pH Asam Lemah | Kimia SMA | Tetty Afianti

Summary

In this chemistry lesson, the topic of weak acid solutions and their pH determination is discussed. The teacher explains the definition of weak acids, their ionization, and how to calculate the pH using formulas that involve ionization constants (Ka) and molarity (M). Several examples are given, including step-by-step solutions to pH calculation problems. The teacher also explains how to calculate ionization percentages and mass requirements for certain concentrations, offering clear formulas and practical methods. The session concludes with practice problems to reinforce learning.

Takeaways

• 😀 As weak acids only partially ionize in solution, they have a low degree of ionization (denoted by α).
• 😀 Common examples of weak acids include acetic acid (CH3COOH), phosphoric acid (H3PO4), and hydrogen cyanide (HCN).
• 😀 The pH of a weak acid solution can be determined using a formula similar to that for strong acids, involving ionization constants and concentration.
• 😀 The ionization constant (Ka) and degree of ionization (α) help define the strength of a weak acid.
• 😀 The concentration of H+ ions in a weak acid solution can be calculated using the formula √(Ka * M), where M is molarity.
• 😀 The pH formula for weak acids is similar to strong acids, but it incorporates the H+ concentration calculated from ionization constants.
• 😀 If the Ka value is high, the acid is stronger, meaning it dissociates more into H+ ions.
• 😀 The degree of ionization α increases as the acid becomes more dilute (lower concentration).
• 😀 In examples, the pH of weak acids is calculated by finding the H+ concentration and then using the formula pH = -log[H+].
• 😀 Practical examples and step-by-step calculations were provided, including determining the pH of weak acid solutions and calculating percent ionization.
• 😀 The transcript also provides exercises for practicing these concepts, like calculating the pH of HBrO and determining the percent ionization of HCN.

Q & A

What is the definition of a weak acid?

A weak acid is an acid that only partially ionizes in solution, meaning it has a low degree of ionization, often represented by the symbol 'Alpha'.

Can you provide some examples of weak acids?

Examples of weak acids include acetic acid (CH3COOH), phosphoric acid (H3PO4), hydrocyanic acid (HCN), sulfurous acid (H2SO3), and carbonic acid (H2CO3).

How is pH calculated for weak acids?

For weak acids, pH is calculated using the formula: pH = -log[H+], where [H+] is the concentration of hydrogen ions, which can be derived from the ionization constant (Ka) and the molarity (M) of the acid.

What is the relationship between Ka and the strength of an acid?

The ionization constant (Ka) determines the strength of an acid. A larger Ka value indicates a stronger acid because it dissociates more in solution, producing more hydrogen ions (H+).

How do you determine the degree of ionization (Alpha) for a weak acid?

The degree of ionization (Alpha) for a weak acid can be determined using the formula Alpha = √(Ka / M), where Ka is the ionization constant and M is the molarity of the acid.

What does it mean when an acid has a small value for Alpha?

A small value for Alpha indicates that the acid is very weak and ionizes very little in solution, meaning it has a low concentration of hydrogen ions.

In the example given, how is the pH calculated for a 0.1 M weak acid with a Ka of 4 × 10^-9?

The pH is calculated by first finding the concentration of H+ ions using the formula √(Ka × M), which results in 2 × 10^-5 M for H+. Then, pH is calculated using pH = -log[H+], resulting in a pH of 5.

How do you calculate the percentage ionization of an acid in a solution?

The percentage ionization is calculated by the formula: Percentage Ionization = (Alpha × 100%), where Alpha is the degree of ionization. For example, if Alpha is 0.01, the ionization is 1%.

In the case of a weak acid with a concentration of 0.01 M and Ka = 10^-5, what is the pH?

To calculate the pH, first determine the H+ concentration using √(Ka × M), resulting in 10^-3.5 M for H+. Then, pH = -log(10^-3.5), which gives a pH of 3.5.

How do you determine the mass of acetic acid needed to prepare a specific pH solution?

To determine the mass of acetic acid required, first calculate the molarity (M) from the given pH, use the formula M = √(Ka × M), then calculate the mass using the formula: Mass = M × Mr × Volume, adjusting the units as necessary.