Problem 47 - Chemistry The Molecular Science

Question

Without doing any calculations, assign each of these 0.10-M aqueous solutions to one of these pH ranges: \(\mathrm{pH} 2 ; \mathrm{pH}\) between 2 and \(6 ; \mathrm{pH}\) between 6 and 8 \(\mathrm{pH}\) between 8 and \(12 ; \mathrm{pH} 12 .\) (a) \(\mathrm{HNO}_{2}\) (b) \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) NaF (d) \(\mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\) (e) \(\mathrm{BaO}\) (f) \(\mathrm{KHSO}_{4}\) (g) \(\mathrm{NaHCO}_{3}\) (h) \(\mathrm{BaCl}_{2}\)

Step-by-Step Solution

Answer

HNO2: pH 2-6; NH4Cl: pH 2-6; NaF: pH 8-12; Mg(CH3COO)2: pH 8-12; BaO: pH 12; KHSO4: pH 2-6; NaHCO3: pH 8-12; BaCl2: pH 6-8.

Step 1: Classify the nature of each compound

Determine whether each compound is an acid, base, or neutral salt. For example, \( \mathrm{HNO}_2 \) is a weak acid, \( \mathrm{NH}_4 \mathrm{Cl} \) is an acidic salt, \( \mathrm{NaF} \) is a basic salt, \( \mathrm{Mg(CH}_3\mathrm{COO})_2 \) is a basic salt, \( \mathrm{BaO} \) is a strong base, \( \mathrm{KHSO}_4 \) is an acidic salt, \( \mathrm{NaHCO}_3 \) is a basic salt, and \( \mathrm{BaCl}_2 \) is a neutral salt.

Step 2: Assign pH range based on compound type

Based on whether the compound is acidic, basic, or neutral, assign a pH range. Strong acids have a pH around 2, weak acids fall between \(2\) and \(6\), neutral solutions have a pH between \(6\) and \(8\), basic solutions fall between \(8\) and \(12\), and strong bases have a pH around \(12\).

Step 3: Provide the pH ranges for each solution

- \( \mathrm{HNO}_2 \): weak acid, pH between \(2\) and \(6\)- \( \mathrm{NH}_4 \mathrm{Cl} \): acidic salt, pH between \(2\) and \(6\)- \( \mathrm{NaF} \): basic salt, pH between \(8\) and \(12\)- \( \mathrm{Mg(CH}_3\mathrm{COO})_2 \): basic salt, pH between \(8\) and \(12\)- \( \mathrm{BaO} \): strong base, pH \(12\)- \( \mathrm{KHSO}_4 \): acidic salt, pH between \(2\) and \(6\)- \( \mathrm{NaHCO}_3 \): basic salt, pH between \(8\) and \(12\)- \( \mathrm{BaCl}_2 \): neutral salt, pH between \(6\) and \(8\)

Key Concepts

Acidic Solutions

An acidic solution has more hydrogen ions ( H^+ ) than hydroxide ions ( OH^- ). The pH scale is used to measure how acidic or basic an aqueous solution is. The pH scale ranges from 0 to 14, where a lower pH indicates a more acidic solution.
Understanding where a solution falls on this scale helps in predicting its chemical behavior.

• Acidic solutions have a pH less than 7.
• Strong acids, like hydrochloric acid (HCl), have very low pH values, around 1 or 2.
• Weak acids, such as acetic acid (CH₃COOH), have pH values between 2 and 6.

Acids can be strong or weak depending on their ability to dissociate in water. Strong acids completely dissociate, releasing more hydrogen ions, whereas weak acids partially do so. Identifying whether a solution is composed of a weak or strong acid helps determine its position on the pH scale. For example, HNO_2 is a weak acid resulting in a pH range from 2 to 6.

Basic Solutions

Basic solutions, also known as alkaline solutions, have more hydroxide ions ( OH^- ) than hydrogen ions ( H^+ ). These solutions typically have pH values greater than 7 and can be recognized by their slippery feel and ability to turn red litmus paper blue.
A pH above 7 indicates a basic nature, and the strength of the base will dictate just how high the pH value is.

• Basic solutions have a pH greater than 7.
• Strong bases, such as sodium hydroxide (NaOH), can have pH values close to 14.
• Weak bases have pH values between 8 and 12.

Some salts can also lead to basic solutions in water due to their ability to hydrolyze, releasing hydroxide ions. For instance, NaF and Mg(CH_3COO)_2 are salts that create basic solutions with pH values between 8 and 12.

Neutral Salts

Neutral salts arise when an acid reacts with a base, forming a salt and water. In a neutral salt solution, neither hydrogen ion ( H^+ ) or hydroxide ion ( OH^- ) predominates.
As a result, these solutions have a pH close to 7, indicating neutrality.

• Neutral salt solutions are commonly formed by strong acids and strong bases.
• The pH of neutral salts is typically around 7, indicating a balance between H^+ and OH^- ions.
• Examples include sodium chloride (NaCl), which forms a neutral solution.

Sometimes, salts that are considered neutral can have slight effects on the pH due to subtle hydrolysis effects, but these are often negligible. For instance, BaCl_2 is considered a neutral salt, placing it in the pH range of 6 to 8.

Aqueous Solutions

Aqueous solutions are formed when a substance is dissolved in water. The interaction between the solute and water molecules can significantly impact the pH of the solution.
Understanding the behavior of aqueous solutions aids in anticipating how different compounds will influence their pH levels.

• The term "aqueous" indicates the presence of water as the solvent.
• Aqueous solutions can be acidic, basic, or neutral.
• Water's amphiprotic nature allows it to act as either an acid or base depending on the other substances present.

Many of the properties of solutions, such as conductivity and reactivity, depend on whether the solution is acidic, basic, or neutral. Knowing these properties helps in many scientific applications, from chemistry experiments to biological processes.