Describe how you would prepare each of the following aqueous solutions from water and solid KBr: b. 125 g of 0.180 m KBr.

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Calculate the moles of KBr needed: Use the formula for molality, \( m = \frac{\text{moles of solute}}{\text{kg of solvent}} \), to find the moles of KBr required.

Determine the mass of water: Since the solution is 125 g, subtract the mass of KBr from 125 g to find the mass of water.

Convert the mass of water to kilograms: This is necessary because molality is defined in terms of kilograms of solvent.

Weigh the calculated mass of KBr: Use a balance to measure the exact mass of KBr needed.

Dissolve the KBr in water: Add the KBr to the calculated mass of water and stir until the KBr is completely dissolved.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molarity and Molality

Molarity (M) is a measure of concentration defined as the number of moles of solute per liter of solution, while molality (m) is defined as the number of moles of solute per kilogram of solvent. In this question, the solution is specified in molality, which is important for understanding how to prepare the solution based on the mass of KBr and the mass of water used.

Calculating Moles

To prepare a solution, it is essential to calculate the number of moles of solute required. The number of moles can be determined using the formula: moles = mass (g) / molar mass (g/mol). For KBr, the molar mass is approximately 119 g/mol, which will be used to convert the mass of KBr into moles for the desired molality.

Solution Preparation

Preparing a solution involves dissolving a calculated amount of solute in a specific amount of solvent. For a 0.180 m KBr solution, you would need to dissolve the calculated moles of KBr in enough water to achieve the desired molality, ensuring that the total mass of the solvent is accurate. This process requires careful measurement and mixing to ensure uniformity.

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