A solid sample of Zn(OH)2 is added to 0.350 L of 0.500 M aqueous HBr. The solution that remains is still acidic. It is then titrated with 0.500 M NaOH solution, and it takes 88.5 mL of the NaOH solution to reach the equivalence point. What mass of Zn(OH)2 was added to the HBr solution?

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Determine the initial moles of HBr in the solution using the formula: \( \text{moles of HBr} = M \times V \), where \( M = 0.500 \text{ M} \) and \( V = 0.350 \text{ L} \).

Calculate the moles of NaOH used in the titration using the formula: \( \text{moles of NaOH} = M \times V \), where \( M = 0.500 \text{ M} \) and \( V = 0.0885 \text{ L} \).

Since the solution is acidic after adding Zn(OH)₂, determine the moles of HBr that reacted with NaOH by recognizing that at the equivalence point, moles of NaOH = moles of HBr remaining.

Calculate the moles of HBr that reacted with Zn(OH)₂ by subtracting the moles of HBr that reacted with NaOH from the initial moles of HBr.

Use the stoichiometry of the reaction \( \text{Zn(OH)}_2 + 2\text{HBr} \rightarrow \text{ZnBr}_2 + 2\text{H}_2\text{O} \) to find the moles of Zn(OH)₂ that reacted, and then convert this to mass using the molar mass of Zn(OH)₂.

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Key Concepts

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

Acid-Base Neutralization

Acid-base neutralization is a chemical reaction where an acid reacts with a base to produce water and a salt. In this context, HBr, a strong acid, reacts with NaOH, a strong base, to reach the equivalence point, where the amount of acid equals the amount of base. Understanding this concept is crucial for calculating the amount of Zn(OH)2 that reacted with HBr.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced equations. In this problem, stoichiometric relationships will help determine how much Zn(OH)2 was present based on the volume and concentration of NaOH used during titration. This concept is essential for converting moles of NaOH to moles of Zn(OH)2.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole. To find the mass of Zn(OH)2 added to the solution, it is necessary to calculate the moles of Zn(OH)2 from the stoichiometry of the reaction and then convert that to mass using its molar mass. This concept is fundamental for translating moles into grams.

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