(a) The EPA threshold for acceptable levels of lead ions in water is 615 ppb. What is the molarity of an aqueous solution with a concentration of 15 ppb?

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Understand the concept of parts per billion (ppb). It is a unit of concentration that denotes the number of parts of a substance in a billion parts of the solution. For lead ions in water, 15 ppb means 15 grams of lead ions in 1 billion grams of water.

Convert the concentration from ppb to grams per liter. Since 1 billion grams of water is approximately 1 million liters (assuming the density of water is 1 g/mL), 15 ppb is equivalent to 15 grams of lead ions per 1 million liters of water.

Calculate the number of moles of lead ions. Use the molar mass of lead (Pb), which is approximately 207.2 g/mol. The formula to find moles is: \( \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \).

Determine the molarity of the solution. Molarity (M) is defined as the number of moles of solute per liter of solution. Use the formula: \( M = \frac{\text{moles of solute}}{\text{liters of solution}} \).

Combine the calculations to find the molarity. Use the number of moles calculated in step 3 and the volume in liters from step 2 to find the molarity of the solution.

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Key Concepts

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

Parts Per Billion (ppb)

Parts per billion (ppb) is a unit of measurement used to describe the concentration of a substance in a solution. It indicates how many parts of a substance are present in one billion parts of the total solution. In the context of the question, 15 ppb means that there are 15 micrograms of lead ions in one liter of water.

Molarity

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. It is expressed in moles per liter (mol/L). To convert ppb to molarity, one must first convert the mass of the solute to moles using its molar mass and then divide by the volume of the solution in liters.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). For lead (Pb), the molar mass is approximately 207.2 g/mol. This value is essential for converting the mass of lead ions in ppb to moles, which is necessary for calculating the molarity of the solution.

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