Determining Molar Concentration and pNO3 in Mixture Solutions

In analytical chemistry and environmental science, it is often necessary to determine the concentrations of specific ions in a mixture of solutions. This is particularly useful in understanding chemical reactions, monitoring water quality, and ensuring the safety and efficacy of various products. This article will guide you through the process of calculating the molar concentration of NO3- (nitrate ion) in a solution prepared by mixing two distinct solutions. Additionally, we will explore how to calculate the pNO3 for this mixture.

Understanding the Problem

Given two solutions, we need to determine the combined concentration of nitrate ions after mixing them. The first solution consists of 50.0 mL of 0.050 M KNO3, and the second solution contains 40.0 mL of 0.075 M NaNO3.

Step-by-Step Solution

Step 1: Calculating Moles of Nitrate Ions from Each Solution

First, we calculate the moles of NO3- in each solution. The nitrate ion (NO3-) is present in both KNO3 and NaNO3 in a 1:1 molar ratio.

KNO3 Solution:

[text{Moles of } NO_3^- text{Molarity} times text{Volume (in L)} 0.050 text{ mol/L} times 0.050 text{ L} 0.0025 text{ mol}]

NaNO3 Solution:

[text{Moles of } NO_3^- text{Molarity} times text{Volume (in L)} 0.075 text{ mol/L} times 0.040 text{ L} 0.0030 text{ mol}]

Step 2: Calculating the Total Moles of Nitrate Ions

To find the total moles of nitrate ions in the mixture:

[text{Total moles of } NO_3^- 0.0025 text{ mol} 0.0030 text{ mol} 0.0055 text{ mol}]

Step 3: Calculating the Total Volume of the Mixture

The total volume of the mixture is the sum of the volumes of the two solutions:

[text{Total volume} 50.0 text{ mL} 40.0 text{ mL} 90.0 text{ mL} 0.090 text{ L}]

Step 4: Calculating the Molar Concentration of Nitrate Ions

Now, we calculate the molar concentration of nitrate ions in the mixture:

[text{Concentration of } NO_3^- frac{text{Total moles of } NO_3^-}{text{Total volume}} frac{0.0055 text{ mol}}{0.090 text{ L}} approx 0.0611 text{ M}]

Step 5: Calculating the pNO3 for the Mixture

The pNO3 is the negative logarithm of the molar concentration of NO3- ions:

[text{pNO3} -log[NO_3^-] -log 0.0611 approx 1.214]

Conclusion and Practical Applications

The molar concentration of NO3- in the mixture is approximately 0.0611 M, and the pNO3 value is around 1.214. These values are critical for understanding the nature of the solution and can be used in various applications, including environmental monitoring, chemical analysis, and scientific research.

Understanding the concepts of molar concentration and the calculation of pH (represented here by pNO3) is essential for anyone involved in chemistry, environmental science, and related fields. By following these steps, you can accurately determine the concentrations of ions in complex mixtures, ensuring that your experiments and analyses are both precise and reliable.