Describe how you would prepare each of the following aqueous solutions, starting with 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. Rearrange the formula to find moles: \( \text{moles of KBr} = m \times \text{kg of solvent} \).

Determine the mass of water needed: Since the problem specifies 125 g of solution, convert this mass to kilograms (0.125 kg) and use it as the mass of the solution. Subtract the mass of KBr (which you will calculate next) from the total mass of the solution to find the mass of water.

Calculate the mass of KBr: Use the molar mass of KBr (approximately 119 g/mol) to convert the moles of KBr (calculated in step 1) to grams.

Mix the KBr with water: Weigh the calculated mass of KBr and dissolve it in the calculated mass of water (from step 2) to prepare the solution.

Ensure complete dissolution: Stir the mixture until all the KBr is completely dissolved in the water, ensuring a homogeneous solution.

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Key Concepts

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

Molarity

Molarity (M) is a measure of concentration defined as the number of moles of solute per liter of solution. It is crucial for preparing solutions, as it allows chemists to quantify how much solute is needed to achieve a desired concentration. In this case, a 0.180 M solution of KBr means that there are 0.180 moles of KBr in every liter of the solution.

Moles and Molar Mass

To prepare a solution, it is essential to understand the relationship between moles and molar mass. The molar mass of KBr is approximately 119.0 g/mol, which means that one mole of KBr weighs 119.0 grams. To find out how many grams are needed for a specific molarity, you can use the formula: grams = moles × molar mass.

Dilution and Solution Preparation

Preparing a solution involves dissolving a specific amount of solute in a solvent, typically water, to achieve the desired concentration. For the given problem, you would first calculate the number of moles required for 125 g of a 0.180 M KBr solution, then weigh out the appropriate mass of KBr, and finally dissolve it in enough water to reach the total volume of the solution, ensuring thorough mixing.

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