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Ch.13 - Solutions & Their Properties
Chapter 13, Problem 79

Lactose, C12H22O11, is a naturally occurring sugar found in mammalian milk. A 0.335 M solution of lactose in water has a density of 1.0432 g/L at 20 °C. What is the concentra- tion of this solution in the following units? (b) Mass percent

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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. In this case, a 0.335 M solution of lactose indicates that there are 0.335 moles of lactose in every liter of the solution. Understanding molarity is essential for converting between different concentration units, such as mass percent.
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Density

Density is the mass of a substance per unit volume, typically expressed in grams per liter (g/L) or grams per cubic centimeter (g/cm³). The density of the lactose solution at 1.0432 g/L is crucial for calculating the mass of the solution, which is necessary for determining mass percent. It allows us to relate the volume of the solution to its total mass.
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Mass Percent

Mass percent is a way of expressing the concentration of a component in a mixture, calculated as the mass of the solute divided by the total mass of the solution, multiplied by 100. To find the mass percent of lactose in the solution, one must first determine the mass of lactose and the total mass of the solution using the molarity and density values provided.
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