General Chemistry II: Exam #2 Study Guide – CHEM116

Gravimetric Methods

Calculating Moles, Molar Mass, and Hydrate Formulas

Gravimetric analysis is a quantitative method used to determine the amount of an analyte based on the mass of a solid. It is commonly applied to hydrates and compounds containing water of crystallization.

• Mole Calculation: The mole is the SI unit for amount of substance. To calculate moles, use the formula:

$\text{Moles} = \frac{\text{Mass (g)}}{\text{Molar Mass (g/mol)}}$

• Molar Mass: The sum of the atomic masses of all atoms in a molecule or formula unit.

• Hydrate Formula: Hydrates are compounds that contain water molecules within their crystal structure. The formula is written as Compound·nH2O, where n is the number of water molecules per formula unit.

• Desiccator and Hot Plate: Used to dry samples and remove water for accurate mass measurements.

Example: If 2.50 g of a hydrate yields 1.50 g of anhydrous salt after heating, the mass of water lost is 1.00 g. Calculate moles of water and salt to determine the hydrate formula.

Empirical Formula of Magnesium Oxide

Determining Empirical Formulas from Experimental Data

The empirical formula represents the simplest whole-number ratio of elements in a compound. Magnesium oxide is commonly used in experiments to illustrate this concept.

• Experimental Determination: Measure the mass of magnesium and oxygen that react to form magnesium oxide.

• Calculation: Convert masses to moles, then find the simplest ratio.

$\text{Empirical Ratio} = \frac{\text{Moles of Mg}}{\text{Moles of O}}$

Example: If 0.24 g Mg reacts with 0.16 g O, calculate moles and determine the empirical formula.

Lewis Structures

Electron Arrangement and Molecular Geometry

Lewis structures are diagrams that show the bonding between atoms and the lone pairs of electrons in a molecule. They are essential for predicting molecular shape and reactivity.

• Valence Electrons: The electrons in the outermost shell of an atom, involved in bonding.

• Drawing Lewis Structures: Count valence electrons, arrange atoms, and distribute electrons to satisfy the octet rule.

• Formal Charge: Used to determine the most stable Lewis structure. Calculated as:

$\text{Formal Charge} = \text{Valence electrons} - (\text{Lone pair electrons} + \frac{1}{2}\text{Bonding electrons})$

• Resonance: Some molecules have multiple valid Lewis structures; the actual structure is a resonance hybrid.

• AXE Notation: Describes electron domains: A = central atom, X = bonded atoms, E = lone pairs.

• Exceptions to the Octet Rule: Some molecules (e.g., BF3, SF6) do not follow the octet rule due to expanded or incomplete octets.

Example: Draw the Lewis structure for CO2 and determine its geometry.

Limiting Excess Reagents

Stoichiometry and Yield Calculations

In chemical reactions, the limiting reagent is the reactant that is completely consumed first, determining the maximum amount of product formed.

• Identifying Limiting Reagent: Compare the mole ratio of reactants used to the balanced equation.

• Theoretical Yield: The maximum amount of product that can be formed from the limiting reagent.

• Actual Yield: The amount of product actually obtained from the reaction.

• Percent Yield: Calculated as:

$\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$

Example: If 5.0 g of A reacts with 10.0 g of B to produce 8.0 g of C, determine the limiting reagent and percent yield.

Chemical Reactions

Types and Equations of Chemical Reactions

Chemical reactions involve the transformation of substances through breaking and forming chemical bonds. Understanding reaction types and equations is fundamental in chemistry.

• Writing Equations: Use chemical formulas and balance equations to obey the law of conservation of mass.

• Molecular, Ionic, and Net Ionic Equations: Molecular equations show all reactants and products; ionic equations show ions in solution; net ionic equations show only species that change during the reaction.

• Precipitation Reactions: Occur when two solutions are mixed and an insoluble solid (precipitate) forms.

• Solubility Rules: Used to predict whether a precipitate will form.

Example: Mixing AgNO3 and NaCl forms AgCl precipitate. Write the net ionic equation.

Terms

Key Vocabulary for Exam Preparation

• Empirical Formula: The simplest whole-number ratio of elements in a compound.

• Molecular Formula: The actual number of atoms of each element in a molecule.

• Lewis Structure: Diagram showing bonds and lone pairs in a molecule.

• Electron Pair and Electron Geometries: The arrangement of electron domains around a central atom.

• Precipitate: An insoluble solid formed in a chemical reaction.

• Filtration: Separation technique to remove solids from liquids.

• Buchner Filter System: Apparatus used for vacuum filtration.

• Metathesis Reaction: Double displacement reaction where ions exchange partners.

Tables

VSEPR Geometries and Bond Angles

The following table summarizes electron domain geometries, molecular shapes, and approximate bond angles according to VSEPR theory:

Domain Groups	Bonding Groups	Lone Pairs	Domain Geometry	Molecular Geometry	Approximate Bond Angle
2	2	0	Linear	Linear	180°
3	3	0	Trigonal planar	Trigonal planar	120°
3	2	1	Trigonal planar	Bent	~120°
4	4	0	Tetrahedral	Tetrahedral	109.5°
4	3	1	Tetrahedral	Trigonal pyramidal	~109.5°
4	2	2	Tetrahedral	Bent	~109.5°
5	5	0	Trigonal bipyramidal	Trigonal bipyramidal	90°, 120°
5	4	1	Trigonal bipyramidal	Seesaw	~90°, ~120°
5	3	2	Trigonal bipyramidal	T-shaped	~90°
5	2	3	Trigonal bipyramidal	Linear	180°
6	6	0	Octahedral	Octahedral	90°
6	5	1	Octahedral	Square pyramidal	~90°
6	4	2	Octahedral	Square planar	90°

Solubility Rules for Ionic Compounds in Water

Solubility rules help predict whether an ionic compound will dissolve in water or form a precipitate.

Compounds Containing the Following Ions: Are Generally Soluble	Exceptions
Li+, Na+, K+, NH4+	None
NO3-, C2H3O2-	None
Cl-, Br-, I-	When these ions pair with Ag+, Hg22+, or Pb2+, the resulting compounds are insoluble.
SO42-	When these ions pair with Sr2+, Ba2+, Pb2+, Ag+, or Ca2+, the resulting compounds are insoluble.

Compounds Containing the Following Ions: Are Generally Insoluble	Exceptions
OH-, S2-	When these ions pair with Li+, Na+, K+, or NH4+, the resulting compounds are soluble. S2- is also soluble with Ca2+, Sr2+, Ba2+. OH- is slightly soluble with Ca2+, Sr2+, Ba2+.
CO32-, PO43-	When these ions pair with Li+, Na+, K+, or NH4+, the resulting compounds are soluble.

Additional info: These tables are essential for predicting molecular shapes and solubility in aqueous reactions, which are key topics in General Chemistry.