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Ch.4 - Reactions in Aqueous Solution
Chapter 4, Problem 101a

Hard water contains Ca2+, Mg2+, and Fe2+, which interfere with the action of soap and leave an insoluble coating on the insides of containers and pipes when heated. Water softeners replace these ions with Na+. Keep in mind that charge balance must be maintained. (a) If 1500 L of hard water contains 0.020 M Ca2+ and 0.0040 M Mg2+, how many moles of Na+ are needed to replace these ions?

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Key Concepts

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

Ion Concentration and Molarity

Molarity (M) is a measure of concentration defined as the number of moles of solute per liter of solution. In this context, the concentrations of Ca2+ and Mg2+ ions in hard water are given in molarity, which allows us to calculate the total number of moles present in a specific volume of water. Understanding molarity is essential for determining how many moles of Na+ are needed to replace these ions.
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Charge Balance in Solutions

Charge balance refers to the principle that in a neutral solution, the total positive charge must equal the total negative charge. In the case of water softening, replacing Ca2+ and Mg2+ (both divalent cations) with Na+ (a monovalent cation) requires careful calculation to ensure that the overall charge remains balanced. This concept is crucial for understanding how to effectively soften water without altering its overall ionic balance.
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Stoichiometry in Chemical Reactions

Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced equations. In this scenario, stoichiometric principles are applied to determine the exact number of moles of Na+ needed to replace the moles of Ca2+ and Mg2+ in the hard water. This concept is fundamental for solving problems related to ion exchange and ensuring that the correct amounts of substances are used.
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Related Practice
Textbook Question

Bronze is a solid solution of Cu(s) and Sn(s); solutions of metals like this that are solids are called alloys. There is a range of compositions over which the solution is considered a bronze. Bronzes are stronger and harder than either copper or tin alone. (b) Based on part (a), calculate the concentration of the solute metal in the alloy in units of molarity, assuming a density of 7.9 g/cm3.

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Textbook Question

Bronze is a solid solution of Cu(s) and Sn(s); solutions of metals like this that are solids are called alloys. There is a range of compositions over which the solution is considered a bronze. Bronzes are stronger and harder than either copper or tin alone. (c) Suggest a reaction that you could do to remove all the tin from this bronze to leave a pure copper sample. Justify your reasoning.

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Textbook Question

Neurotransmitters are molecules that are released by nerve cells to other cells in our bodies, and are needed for muscle motion, thinking, feeling, and memory. Dopamine is a common neurotransmitter in the human brain. (c) Experiments with rats show that if rats are dosed with 3.0 mg/kg of cocaine (that is, 3.0 mg cocaine per kg of animal mass), the concentration of dopamine in their brains increases by 0.75 mM after 60 seconds. Calculate how many molecules of dopamine would be produced in a rat (average brain volume 5.00 mm3) after 60 seconds of a 3.0 mg/kg dose of cocaine.

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Textbook Question

Hard water contains Ca2+, Mg2+, and Fe2+, which interfere with the action of soap and leave an insoluble coating on the insides of containers and pipes when heated. Water softeners replace these ions with Na+. Keep in mind that charge balance must be maintained. (b) If the sodium is added to the water softener in the form of NaCl, how many grams of sodium chloride are needed?

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Textbook Question

Citric acid, C6H8O7, is a triprotic acid. It occurs naturally in citrus fruits like lemons and has applications in food flavouring and preservatives. A solution containing an unknown concentration of the acid is titrated with KOH. It requires 23.20 mL of 0.500 M KOH solution to titrate all three acidic protons in 100.00 mL of the citric acid solution. Write a balanced net ionic equation for the neutralization reaction.

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Textbook Question

Citric acid, C6H8O7, is a triprotic acid. It occurs naturally in citrus fruits like lemons and has applications in food flavouring and preservatives. A solution containing an unknown concentration of the acid is titrated with KOH. It requires 23.20 mL of 0.500 M KOH solution to titrate all three acidic protons in 100.00 mL of the citric acid solution. Calculate the molarity of the citric acid solution.

2019
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