Pemantapan PTS Genap Kelas XI Part 1 | Kimia SMA | Tetty Afianti

Summary

This video is a chemistry tutorial for 11th-grade students, focusing on practice questions for the mid-term exam. The instructor explains key concepts in acid-base theory, emphasizing the Bronsted-Lowry theory of acids and bases. Through multiple example reactions, the teacher illustrates how to identify acids and bases based on proton donation and acceptance. The video also covers calculating pH for strong and weak acids and bases, titration calculations, and understanding hydrolysis in salts. The lesson aims to help students prepare for their upcoming exam by clarifying difficult concepts and solving problems step by step.

Takeaways

• 😀 Bronsted-Lowry Acid-Base Theory: Acids are proton donors (H+), and bases are proton acceptors.
• 😀 In reactions, the substance with more H+ ions is considered the acid, and the one with fewer H+ ions is the base.
• 😀 The first reaction involves H2PO4- and HPO4^2-, where H2PO4- acts as the acid, and HPO4^2- as the base.
• 😀 For the second reaction, H2S is the acid, and KHSO4 is the base, as H2S has more H+ ions.
• 😀 In the third reaction, H2SO4 is the acid, and HCO3- is the base, based on the number of H+ ions.
• 😀 The fourth reaction discusses NH3 (acting as the base) and NH4+, where NH4+ is the acid due to its higher H+ ion count.
• 😀 pH calculations: For strong acids like H2SO4, pH can be found using the formula pH = -log [H+], which in this case results in pH = 1.
• 😀 For weak bases like NH4OH, pH is calculated using the formula for weak base dissociation, yielding a pH of 10.
• 😀 In titration problems, the volume of base and acid can be used to find the molarity of the base in titration using the formula M1V1 = M2V2.
• 😀 In titration experiments, NaOH concentration was found to be 0.25M, and the amount of NaOH in the titration was calculated to be 5 grams.
• 😀 For the topic of hydrolysis, KCl (formed from strong acid and base) does not undergo hydrolysis, while NH4Cl (formed from weak acid and base) undergoes partial hydrolysis.
• 😀 NH4CN, formed from weak acid and weak base, undergoes total hydrolysis, unlike other salts formed from strong acids or bases.

Q & A

What is the Bronsted-Lowry theory of acids and bases?

According to the Bronsted-Lowry theory, acids are substances that donate protons (H+ ions), while bases are substances that accept protons (H+ ions).

How can we identify acids and bases in a reaction based on the Bronsted-Lowry theory?

In a reaction, the substance that has more H+ ions is the acid, and the substance that has fewer H+ ions is the base. For example, in the reaction between H2PO4- and HPO4^2-, H2PO4- acts as the acid, and HPO4^2- acts as the base.

What is the role of H2SO4 in the reactions discussed?

H2SO4 (sulfuric acid) acts as an acid in the reactions discussed, donating protons (H+) to other substances.

How do we calculate the pH of a strong acid like H2SO4?

For a strong acid like H2SO4, the pH is calculated by using the formula pH = -log[H+], where [H+] is the concentration of hydrogen ions. For H2SO4, the concentration of H+ is doubled because each molecule of H2SO4 releases two protons.

How do we calculate the pH of a weak base like NH4OH?

For a weak base like NH4OH, the pH is calculated by determining the concentration of OH- ions using the base dissociation constant (Kb), and then converting that to pOH. Finally, pH is found using the formula pH = 14 - pOH.

What is the purpose of titration in acid-base reactions?

Titration is used to determine the concentration of an unknown solution by reacting it with a solution of known concentration. In this case, NaOH (a base) is titrated with H2SO4 (an acid) to determine the concentration of NaOH.

How do we determine the concentration of NaOH in a titration experiment?

To determine the concentration of NaOH, we use the titration formula: Ma * Va = Mb * Vb, where Ma and Va are the molarity and volume of the acid (H2SO4), and Mb and Vb are the molarity and volume of the base (NaOH). By rearranging the formula, we can calculate the concentration of NaOH.

What is the result of titrating NaOH with H2SO4, based on the experiment?

In the titration experiment, the average volume of NaOH required to neutralize 25 mL of 0.1 M H2SO4 was found to be 20 mL. This was used to calculate the concentration of NaOH as 0.25 M.

What is the difference between hydrolysis total and hydrolysis partial?

Hydrolysis total occurs when both components of the salt (acid and base) are weak and react completely with water. Hydrolysis partial happens when one component is strong (either the acid or the base) and the other is weak, leading to partial hydrolysis.

Which salt in the example undergoes total hydrolysis?

NH4CN (ammonium cyanide) undergoes total hydrolysis because both its acid (HCN) and base (NH4OH) are weak, leading to complete hydrolysis.