EDUCA PE | ENSINO MÉDIO | QUÍMICA | 3º ANO | TEORIA DE ÁCIDOS E BASES E AS MEDIDAS DE PH (Parte 2)

Summary

In this chemistry lesson, Professor Emanuel Maresco explains the concepts of ionic equilibrium, focusing on the calculation of molar concentration of H+ ions corresponding to pH values. The lesson covers the pH of weak acid and weak base solutions, as well as buffer solutions. Key topics include applying logarithms and antilogarithms to determine concentrations, understanding the characteristics of weak acids and bases, and using formulas for pH calculations in various chemical contexts. Practical examples, such as those involving acerola juice and hydrocyanic acid, help illustrate these concepts in real-world scenarios.

Takeaways

• 😀 pH is a measure of the concentration of hydrogen ions (H+) in a solution and can be calculated using logarithms.
• 😀 To calculate the molar concentration of H+ from pH, three rules are applied: changing the sign of the pH, adjusting the characteristic, and using the antilog.
• 😀 For pH values like 3.4, converting to negative (-3.4) and applying the rules gives the molar concentration of H+ as 4 × 10⁻⁴ M.
• 😀 Weak acids and weak bases don't dissociate fully in solution, and their pH calculations depend on their ionization constants (Ka or Kb).
• 😀 The molar concentration of H+ for weak acids can be found using the formula: H+ = √(Ka × M), where M is the molarity of the acid.
• 😀 For weak acid solutions, you can also use the formula pH = 1/2 * (pKa - log M) for pH calculation.
• 😀 For weak base solutions, the pH can be calculated similarly, but the formula involves 14 minus half of pKb plus half of the log of the base's molarity.
• 😀 Buffer solutions resist changes in pH when small amounts of acids or bases are added. They are typically made from a weak acid and its salt, or a weak base and its salt.
• 😀 In acidic buffer solutions, the pH is calculated using the equation: pH = pKa - log([acid]/[salt]).
• 😀 In basic buffer solutions, the pH is calculated using the equation: pH = 14 - (pKb + log([base]/[salt])).
• 😀 The pH of weak acids, weak bases, and buffer solutions are typically found using logarithmic relationships involving ionization constants and molarity.

Q & A

What is the first step in calculating the molar concentration of H+ from a given pH value?

The first step is to change the sign of the pH value.

What is the role of the characteristic and mantissa when calculating the molar concentration of H+?

The characteristic is the whole number part of the pH, and the mantissa is the decimal part. In the calculation, you add and subtract the characteristic of the immediately superior number to simplify the calculation.

What does the term 'antilogarithm' mean in the context of calculating the molar concentration of H+?

The antilogarithm is the opposite of the logarithm. In this case, it is used to calculate the molar concentration of H+ from the pH value.

In the example with acerola juice (pH = 3.4), how is the molar concentration of H+ calculated?

First, change the sign of the pH to -3.4, then add and subtract 4 to get 0.6. The antilogarithm of 0.6 is 4, and the antilogarithm of -4 is 10^-4. Multiplying these gives the molar concentration as 4 × 10^-4 mol/L.

What formula is used to calculate the pH of a solution of weak acids or weak bases?

For weak acids, the formula used is pH = 1/2 (pKa - log of molarity). For weak bases, pH is calculated using the formula pH = 14 - 1/2 (pKb + log of molarity).

How do you identify a weak acid?

A weak acid can be identified by three characteristics: 1) Its degree of ionization is less than 5%, 2) It doesn't contain halogens (like fluorine, chlorine, or bromine), and 3) It is an oxyacid where the subtraction of oxygen minus hydrogen equals zero (e.g., boric acid).

What is the pH calculation for a 0.02 mol/L solution of hydrocyanic acid (Ka = 4.9 x 10^-10)?

Using the formula h+ = sqrt(Ka × M), and applying logarithmic calculations, the pH for hydrocyanic acid is approximately 5.5.

What distinguishes weak bases from strong bases?

Weak bases are typically bases that do not contain alkali metals (group 1A) or alkaline earth metals (group 2A), except for beryllium and magnesium. Examples include aluminum hydroxide and silver hydroxide.

How do you calculate the pH of a buffer solution?

The pH of an acidic buffer solution is calculated using the formula: pH = pKa - log(molarity of acid/salt). For a basic buffer solution, the formula is: pH = 14 - pKb + log(molarity of base/salt).

What are the key differences between acidic and basic buffer solutions?

An acidic buffer solution is formed by a weak acid and a salt derived from a strong base (e.g., acetic acid and sodium acetate). A basic buffer solution is formed by a weak base and a salt from a strong acid (e.g., ammonium hydroxide and ammonium chloride).