Question

An aqueous solution of \(1 \mathrm{M} \mathrm{NaCl}\) and \(\mathrm{lM} \mathrm{HCl}\) is: (a) Not a buffer but \(\mathrm{pH}<7\) (b) Not a buffer but \(\mathrm{pH}>7\) (c) A buffer with \(\mathrm{pH}<7\) (d) A buffer with \(\mathrm{pH}>7\)

Short answer

(a) Not a buffer but \( \text{pH} < 7 \).

Step-by-step solution

Define Buffer Solution

A buffer solution is comprised of a weak acid and its conjugate base or a weak base and its conjugate acid. It resists changes in pH upon the addition of small amounts of acid or base.

Analyze Given Solution Components

In the given solution, we have 1M NaCl, a salt, and 1M HCl, a strong acid. A buffer requires a weak acid and its conjugate base (or vice versa), but HCl is a strong acid, and NaCl doesn't act as an acid or base.

Determine the Nature of the Solution

Since the solution contains a strong acid (HCl) and a neutral salt (NaCl), it cannot act as a buffer. Instead, the pH of the solution will be determined largely by the 1M HCl.

Calculate the pH of the Solution

The concentration of HCl is 1M, and since HCl fully dissociates in water, the concentration of hydrogen ions [H+] will also be 1M. The pH is calculated using the formula: \[ pH = -\log[H^+]= -\log(1) = 0 \]Therefore, the solution's pH is less than 7.

Select the Correct Answer

The solution is not a buffer because it does not contain a weak acid and its conjugate base. With a pH of 0, the solution satisfies the condition for "not a buffer but \( \text{pH} < 7 \)."

Key concepts

pH Calculation

Understanding how to calculate the pH of a solution is fundamental in acid-base chemistry. The pH quantifies the acidity or basicity of a solution. A lower pH value, less than 7, indicates an acidic solution, while a value greater than 7 suggests a basic solution. The formula to calculate pH is quite simple:
\[\text{pH} = -\log[H^+]\]
Here, \([H^+]\) represents the concentration of hydrogen ions in moles per liter. When you encounter a strong acid like HCl, understanding that it completely dissociates in water is crucial. Therefore, for a 1M HCl solution, the hydrogen ion concentration, \([H^+]\), equals 1M. When you apply the formula:

• If \([H^+]\) equals 1M, then pH = \(-\log(1) = 0\).

Thus, the calculated pH is 0, easily confirming that the solution is acidic since its pH is less than 7. This simple yet powerful calculation helps you grasp the acidity or basicity of various solutions quickly.

Acid-Base Chemistry

Acid-base chemistry is the study of acidic and basic molecules and how they interact in solution. A critical aspect of this field involves understanding how these substances influence the pH of solutions. An acid is a molecule that can donate a proton (H+), whereas a base can accept a proton. The strength of an acid or base refers to its degree of ionization in water.
- **Strong Acids/Bases**: These substances completely dissociate into ions in aqueous solutions.- **Weak Acids/Bases**: These do not fully ionize, setting up an equilibrium between the forward and reverse reactions.NaCl, mentioned in the exercise, behaves neutrally in water, meaning it doesn't affect the pH or act as a weak or strong acid/base. On the other hand, HCl is a prototypical strong acid. It disassociates entirely in water:\[\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-\]
This reaction makes it incapable of forming a buffer solution, which requires a weak acid or base and its conjugate. Acid-base chemistry concepts are fundamental in predicting and explaining the behavior of solutions and reactions in chemistry.

Strong Acids

Strong acids are acids that completely dissociate in water, releasing hydrogen ions (H+) into the solution. This total dissociation distinguishes them from weak acids, which only partially dissociate. Common examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).
When you look at a solution like 1M HCl, you should expect it to behave predictably by fully ionizing:

• The hydrogen ions contribute a high concentration, leading to a low pH.
• Due to complete dissociation, pH calculations are straightforward as they directly reflect the initial concentration of the acid.

These properties make strong acids vital in various industries and laboratories, known for their effective and clear-cut chemical behavior. Knowing that HCl dissociates completely explains why our initial solution cannot form a buffer; buffers require partially dissociated weak acids or bases. This concept of strong acids helps us predict outcomes in chemical solutions efficiently.