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How to Calculate pH: Easy Steps for Strong and Weak Acids & Bases

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How to Calculate pH: Easy Steps for Strong and Weak Acids & Bases
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Manuela Aristizabal

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How to Calculate pH: Easy Steps for Strong and Weak Acids & Bases

• pH is a measure of acidity or basicity of a solution, calculated using the concentration of hydrogen ions.
• The document covers calculations for pH of strong acids, strong bases, and weak acids and bases.
• For strong acids and bases, pH is directly calculated from concentration, while weak acids and bases require equilibrium considerations.
• The pH scale ranges from 0 to 14, with 7 being neutral, below 7 acidic, and above 7 basic.
• Examples are provided for calculating pH of hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions.

1/7/2024

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pH of Water and Weak Acids

This section discusses the pH of pure water and weak acids. For water, the self-ionization equilibrium is considered, where the concentration of H3O+ equals OH-. The pH of pure water is calculated to be 7.

For weak acids, which partially dissociate in water, an equilibrium approach is used. The general equation for a weak acid HA is:

HA + H2O ⇌ H3O+ + A-

The equilibrium constant Ka is used to determine the concentration of H3O+. For weak acids, an approximation method is often used when the acid concentration is much larger than Ka.

Formula: [H3O+] ≈ √(Ka × initial acid concentration)

An example is provided for calculating the pH of 500 mL of 0.250 M acetic acid (CH3COOH) with Ka = 1.75 × 10^-5:

Example: Using the quadratic equation or approximation method, [H3O+] is calculated, and pH = -log[H3O+]

This demonstrates the more complex equilibrium considerations required for weak acid pH calculations.

PH Acidos fuertes •monoprotico (solo 1 proton) HCI (a)+ H₂O (1) n H3O+ n HCI = → H30*(c) + Cl (ac) 1 n H30+ = n HCI molaridad: → Diprótico (

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pH of Strong Bases

This section covers the calculation of pH for strong bases. Strong bases fully dissociate in water, releasing hydroxide ions (OH-). The pH calculation depends on the number of hydroxide ions released by the base.

For monobasic compounds like NaOH, which release one OH- ion, the concentration of OH- equals the base concentration. The pOH is calculated first, then converted to pH:

Formula: pOH = -log[OH-] = -log[base concentration] pH = 14 - pOH

For dibasic compounds like Ca(OH)2, which release two OH- ions, the concentration of OH- is twice the base concentration:

Formula: pOH = -log[2 × base concentration] pH = 14 - pOH

For polybasic compounds releasing n OH- ions, the general formula is:

Formula: pOH = -log[n × base concentration] pH = 14 - pOH

An example is provided for calculating the pH of 100 mL of 0.1 M NaOH:

Example: For 0.1 M NaOH, pOH = -log[0.1] = 1, pH = 14 - 1 = 13

This illustrates the application of pH calculations for strong bases.

PH Acidos fuertes •monoprotico (solo 1 proton) HCI (a)+ H₂O (1) n H3O+ n HCI = → H30*(c) + Cl (ac) 1 n H30+ = n HCI molaridad: → Diprótico (

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pH of Weak Bases

This final section covers the calculation of pH for weak bases. Weak bases partially dissociate in water, establishing an equilibrium. The general equation for a weak base B is:

B + H2O ⇌ BH+ + OH-

The equilibrium constant Kb is used to determine the concentration of OH-. Similar to weak acids, an approximation method can be used when the base concentration is much larger than Kb.

Formula: [OH-] ≈ √(Kb × initial base concentration)

An example is provided for calculating the pH of 250 mL of 0.250 M ammonia (NH3) with Kb = 1.75 × 10^-5:

Example: Using the quadratic equation or approximation method, [OH-] is calculated, then pOH = -log[OH-], and finally pH = 14 - pOH

This example illustrates the process of calculating pH for a weak base, considering the equilibrium and using the relationship between pH and pOH.

PH Acidos fuertes •monoprotico (solo 1 proton) HCI (a)+ H₂O (1) n H3O+ n HCI = → H30*(c) + Cl (ac) 1 n H30+ = n HCI molaridad: → Diprótico (

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pH of Strong Acids

This section explains how to calculate the pH of strong acids. Strong acids fully dissociate in water, releasing hydrogen ions (H+) or hydronium ions (H3O+). The pH calculation depends on the number of protons released by the acid.

For monoprotic acids like HCl, which release one proton, the concentration of H3O+ equals the concentration of the acid. The pH is calculated using the formula:

Formula: pH = -log[H3O+] = -log[acid concentration]

For diprotic acids like H2S, which release two protons, the concentration of H3O+ is twice the acid concentration. The pH formula becomes:

Formula: pH = -log[2 × acid concentration]

For polyprotic acids releasing n protons, the general formula is:

Formula: pH = -log[n × acid concentration]

An example is provided for calculating the pH of 100 mL of 0.1 M HCl:

Example: For 0.1 M HCl, pH = -log[0.1] = 1

This demonstrates how to apply the pH formula for a strong monoprotic acid.

PH Acidos fuertes •monoprotico (solo 1 proton) HCI (a)+ H₂O (1) n H3O+ n HCI = → H30*(c) + Cl (ac) 1 n H30+ = n HCI molaridad: → Diprótico (

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