Household bleach is a 5.0 mass % aqueous solution of sodium hypochlorite, NaOCl. What is the molality of the bleach? What is the mole fraction of NaOCl in the bleach?

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Determine the mass of sodium hypochlorite (NaOCl) in 1000 g of bleach solution. Since the solution is 5.0 mass % NaOCl, calculate 5.0% of 1000 g.

Calculate the molar mass of NaOCl by adding the atomic masses of sodium (Na), oxygen (O), and chlorine (Cl).

Convert the mass of NaOCl obtained in step 1 to moles using the molar mass calculated in step 2.

Calculate the molality of the solution by dividing the number of moles of NaOCl by the mass of the solvent (water) in kilograms. Remember that the total mass of the solution is 1000 g, and you need to subtract the mass of NaOCl to find the mass of water.

To find the mole fraction of NaOCl, divide the moles of NaOCl by the total moles in the solution. This includes the moles of NaOCl and the moles of water (calculated by dividing the mass of water by the molar mass of water, approximately 18.015 g/mol).

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Key Concepts

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

Mass Percent Concentration

Mass percent concentration is a way to express the concentration of a solute in a solution. It is calculated as the mass of the solute divided by the total mass of the solution, multiplied by 100. In this case, a 5.0 mass % solution of sodium hypochlorite means that there are 5 grams of NaOCl in every 100 grams of the solution.

Molality

Molality is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. It is particularly useful in situations where temperature changes may affect volume, as it is based on mass rather than volume. To calculate molality, one must first determine the moles of NaOCl and the mass of the solvent (water) in kilograms.

Mole Fraction

Mole fraction is a way to express the concentration of a component in a mixture, defined as the ratio of the number of moles of that component to the total number of moles of all components in the mixture. For the bleach solution, the mole fraction of NaOCl can be calculated by determining the moles of NaOCl and the moles of water, then using these values to find the fraction of the total moles that NaOCl represents.

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