm of a number.
Step 3. Understand the meaning of concentration
This is the number of particles of a compound that are dissolved in a solution. Concentration is commonly described by molarity. Concentration is indicated as moles per unit volume (m / v or M). Inside the chemistry laboratories, the concentration of the available solutions is shown on the bottle. In chemistry problems, concentration is usually a given that is given.
Method 2 of 3: Use Concentration to Calculate pH
Step 1. Remember the pH equation
The pH is described by the following equation: pH = -log [H3O +]. Make sure you know what the terms in that equation represent. Identify the term for concentration.
In chemistry, square brackets usually mean "concentration of". So the pH equation should be read as: "The pH is equal to the negative logarithm of the hydronium ion concentration"
Step 2. Identify the concentration value
Read the text of the chemistry problem you need to solve to find out the concentration of an acid or base. Write down the entire equation on paper by substituting the known values for the related variables. To avoid confusion, always report the units of measurement.
For example, if the concentration is 1.05 * 105 M, the equation for calculating the pH will be as follows: pH = -log [1.05 * 105 M]
Step 3. Solve the equation
To do this, you will be obliged to use a scientific calculator. First press the "-" key, followed by the "log" key. "-Log" should appear on the calculator display. Press the key relating to the opening parenthesis and enter the concentration. When present, do not forget to indicate the exponent. At the end close the parenthesis. The following formula "-log (1.05x10 ^ 5)" should appear on the calculator display. Press the key to perform the calculation, the result should be: pH = 5.
Method 3 of 3: Use pH to Calculate Concentration
Step 1. Identify unknown variables
First write the equation to calculate the pH. Continue by identifying the values you know and report them directly below the equation. For example, if you know that the pH value is equal to 10, 1, write it directly on the paper immediately after writing the equation to calculate it.
Step 2. Set up the equation
This step requires extensive knowledge of algebra. To calculate the concentration from the pH, you need to set up the equation so that the concentration is isolated within one member. It starts by shifting the pH in one member and the hydronium ion concentration in the other. Remember to keep the sign of the logarithm when you move it to the other member, and that from negative it will have to be transformed into positive. Now subtract the pH from the left side and set it as the negative exponent of the right side.
Our original equation, pH = -log [H3O +] will become + [H3O +] = log-pH. Note that the pH value has become an inverse logarithm. Now you can replace the pH value within the equation with 10, 1.
Step 3. Proceed to solve the equation
When working with inverse logarithms the process of calculations to follow is unique. Remember that the logarithm is a base 10 multiplication. To enter the equation into the calculator, type the number 10. Now press the "EXP" key then type the "-" followed by the known pH value. At the end, press the key to perform the calculation.
In our example we know that the pH is equal to 10, 1. So we will have to type "10" and press the "EXP" key. Now press the "-" key since the exponent of our equation is negative. Finally enter the pH value ie "10, 1". At the end, press the key to perform the calculations. As a result we should get "1e-100". This means that the concentration is equal to 1.00 * 10-100 M.
Step 4. Analyze the result
Is the solution obtained in the previous step correct? We know that a solution with a pH of 10.1 is very basic, so the concentration of hydronium ions is very low. This means that the concentration number is tiny, so the solution obtained is correct.
