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In any chemical reaction, the limiting reactant is the substance that is completely consumed first, preventing further reaction from taking place. Identifying the limiting reactant is crucial because it determines the amount of product that can be formed in the reaction. In this exercise, you are tasked with identifying the limiting reactant when mixing hydrochloric acid (HCl) and ammonium chloride (NH4Cl).

To find the limiting reactant, start by calculating the moles of each reactant. You will do this using the molarity and volume of each solution:

• Moles of HCl: n( HCl ) = 0.5 M 脳 20 ml / 1000 = 0.01 moles
• Moles of NH4Cl: n( NH4Cl ) = 0.1 M 脳 80 ml / 1000 = 0.008 moles

Upon comparing these amounts, you will observe that NH4Cl provides fewer moles than HCl does. Therefore, NH4Cl will be entirely used up in the reaction, making it the limiting reactant. Remember, the limiting reactant is not always the one in the smallest initial quantity, but the one that determines the maximum amount of product because of the stoichiometry of the reaction.

Molarity is an important concept in chemistry that expresses the concentration of a solution. It is defined as the number of moles of a solute divided by the volume of the solution in liters. Molarity is used to conveniently express concentrations in the lab and is crucial when performing reactions that depend on precise amounts of reactants and products.

In this exercise, you are dealing with two solutions: HCl and NH4Cl. You need to calculate their molarity to determine the number of moles involved in the reaction. Calculations here are based on:

• Moles of a compound = Molarity 脳 Volume
• Remember to convert all volumes to liters for consistency.

For example, when calculating for HCl:
You have 20 ml of 0.5 M HCl. Convert 20 ml to liters by dividing by 1000. Then multiply by 0.5 to get the moles of HCl, which equals 0.01 moles.

Similarly, for NH4Cl, convert 80 ml to 0.08 L, multiply by 0.1 M, and find 0.008 moles. Understanding how to calculate molarity and moles directly impacts the ability to identify the limiting reactant and directly influences the calculation of concentrations after a reaction.

The pH scale is a measure of the acidity or basicity of a solution. In general, calculating the pH of mixtures involves understanding the concentration of hydrogen ions (H鈦�) remaining after the reaction. It reflects the completeness of the limiting reactant's consumption and any excess reactant.

In this exercise, once the limiting reactant NH4Cl is consumed, calculate the remaining concentration of H鈦� ions from HCl. Use the change in moles due to reaction:

• Initial moles of H鈦� provided by HCl is 0.01
• Moles of H鈦� reacting with NH4Cl is 0.008
• Therefore, remaining H鈦� = 0.01 0.008 = 0.002
• Total solution volume is 100 ml or 0.1 L, giving a concentration of 0.02 M

With the concentration known, calculate the pH using the formula:

\[\text{pH} = -\log_{10}(\text{Concentration of } \text{H}^+)\]Here, \(-\log_{10}(0.02) \approx 1.7\). Understanding how to compute and interpret these values is key not only to knowing the acidity of a solution but also for predicting reaction behavior.