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Ch.9 Solutions
McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.9 SolutionsProblem 50
Chapter 9, Problem 50

How many moles of each substance are needed to prepare the following solutions?
a. 50.0 mL of 8.0% (m/v) KCl (MW = 74.55 g/mol)
b. 200.0 mL of 7.5% (m/v) acetic acid (MW = 60.05 g/mol)

Verified step by step guidance
1
Step 1: Understand the problem. The (m/v) percentage indicates the mass of solute (in grams) per 100 mL of solution. To find the moles of solute, we need to calculate the mass of solute first using the given volume and percentage, then convert the mass to moles using the molar mass (MW).
Step 2: For part (a), calculate the mass of KCl required. Use the formula: \( \text{Mass of solute (g)} = \text{(m/v percentage)} \times \frac{\text{Volume of solution (mL)}}{100} \). Substitute \( 8.0\% \) and \( 50.0 \ \text{mL} \) into the formula.
Step 3: Convert the mass of KCl obtained in Step 2 to moles using the molar mass of KCl. Use the formula: \( \text{Moles of KCl} = \frac{\text{Mass of KCl (g)}}{\text{Molar mass of KCl (g/mol)}} \). Substitute the calculated mass and \( 74.55 \ \text{g/mol} \).
Step 4: For part (b), calculate the mass of acetic acid required. Use the same formula as in Step 2: \( \text{Mass of solute (g)} = \text{(m/v percentage)} \times \frac{\text{Volume of solution (mL)}}{100} \). Substitute \( 7.5\% \) and \( 200.0 \ \text{mL} \) into the formula.
Step 5: Convert the mass of acetic acid obtained in Step 4 to moles using the molar mass of acetic acid. Use the formula: \( \text{Moles of acetic acid} = \frac{\text{Mass of acetic acid (g)}}{\text{Molar mass of acetic acid (g/mol)}} \). Substitute the calculated mass and \( 60.05 \ \text{g/mol} \).

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Key Concepts

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

Molarity and Moles

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. To find the number of moles, one can use the formula: moles = mass (g) / molar mass (g/mol). Understanding how to convert between mass and moles is essential for preparing solutions accurately.
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Percent by Mass/Volume (m/v)

Percent by mass/volume (m/v) is a way to express the concentration of a solution, indicating the grams of solute per 100 mL of solution. For example, an 8.0% (m/v) KCl solution contains 8.0 grams of KCl in 100 mL of solution. This concept is crucial for calculating the required mass of solute for a given volume of solution.
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Dilution and Solution Preparation

Dilution involves reducing the concentration of a solute in a solution, typically by adding more solvent. When preparing solutions, it is important to calculate the correct amount of solute needed based on the desired concentration and volume. This ensures that the final solution meets the specified concentration requirements.
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Related Practice
Textbook Question

What is the mass/volume percent concentration of the following solutions?

a. 0.078 mol KCl in 75 mL of solution

b. 0.044 mol sucrose (C12H22O11) in 380 mL of solution

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Nalorphine, a relative of morphine, is used to combat withdrawal symptoms in heroin users. How many milliliters of a 0.40% (m/v) solution of nalorphine must be injected to obtain a dose of 1.5 mg?

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How is volume/volume percent concentration defined and for what types of solutions is it typically used?

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Textbook Question

If you had only 23 g of KOH remaining in a bottle, how many milliliters of 10.0% (m/v) solution could you prepare? How many milliliters of 0.25 M solution?

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Textbook Question

A dilute aqueous solution of boric acid, H3BO3 is often used as an eyewash. How would you prepare 500.0 mL of a 0.50% (m/v) boric acid solution?

Textbook Question

Sodium thiosulfate (Na2S2O3) the major component in photographic fixer solution, reacts with silver bromide to dissolve it according to the following reaction:

AgBr(s) + 2 Na2S2O3(aq) → Na3Ag(S2O3)2(aq) + NaBr(aq)

b. How many mL of 0.02 M Na2S2O3 contain this number of moles?