Chem 101: Stoichiometry and Molarity 5b

Stoichiometry Overview

General Overview

  • Course: Chem 101

  • Instructor: Prof. Drew Musacchio

  • Institution: The University at Buffalo, The State University of New York

  • Chapter Overview: Introduction to key concepts around stoichiometry, including limiting reagents and percent yield.

Chapter 5a: Stoichiometry, Limiting Reagent, Percent Yield

  • Key Concepts:

    • Understanding how to convert between different compounds in a chemical reaction.

    • Determining which reagent controls how far the reaction proceeds.

Chapter 5b: Concentration (Molarity), Dilutions, Solution Stoichiometry, Titrations

  • Key Concepts:

    • Conversion between volume and moles of substances.

    • Application of the dilution equation.

    • Stoichiometric calculations involving the volumes of materials used in reactions.

Molarity

1. Definitions

  • Solution: A homogeneous mixture composed of two or more substances, such as saltwater (NaCl + H2O).

    • Solvent: The major component of the solution (e.g., H2O in saltwater).

    • Solute: The substance that is being dissolved in the solution (e.g., NaCl).

  • Concentration: The amount of solute in a given quantity of solvent.

  • Molarity (M): A specific type of concentration that measures moles of solute in a liter of solution.

    • Formula:
      Molarity = \frac{mol\,of\,solute}{L\,of\,solution}

  • Critical Chemistry Accounting Unit: Understanding Molarity is crucial for lab measurements where concentration impacts reactions.

2. Example of Molarity

  • Comparison: Two solutions may have similar quantities of caffeine, but their volumes differ, resulting in different concentrations.

Molarity Practice Problems

Problem 1: Molarity Calculation

  • Given: 7.5 mol of NaCl dissolved in 1.5 L of water.

  • Calculation:
    M = \frac{7.5\, mol\, NaCl}{1.5\, L} = 5.0\, M\, NaCl

Problem 2: Concentration from Volume

  • Question: Determine how many moles of NaNO3 are contained in 538 mL of a 1.94 M solution of NaNO3.

Problem 3: Volume from Molarity

  • Given: 0.597 M solution of AlCl3 that contains 2.45 mol of AlCl3.

  • Question: Calculate the volume of the solution.

Molarity of Ions Practice Problems

Problem 1: Molarity Calculation

  • Given: 7.93 g of Fe2(CO3)3 (Molar Mass = 291.73 g/mol) dissolved in 680 mL of solution.

  • Molarity of Solution: Calculate the molarity of Fe2(CO3)3 in solution and the molarity of its ions:

    • Fe3+: 0.0400 M

    • CO3^2-: 0.120 M

Dilution

1. Concept of Dilution

  • Definition: Dilution is the process of making a new solution from a concentrated solution.

2. Dilution of Concentrated Solutions

  • Equation for Dilution: M1V1 = M2V2

    • Where:

    • $M_1$ = initial molarity

    • $V_1$ = initial volume

    • $M_2$ = final molarity

    • $V_2$ = final volume

3. Dilution Practice Problem

  • Given: A solution of 7.93 g of Fe2(CO3)3 in 680 mL and asked to calculate the volume needed to prepare a 0.0100 M solution.

  • Answer: 75.0 mL needed from the original solution.

Solution Stoichiometry

1. Basic Stoichiometric Principles

  • Balanced Equations: These equations provide the necessary stoichiometric relationships in moles.

  • Methodology: Calculate the moles of substance A from the volume of solution using their molarity.

    • Conversion Steps:
      \text{Given: Volume of A} \Rightarrow \text{Molarity of A} \Rightarrow \text{Stoichiometry} \Rightarrow \text{Moles of B}

Titration

1. Definition of Titration

  • Titration: A laboratory technique used to determine the moles of a substance through reaction with a solution of known concentration.

    • Chemical Equation Example:
      H^+ + OH^- \rightarrow H_2O

    • Key Point: At the equivalence point, moles of H+ equal moles of OH-.

2. Methodology of Titration

  • React a solution of unknown concentration with a solution of known concentration to find the concentration of the unknown.

Titration Practice Problems

Problem 1: Finding Concentration of HCl

  • Given: A titration of 25.0 mL HCl with a 2.00 M NaOH standard solution; buret readings from 2.17 mL to 39.42 mL.

  • Question: What is the concentration of HCl?

Problem 2: Molarity of Hydrochloric Acid

  • Given: A 32.0 mL sample of HCl requires 54.7 mL of a 0.231 M barium hydroxide for complete neutralization.

  • Answer: The molarity of hydrochloric acid is 0.790 M.