Ch.17 - Additional Aspects of Aqueous Equilibria

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.17 - Additional Aspects of Aqueous Equilibria

Problem 90a

Chapter 17, Problem 90a

A sample of 0.2140 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.0950 M NaOH. The acid required 30.0 mL of base to reach the equivalence point. (a) What is the molar mass of the acid?

Verified step by step guidance

Step 1: Calculate the moles of NaOH used in the titration. The molarity of a solution is defined as the moles of solute per liter of solution. Therefore, to find the moles of NaOH, multiply the volume of the NaOH solution (in liters) by its molarity.

Step 2: Since the unknown acid is monoprotic, it means that one mole of the acid reacts with one mole of NaOH. Therefore, the moles of the unknown acid are equal to the moles of NaOH calculated in step 1.

Step 3: The molar mass of a substance is defined as the mass of the substance divided by the number of moles. Therefore, to find the molar mass of the unknown acid, divide the mass of the acid (in grams) by the moles of the acid calculated in step 2.

Step 4: The result from step 3 will give you the molar mass of the unknown acid in grams per mole (g/mol).

Step 5: Check your answer to make sure it makes sense. The molar mass should be a positive number, and it should be within a reasonable range for a monoprotic acid.

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Key Concepts

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

Monoprotic Acid

A monoprotic acid is an acid that can donate only one proton (H+) per molecule during a chemical reaction. This characteristic simplifies titration calculations, as the amount of base required to neutralize the acid directly correlates to the number of moles of the acid present in the solution.

Titration and Equivalence Point

Titration is a quantitative analytical method used to determine the concentration of a solute in a solution. The equivalence point is reached when the amount of titrant added is stoichiometrically equivalent to the amount of substance being titrated, indicating that the reaction is complete. This point is crucial for calculating the molar mass of the unknown acid.

Molar Mass Calculation

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). To calculate the molar mass of the unknown acid, one must first determine the number of moles of acid neutralized by the titrant using the volume and concentration of the NaOH solution, and then relate this to the mass of the acid sample.

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Textbook Question

A sample of 0.1687 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.1150 M NaOH. The acid required 15.5 mL of base to reach the equivalence point. (b) After 7.25 mL of base had been added in the titration, the pH was found to be 2.85. What is the Ka for the unknown acid?

The acid–base indicator bromcresol green is a weak acid. The yellow acid and blue base forms of the indicator are present in equal concentrations in a solution when the pH is 4.68. What is the pKa for bromcresol green?

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Textbook Question

Suppose you want to do a physiological experiment that calls for a pH 6.50 buffer. You find that the organism with which you are working is not sensitive to the weak acid H2A 1Ka1 = 2 * 10-2; Ka2 = 5.0 * 10-72 or its sodium salts. You have available a 1.0 M solution of this acid and a 1.0 M solution of NaOH. How much of the NaOH solution should be added to 1.0 L of the acid to give a buffer at pH 6.50? (Ignore any volume change.)

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