Problem 27 - Chemistry The Science in Context

Question

Describe a solution (solute and concentration) that has a negative pH value.

Step-by-Step Solution

Answer

Question: Describe a solution containing a solute and a specific concentration that results in a negative pH value. Answer: A solution containing hydrochloric acid (HCl) with a concentration greater than 1 M will have a negative pH value, as this strong acid dissociates completely in water, leading to a high hydrogen ion concentration and a negative pH according to the pH = -log[H+] formula.

Step 1: Understanding the pH scale and formula

The pH scale is a measure of the acidity or basicity of a solution, ranging from 0 to 14. A pH value of 7 indicates a neutral solution, while values less than 7 indicate acidic solutions and values greater than 7 indicate basic solutions. The pH of a solution is calculated using the formula: pH = -log[H+], where [H+] is the hydrogen ion concentration in moles per liter (M). The goal in this exercise is to find a solution with a negative pH value, meaning [H+] must be greater than 1 M.

Step 2: Understanding the solute

A solute with a strong acidic nature will be required for the solution to have a negative pH value. For this example, we will use a strong acid, such as hydrochloric acid (HCl). When dissolved in water, HCl dissociates completely to release hydrogen ions (H+) and chloride ions (Cl-). This strong acid will result in a high [H+] and, consequently, a negative pH value given the appropriate concentration.

Step 3: Finding the concentration that results in a negative pH value

As stated earlier, to achieve a negative pH value, the [H+] in the solution should be greater than 1 M. Since HCl completely dissociates in water, the [H+] will be equal to the concentration of HCl. Therefore, we must use a concentration of HCl that is greater than 1 M.

Step 4: Describing the solution with a negative pH value

In conclusion, a solution containing hydrochloric acid (HCl) with a concentration greater than 1 M will have a negative pH value. This strong acid, when dissolved in water, will result in a high hydrogen ion concentration, leading to a negative pH value as calculated by the pH = -log[H+] formula.

Key Concepts

pH Scale

The pH scale is a numerical representation of acidity or basicity in a solution. It typically ranges from 0, representing extreme acidity, to 14, representing extreme basicity, with 7 being neutral, like pure water. The scale is logarithmic, meaning each whole number change represents a tenfold increase or decrease in acidity. So, a substance with a pH of 3 is ten times more acidic than one with a pH of 4. When dealing with highly acidic solutions, it is possible to encounter a pH value that is actually negative. This often confounds students, as negative pH values seem counterintuitive within the 0 to 14 range. However, the logarithmic nature of the scale makes this perfectly valid for extremely acidic solutions with high hydrogen ion concentrations.

Hydrogen Ion Concentration

Hydrogen ion concentration, denoted by \[H+\], is the amount of hydrogen ions present in a solution, measured in moles per liter (M). In aqueous solutions, hydrogen ions are of particular interest because they define the solution's acidity. The relationship between \[H+\] and pH is inverse and logarithmic; as the concentration of hydrogen ions increases, the pH value decreases. This inverse relation underpins the necessity for a high \[H+\] to achieve a negative pH. A concentration greater than 1 mole per liter (1M) will lead to negative pH values upon applying the formula pH = -log\[H+\].

Strong Acids

Strong acids are substances that completely dissociate in water, releasing all their hydrogen ions into the solution. This complete dissociation is why they are capable of creating solutions with very low pH values, including negative pH. Examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). It is crucial to note that not all strong acids will result in a negative pH by default but can do so at high concentrations because of their complete ionization in the solution. This characteristic is essential for students to understand when tackling problems involving pH calculations.

Hydrochloric Acid (HCl)

Hydrochloric acid (HCl) is a commonly used strong acid, often referenced when studying chemistry and pH levels. When dissolved in water, HCl dissociates completely into hydrogen ions (H+) and chloride ions (Cl-), making it an excellent example of a strong acid. To visualize the significance of HCl in terms of pH values, consider a bottle of concentrated HCl with a concentration greater than 1 M. This concentration grants it a negative pH value due to the abundance of released hydrogen ions. Hydrochloric acid is not only fundamental in academic exercises but also plays a significant role in various industries and our digestive system, where it aids in food digestion.