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Chapter 3

Stoichiometry

Shaun Williams, PhD

Chemical Stoichiometry

Counting by Weighing

Counting by Weighing

Exercise 1

A pile of marbles weigh 394.80 g. 10 marbles weigh 37.60 g. How many marbles are in the pile?

Exercise 1 - Answer

A pile of marbles weigh 394.80 g. 10 marbles weigh 37.60 g. How many marbles are in the pile?

$$ \text{Avg. Mass of 1 Marble}=\frac{37.60\,\chem{g}}{10\,\chem{marbles}} = 3.76\,\bfrac{\chem{g}}{\chem{marble}} $$ $$ \frac{394.80\,\chem{g}}{3.76\,\bfrac{\chem{g}}{\chem{marble}}} = 105\,\chem{marbles} $$

Atomic Masses

Atomic Masses

Average Atomic Mass for Carbon

A photograph of a mass spectrometer and a diagram showing the parts of a mass spectrometer.

Exercise 2

An element consists of 62.60% of an isotope with mass 186.956 u and 37.40% of an isotope with mass 184.953 u.

Calculate the average atomic mass and identify the element

Exercise 2 - Answer

An element consists of 62.60% of an isotope with mass 186.956 u and 37.40% of an isotope with mass 184.953 u.

Calculate the average atomic mass and identify the element

$$ 186.2\,\chem{u} $$ $$ \text{Rhenium (Re)} $$

The Mole

The Mole

Exercise 3

Calculate the number of iron atoms in a 4.48 mole sample of iron.

Exercise 3 - Answer

Calculate the number of iron atoms in a 4.48 mole sample of iron.

$$ 2.70 \times 10^{24}\,\chem{Fe\,atoms} $$

Molar Mass

Molar Mass

Concept Check!

Calculate the number of copper atoms in a 63.55 g sample of copper.

Concept Check! - Answer

Calculate the number of copper atoms in a 63.55 g sample of copper.

$$ 6.022 \times 10^{23}\,\text{Cu atoms} $$

Percent Composition of Compounds

Percent Composition

Determining the Formula of a Compound

Formulas

Analyzing for Carbon and Hydrogen

A diagram of a device used to sample and measure the products of burning a sample.

Exercise 4

The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about \(146\,\bfrac{\chem{g}}{\chem{mol}}\).

Exercise 4 - Answer

The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about \(146\,\bfrac{\chem{g}}{\chem{mol}}\).

Chemical Equations

Chemical Equation

$$ \chem{C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O} $$

Balancing Chemical Equations

Writing and Balancing the Equation for a Chemical Reaction

  1. Determine what reaction is occurring. What are the reactants, the products, and the physical states involved?
  2. Write the unbalanced equation that summarizes the reaction described in step 1.
  3. Balance the equation by inspection, starting with the most complicated molecule(s). The same number of each type of atom needs to appear on both reactant and product sides. Do NOT change the formulas of any of the reactants or products.

Notice

Stoichiometric Calculations: Amounts of Reactants and Products

Stoichiometric Calculations

Calculating Masses of Reactants and Products in Reactions

  1. Balance the equation for the reaction.
  2. Convert the known mass of the reactant or product to moles of that substance.
  3. Use the balanced equation to set up the appropriate mole ratios.
  4. Use the appropriate mole ratios to calculate the number of moles of the desired reactant or product.
  5. Convert from moles back to grams if required by the problem.

Calculating Masses of Reactants and Products - Graphically

A graphical representation of the steps in the previous slide.

Exercise 4

Consider the following reaction: $$ \chem{P_4(s)+5O_2(g) \rightarrow 2P_2O_5(s)} $$ If 6.25 g of phosphorus is burned, what mass of oxygen does it combine with?

Exercise 4 - Answer

Consider the following reaction: $$ \chem{P_4(s)+5O_2(g) \rightarrow 2P_2O_5(s)} $$ If 6.25 g of phosphorus is burned, what mass of oxygen does it combine with?

$$ 8.07\,\chem{g}\,\chem{O_2} $$

The Concept of Limiting Reactant

Limiting Reactants

A. The Concept of Limiting Reactants

Stoichiometric mixture $$ \chem{N_2(g)+3H_2(g) \rightarrow 2NH_3(g)} $$

A graphical representation of this slide's reaction.

A. The Concept of Limiting Reactants

Limiting reactant mixture $$ \chem{N_2(g)+3H_2(g) \rightarrow 2NH_3(g)} $$

A graphical representation of this slide's reaction.

Notice

Percent Yield

Exercise 6

Consider the following reaction: $$ \chem{P_4(s)+6F_2(g) \rightarrow 4PF_3(g)} $$ What mass of \(\chem{P_4}\) is needed to produce 85.0 g of \(\chem{PF_3}\) if the reaction has a 64.9% yield?

Exercise 6 - Answer

Consider the following reaction: $$ \chem{P_4(s)+6F_2(g) \rightarrow 4PF_3(g)} $$ What mass of \(\chem{P_4}\) is needed to produce 85.0 g of \(\chem{PF_3}\) if the reaction has a 64.9% yield?

$$ 46.1\,\chem{g}\,\chem{P_4} $$

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