Lecture 09_2025

Recap – Molar Mass and Moles

  • Relative Atomic Mass: Weighted average mass of all isotopes of an element relative to 12C.

  • Avogadro's Number: (6.022 \times 10^{23}) – Number of atoms in exactly 12.00 g of 12C.

  • The Mole: Unit of measurement equivalent to Avogadro's number of objects. Formula: ( n = \frac{\text{mass}}{\text{molar mass}} )

Learning Outcomes

By the end of the lecture, students should be able to:

  • Perform stoichiometric calculations:

    • Calculate mass of product from the mass of starting materials and vice versa.

    • Identify the limiting reagent and use this in calculations.

    • Determine the % yield in a reaction based on starting materials.

Stoichiometry

Q: What masses of Na and Cl2 are needed to produce 100 g NaCl?

  • Balanced Equation: [ 2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl} ]

    • Molar masses: ( \text{Na} = 23.0 g/mol, \text{Cl}_2 = 71.0 g/mol, \text{NaCl} = 58.5 g/mol )

  • Calculation:

    1. Determine Moles of NaCl: ( n = \frac{100g}{58.5g/mol} = 1.71moles )

    2. Stoichiometry of Na: ( n_{Na} = 1.71 * \frac{1\text{ mol Na}}{1\text{ mol NaCl}} = 1.71 ext{ moles} )

    3. Calculate Mass of Na: ( m_{Na} = n \times M = 1.71 \times 23.0 = 39.3 g )

    4. For Cl2: ( n_{Cl2} = \frac{1.71}{2} = 0.85 ) moles, mass computed similarly to sodium.

Limiting Reagents

  • Definition: A limiting reagent is the reactant that is fully consumed during a reaction, limiting the amount of product formed.

  • Example Calculation: H2 + F2 → 2HF

    • If 5.00 g H2 (Molar mass: 2.01 g/mol, moles = 2.49) and 10.0 g F2 (Molar mass: 38.0 g/mol, moles = 0.263) are mixed, F2 is the limiting reagent as it limits the formation of HF.

% Yield Calculation

  • Formula: ( \text{% Yield} = \frac{\text{actual mass of product}}{\text{max theoretical mass}} \times 100 )

Example: Yield from Sodium Reaction

  • Given: 5.0 g Na reacts to produce 12.4 g NaCl.

    1. Calculate Theoretical Mass of NaCl: Moles of Na = ( n_{Na} = 0.217 ) moles resulting in 12.7 g NaCl.

    2. Calculate % yield: ( \frac{12.4g}{12.7g} \times 100 = 97.6% )

Questions for Next Lecture

  1. Write a balanced equation for the complete combustion of carbon to form carbon dioxide.

  2. Determine moles of carbon in 100 g of carbon.

  3. Identify limiting reagent in combustion performed in air.

  4. Calculate moles of CO2 produced from complete combustion of 100 g carbon.

  5. Determine mass of CO2 from combustion of 100 g carbon.

  6. Calculate % yield from 348 g of CO2 from combustion of 100 g carbon.