[Chemistry Class Notes] on pH Formula Pdf for Exam

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

[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

[ 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 ]

Conclusion

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.

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