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pH Formula

Last updated date: 25th May 2024
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What is pH Formula?

pH is a scale used in chemistry to describe the acidity or basicity of an aqueous solution. Acidic solutions (those containing a larger concentration of \[H^{+}\] ions) have a lower pH than basic or alkaline solutions. The pH scale ranges from 0 to 14 and any solution having a pH value between 0 to 6 is acidic in nature and any solution having a pH value of 7 is neutral and a solution ranging from 8 to 14 is basic in nature. Hydrochloric acid is the best example of an acid, calcium hydroxide is an example of a base, and sodium chloride is an example of salt.

The activity of the \[H^{+}\] ion in the solution accounts for the value. The negative logarithm of the molar hydronium-ion concentration is defined as pH. 

The pH equation is written as:

\[pH = -log[H_{3}O^{+}]\]

\[pH = -log[H^{+}]\]

Here, we will study the pH value formula, and how pH value is calculated in detail. 

pH Formula Chemistry

The pH scale, which normally spans from 0 to 14 in water, is used to determine the pH of an aqueous solution. A pH of 7 is regarded as neutral. Acidity is defined as a pH of less than 7. A pH of more than 7 is classified as basic. High hydronium concentrations and low hydroxide concentrations characterize acidic solutions. Hydroxide concentrations are high in basic solutions, while hydronium concentrations are low.

pH Calculation Formula of Water

The amphipathic property of water to behave as a proton donor and acceptor facilitates the synthesis of hydronium (\[H_{3}O^{+}\]) and hydroxide ions (OH) during self-ionization. Hydronium ions have the same concentration as hydroxide ions in pure water. The concentrations of both hydronium and hydroxide ions are 1.0107% at 25°C. The ion product of water, \[k_{w}\], is the equilibrium condition for water's self-ionization. Given below is the pH calculation formula:

\[k_{w} = (H_{3}O^{+}) (OH^{-}) = 1.0 \times 10^{-14}\]

\[^{p}K_{w} = pH + pOH = 14\]

Strong Acids and Strong Bases

Strong acids and strong bases in dilute aqueous solutions basically complete their ionization. Self-ionization of water occurs only to a small amount in aqueous solutions of strong acids and strong bases. Water self-ionization is a minor source of hydronium and hydroxide ions since it occurs in such a small amount. Knowing this, we can state that the strong acid is the principal source of hydronium ions when calculating hydronium concentration in an aqueous solution of a strong acid. When calculating the concentration of hydroxide ions in an aqueous solution of a strong base, we can also argue that the strong base is the principal source of hydroxide ions

pH Value Equation

How pH value is calculated for weak acids and weak bases?

Weak acids only partially dissolve in aqueous solutions and attain equilibrium, hence the equilibrium equation for that acid in solution determines how much they dissociate:


\[HA \Leftarrow  = \Rightarrow H^{+} + A^{-}\]

\[K_{a} = [H_{3}O^{+}] [A^{-}]\]

\[ H_{3}O^{+} \]  is the Hydronium Concentration

\[A^{-}\]  is conjugate base concentration

HA  is the Weak Acid concentration

In aqueous solutions, weak bases also only partially dissolve and approach equilibrium. The equilibrium equation for a base in solution is then the partial dissociation equation for that base:


\[ BOH \Leftarrow  = \Rightarrow OH^{-} + B^{+}\]

\[K_{b} = (OH^{-}) (B^{+})\]

\[OH^{-}\] = Hydroxide Concentration

\[B^{+}\] = Ion

BOH is the Weak Base

Applications of pH scale in real life:

  • All living organisms including human beings need a certain pH value to live. For example, all the physiological activities in our body function properly when the pH value is 7 to 7.8 and even our blood is basic in nature as it has high hydrogen ion concentration.

  • When bees and some insects like ants sting us, they release acidic substances into our body which causes us a lot of pain. To treat this, medicines like ointments that are basic in nature are used.

  • Acids play a crucial role in our body to digest the food we intake. Our stomach releases hydrochloric acid to break down the complex food substance and convert the food into energy. 

