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

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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Key Concepts

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

Ion Exchange

Ion exchange is a process where undesirable ions in water, such as Ca²⁺ and Mg²⁺, are replaced with more desirable ions, typically Na⁺ from sodium chloride (NaCl). This process is crucial in water softening, as it reduces the hardness of water, allowing soap to lather more effectively and preventing scale buildup in pipes.
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Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions. In the context of the question, it is used to determine the amount of NaCl needed to replace the hard water ions while maintaining charge balance. Understanding stoichiometric relationships is essential for accurately calculating the required mass of sodium chloride.
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Charge Balance

Charge balance refers to the principle that the total positive charge must equal the total negative charge in a solution. In the case of water softening, when replacing Ca²⁺ and Mg²⁺ ions with Na⁺ ions, it is important to ensure that the overall charge remains neutral. This concept is vital for understanding how many moles of Na⁺ are needed to replace the divalent cations.
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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. (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. (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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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.

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Textbook Question
(c) If 18.65 mL of the caesium hydroxide solution was needed to neutralize a 42.3 mL aliquot of the hydroiodic acid solution, what is the concentration (molarity) of the acid?
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