Understanding the pH of a 10 g/L NaHCO3 Solution

Sodium bicarbonate, or NaHCO3, is a solid at room temperature and is commonly used in its solution form. The pH of a 10 g/L NaHCO3 solution is about 8.3. This article will guide you through the process of calculating this pH, explaining the significance of each step and the underlying chemistry involved.

Calculation Steps for the pH of a 10 g/L NaHCO3 Solution

To determine the pH of a 10 g/L NaHCO3 solution, we need to follow a series of steps involving molar concentration, dissociation constants, and equilibrium concentrations. Here, we will break it down step-by-step.

Step 1: Calculate the Molar Concentration

The first step involves converting the given mass of NaHCO3 to its molar concentration. The molar mass of NaHCO3 is approximately 84.0066 g/mol. Therefore, the molar concentration (c) can be calculated as follows:

c m / M x V 10 g / 84.0066 g/mol x 1 L 0.119 mol/L

Step 2: Determine the Equilibrium Concentration

NaHCO3 is a weak acid that partially dissociates in water to produce HCO3- and OH- ions. The dissociation constant (Kb) for the carbonate ion (CO32-) can be derived from the given pKa value (6.37). The pOH can be calculated as:

pOH pKa log([A-]/[HA]), where A- is CO32- and HA is H2CO3

Since the pOH is related to the pH by the equation pH pOH 14, we can calculate the pH as follows:

pH 14 - pOH 14 - 6.37 log(0.119) 8.3 - 0.05 8.25

Step 3: Calculate the H Concentration

The next step is to find the concentration of H using the acid dissociation constant (Ka) of HCO3- and the concentration of CO32-. The Ka for HCO3- is 4.8 x 10^-11, and the concentration of CO32- is 0.00033 M. The concentration of H can be calculated as follows:

[H ] Ka[HCO3-] / [CO32-] (4.8 x 10^-11) x 0.119 / 0.00033 5.96 x 10^-9 M

Step 4: Calculate the Final pH

Using the definition of pH, which is the logarithm of the hydrogen ion concentration, we can calculate the pH as:

pH -log[H ] -log(5.96 x 10^-9) 8.23

When rounded to one decimal place, the pH is 8.3.

Understanding the Significance of pH in Solution

Although NaHCO3 itself does not have a pH, a solution of NaHCO3 in water does have a pH. The pH of a NaHCO3 solution is determined by the concentration of NaHCO3 dissolved in a given volume of water. Therefore, it is important to understand the relationship between the concentration of NaHCO3 and the resulting pH in order to effectively use or manipulate such solutions.

For example, in the context of buffering, NaHCO3 is used as a component of buffer solutions to maintain a stable pH. The extent to which the pH is affected by changes in concentration of NaHCO3 can be significant, especially in biological and chemical applications.

Conclusion

Understanding the pH of a 10 g/L NaHCO3 solution is crucial for various applications, from chemical synthesis to biological uses. By following the steps outlined above, we can accurately determine the pH and gain insights into the behavior and properties of NaHCO3 in solution.