  • The food we consume and the bacteria present in our mouth are acidic in nature and might cause tooth decay in the long run. To avoid any such complications, toothpaste, which is basic in nature, is used to clean our teeth

  • Buildings, monuments, and aquatic life are badly affected when the acid content in their surroundings increases. 

Solved Example:

1. A solution is 0.055 M HBr. What is the pH of this solution?

Solution: Use the pH equation which is:  \[pH = -log[H_{3}O^{+}]\]

0.055 M HBr, HBr is a strong acid

\[H_{3}O^{+} = 5.5 \times 10^{-2} M\]

\[ pH = -log (5.5 \times 10^{-2}) = 1.26 \]


The pH of an aqueous solution indicates how acidic or basic it is. The concentration of hydrogen ions in the solution is used to calculate it. pH can be calculated through the pH equation. The pH scale ranges from 0 to 14, with 0 being neutral and 14 being acidic. A solution with a pH less than 7 is acidic, a solution with a pH more than 7 is basic, and a pH of 7 is neutral. The pH scale, which ranges from 0 to 14, determines how acidic or basic a thing is. The pH and organic acid content of foods are two important parameters that can influence not only the kind of organisms that survive storage but also their spoiling activity.

Log on to Vedantu to find out more about pH, acids, and bases and develop a strong conceptual foundation. Know what the experts have to say about this topic and gain deeper insights.

FAQs on pH Formula

1. What is Meant By The pH Calculation Formula?

The measure of hydrogen ion concentration used to determine the acidity or alkalinity of a solution is known as pH. The following is the equation for calculating the pH: -log[H+] = pH

2. How Do You Calculate pH and pOH?

Take the negative log of the hydronium ion concentration to compute the pH. Simply subtract the pH from 14 to get the pOH. Take the negative log of the hydroxide ion concentration to determine the pOH. Simply subtract pOH from 14 to get the pH.

3. What is an Acid's pH?

The pH of water is a measurement of how acidic or basic it is. The range is 0 to 14, with 7 being the neutral value. Acidity is indicated by a pH less than 7, while a pH greater than 7 indicates a base. pH is a measurement of the proportion of free hydrogen and hydroxyl ions in water.

4. What are the real-life applications of pH scale and measurement?

pH measures the number of hydrogen ions in a solution and it tells us whether a solution is an acid or a base. The pH scale ranges from 0 to 14. Any solution that has a pH from 1 to 6 is acidic, any solution with a pH value of 7 is neutral and any solution with a pH value between 8 to 14 is basic. Following are the applications of pH :

  • Animals and other living beings need a certain pH to live. For example, all the physiological activities in the human body generally take place within the values of 7 and 7.8. When the value of water increases due to acid rain and the addition of pollutants, aquatic life is threatened as the change in pH value depletes their environment.

  • pH value is important for the proper digestion of food in our stomach. The stomach releases hydrochloric acid with a pH value of 1 to 3 which helps in digesting food and converting it into energy. 

  • The food we consume and the bacteria present in our mouth reduces the pH value. To counter this acidic activity we use toothpaste and tooth powders that are basic. It protects our teeth from tooth decay and other oral problems. 

  • Some animals like ants and honeybees sting us and cause us pain. The acid they release into our body is acidic and to counter the pain, medicines with neutral pH value or basic are used to reduce the pain.

5. What are acids, bases, and salts?

Acid is a chemical substance with a pH value ranging from 1 to 6 and bases are substances with 8 to 14 as pH values. Salts on the other hand are neutral and have a pH value of 7. pH value is the measure of the concentration of hydrogen ions in a substance and it can be measured using litmus papers. A base when it is dissolved in water forms an alkali solution. 

All of these have advantages and disadvantages like, acetic acid is present in vinegar, citric acid present in citrus fruits gives us Vitamin C, lactic acid is present in curd, ascorbic acid is present in Amla, calcium hydroxide occur in lime water and it is also the important component in cave formation, ammonium hydroxide is used in window cleaner, sodium hydroxide is used in soap. To test a solution and know its nature, one can use turmeric as it turns red when the solution is soap or basic. Even our blood has a pH value and it is basic as it has a high concentration of hydrogen ions. When an acid reacts with a base, it is an exothermic reaction that results in a base, water, and heat.