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Ch.5 - Introduction to Solutions and Aqueous Solutions
Chapter 5, Problem 73

The density of a 20.0% by mass ethylene glycol (C2H6O2) solution in water is 1.03 g/mL. Find the molarity of the solution.

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Calculate the mass of ethylene glycol in 1 liter of solution. Since the density is 1.03 g/mL, 1 liter (1000 mL) of solution weighs 1030 grams. With a 20.0% by mass concentration, the mass of ethylene glycol can be calculated as 20.0% of 1030 grams.
Convert the mass of ethylene glycol to moles. The molar mass of ethylene glycol (C<sub>2</sub>H<sub>6</sub>O<sub>2</sub>) is calculated by adding the molar masses of carbon (12.01 g/mol), hydrogen (1.008 g/mol), and oxygen (16.00 g/mol). Use this molar mass to convert the mass of ethylene glycol to moles.
Calculate the volume of the solution in liters. Since the density is given and you have the mass of the solution, use the formula: Volume = Mass / Density.
Calculate the molarity of the solution using the formula: Molarity = Moles of solute / Volume of solution in liters.
Ensure all units are consistent (grams, moles, liters) throughout the calculations to avoid any errors in the final molarity.

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Key Concepts

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

Density

Density is defined as mass per unit volume, typically expressed in grams per milliliter (g/mL) or kilograms per cubic meter (kg/m³). In this context, the density of the solution helps to relate the mass of the solution to its volume, which is essential for calculating molarity. Understanding how to manipulate density allows for the conversion between mass and volume, a crucial step in determining the concentration of a solution.
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Molarity

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution (mol/L). It is a key concept in solution chemistry, allowing chemists to quantify how much solute is present in a given volume of solution. To find the molarity of the ethylene glycol solution, one must calculate the number of moles of ethylene glycol and divide it by the total volume of the solution in liters.
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Mass Percent Concentration

Mass percent concentration is a way to express the concentration of a solution, calculated as the mass of solute divided by the total mass of the solution, multiplied by 100. In this case, the 20.0% by mass indicates that there are 20 grams of ethylene glycol in every 100 grams of solution. This concept is important for determining the amount of solute present, which is necessary for calculating molarity when combined with the solution's density.
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