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Ch.17 - Acids and Bases
Chapter 17, Problem 132

Lakes that have been acidified by acid rain can be neutralized by liming, the addition of limestone (CaCO3). How much limestone (in kg) is required to completely neutralize a 4.3 billion liter lake with a pH of 5.5?

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1
Determine the concentration of hydrogen ions \([H^+]\) in the lake using the pH value: \([H^+] = 10^{-pH}\).
Calculate the total moles of \([H^+]\) in the lake by multiplying the concentration of \([H^+]\) by the volume of the lake in liters.
Write the balanced chemical equation for the neutralization reaction: \(CaCO_3 + 2H^+ \rightarrow Ca^{2+} + CO_2 + H_2O\).
Use stoichiometry to find the moles of \(CaCO_3\) needed, knowing that 1 mole of \(CaCO_3\) neutralizes 2 moles of \(H^+\).
Convert the moles of \(CaCO_3\) to mass in kilograms using the molar mass of \(CaCO_3\) (100.09 g/mol).

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Key Concepts

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

pH and Acidity

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or alkalinity. A pH of 7 is neutral, while values below 7 indicate acidity. In this case, a pH of 5.5 suggests the lake is acidic, which can harm aquatic life. Understanding pH is crucial for determining how much limestone is needed to neutralize the lake.
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Neutralization Reaction

A neutralization reaction occurs when an acid reacts with a base to produce water and a salt, effectively reducing the acidity of the solution. In this scenario, limestone (CaCO3) acts as a base that reacts with the hydrogen ions in the acidic lake water, raising the pH towards neutrality. Knowing the stoichiometry of this reaction is essential for calculating the amount of limestone required.
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Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced equations. To determine how much limestone is needed, one must use the molar mass of CaCO3 and the volume of the lake to find the total moles of hydrogen ions present. This allows for the precise calculation of limestone required to achieve neutralization.
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