Problem 21 - Chemistry The Science in Context

Question

Calculate the number of moles of solute contained in the following volumes of aqueous solutions of four pesticides: a. \(0.400 \mathrm{L}\) of \(0.024 M\) lindane b. \(1.65 \mathrm{L}\) of \(0.473 \mathrm{m} M\) dieldrin c. \(25.8 \mathrm{L}\) of \(3.4 \mathrm{m} M\) DDT d. \(154 \mathrm{L}\) of \(27.4 \mathrm{m} \mathrm{M}\) aldrin

Step-by-Step Solution

Answer

Question: Calculate the number of moles of solute in the specified volumes of aqueous solutions of the following pesticides: a) Lindane (0.400 L, 0.024 M), b) Dieldrin (1.65 L, 0.473 M), c) DDT (25.8 L, 3.4 M), d) Aldrin (154 L, 27.4 M). Answer: a) 0.0096 mol Lindane, b) 0.78045 mol Dieldrin, c) 87.72 mol DDT, d) 4215.6 mol Aldrin.

Step 1: Identify given values

For lindane, the volume of the solution is \(0.400 \mathrm{L}\) and the molarity is \(0.024 M\).

Step 2: Calculate number of moles

Use the formula to calculate the number of moles by multiplying the volume by the molarity: Number of moles (lindane) = \(0.400 \mathrm{L} \times 0.024 M = 0.0096 \mathrm{mol}\) #b. Dieldrin#

Step 1: Identify given values

For dieldrin, the volume of the solution is \(1.65 \mathrm{L}\) and the molarity is \(0.473 M\).

Step 2: Calculate number of moles

Use the formula to calculate the number of moles by multiplying the volume by the molarity: Number of moles (dieldrin) = \(1.65 \mathrm{L} \times 0.473 M = 0.78045 \mathrm{mol}\) #c. DDT#

Step 1: Identify given values

For DDT, the volume of the solution is \(25.8 \mathrm{L}\) and the molarity is \(3.4 M\).

Step 2: Calculate number of moles

Use the formula to calculate the number of moles by multiplying the volume by the molarity: Number of moles (DDT) = \(25.8 \mathrm{L} \times 3.4 M = 87.72 \mathrm{mol}\) #d. Aldrin#

Step 1: Identify given values

For aldrin, the volume of the solution is \(154 \mathrm{L}\) and the molarity is \(27.4 M\).

Step 2: Calculate number of moles

Use the formula to calculate the number of moles by multiplying the volume by the molarity: Number of moles (aldrin) = \(154 \mathrm{L} \times 27.4 M = 4215.6 \mathrm{mol}\)

Key Concepts

Mole Concept

Understanding the mole concept is fundamental in chemistry. It provides a way to quantify the amount of substance present. The mole is a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles.

One mole is defined as the amount of substance that contains as many entities (atoms, ions, molecules, etc.) as there are atoms in 12 grams of pure carbon-12. This number is known as Avogadro's number, approximately equal to \(6.022 \times 10^{23}\).

When calculating the number of moles from a given mass, you would divide the mass by the molar mass (the mass of one mole) of the substance. Conversely, to find the mass from the number of moles, you would multiply the number of moles by the molar mass.

Molar Concentration

Molar concentration, also known as molarity, is one of the most commonly used measures of concentration in chemistry. Molarity is denoted by the symbol \(M\) and is defined as the number of moles of a solute dissolved in one liter of solution.

The formula to calculate molarity is \( M = \frac{n}{V} \), where \(n\) represents the number of moles of the solute, and \(V\) represents the volume of the solution in liters.

To determine the number of moles in a solution using its molarity, you simply multiply the volume of the solution by the molarity, as seen in the textbook solutions for calculating the moles of pesticides in aqueous solutions.

Stoichiometry

Stoichiometry is the section of chemistry that deals with the relative quantities of reactants and products in chemical reactions. It is based on the conservation of mass and the concept of the mole. In practice, stoichiometry is used to calculate amounts of reactants needed or products formed in a given chemical reaction.

The stoichiometric coefficients from a balanced chemical equation give the ratios of moles of each substance that react or are formed. When performing stoichiometric calculations, one often starts with a known amount of a substance and uses molar relationships to convert to the mass or volume of another substance involved in the reaction.

Aqueous Solutions

An aqueous solution is one where water acts as the solvent. It is the medium in which the ions and molecules of a solute are dispersed. Solutions come in various concentrations, where the solute interacts with the solvent at a molecular level.

In chemistry, many reactions occur in aqueous solutions, and understanding the concentration of solutions is crucial. The molarity of an aqueous solution affects properties such as boiling point, vapor pressure, and osmotic pressure, as well as the rates of reaction and equilibrium positions in chemical reactions.