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CHEM 111 Exam 3: Step-by-Step Study Guidance

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Q1. Determine the formal charge of the most electronegative element in nitrogen trihydride (NH3).

Background

Topic: Formal Charge and Electronegativity

This question tests your understanding of how to calculate formal charge and identify the most electronegative atom in a molecule.

Key Terms and Formulas

  • Formal charge: The charge assigned to an atom in a molecule, assuming electrons in bonds are shared equally.

  • Electronegativity: The tendency of an atom to attract electrons in a bond. In NH3, nitrogen is more electronegative than hydrogen.

Formal charge formula:

Step-by-Step Guidance

  1. Draw the Lewis structure for NH3. Identify the central atom and the surrounding atoms.

  2. Determine the number of valence electrons for nitrogen and hydrogen.

  3. Count the number of non-bonding (lone pair) electrons and bonding electrons on the nitrogen atom.

  4. Apply the formal charge formula to nitrogen, the most electronegative atom in NH3.

Try solving on your own before revealing the answer!

Q2. Select the element with the lowest electronegativity from the following: Mn, Os, As, Sn, K, Ra, Fe.

Background

Topic: Electronegativity Trends

This question tests your knowledge of periodic trends, specifically how electronegativity changes across periods and down groups.

Key Terms

  • Electronegativity: A measure of an atom's ability to attract electrons in a chemical bond.

  • Electronegativity generally decreases down a group and increases across a period (left to right).

Step-by-Step Guidance

  1. Locate each element on the periodic table.

  2. Recall the trend: electronegativity decreases as you move down a group and increases as you move from left to right across a period.

  3. Identify which element is furthest down and to the left on the periodic table.

  4. Compare the positions of all the given elements to determine which has the lowest electronegativity.

Try solving on your own before revealing the answer!

Q3. Provide the correct name for CS2.

Background

Topic: Naming Binary Covalent Compounds

This question tests your ability to apply the rules for naming binary molecular (covalent) compounds.

Key Terms and Rules

  • Binary covalent compounds are named using prefixes to indicate the number of each atom.

  • The first element keeps its name; the second element ends with "-ide."

  • CS2 contains one carbon and two sulfur atoms.

Step-by-Step Guidance

  1. Identify the elements present and their quantities.

  2. Apply the appropriate prefix for the number of sulfur atoms.

  3. Combine the names, ensuring the second element ends with "-ide."

Try solving on your own before revealing the answer!

Q4. Convert 39.119 K to degrees Fahrenheit (°F). Report your answer to NO decimal places and do not include units.

Background

Topic: Temperature Conversions

This question tests your ability to convert temperatures between Kelvin, Celsius, and Fahrenheit scales.

Key Formulas

Step-by-Step Guidance

  1. Subtract 273.15 from the Kelvin temperature to convert to Celsius.

  2. Multiply the Celsius temperature by .

  3. Add 32 to the result to convert to Fahrenheit.

  4. Round your answer to the nearest whole number as instructed.

Try solving on your own before revealing the answer!

Q5. Identify the VSEPR geometry of O3 (ozone).

Background

Topic: VSEPR Theory and Molecular Geometry

This question tests your ability to use the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict molecular shapes.

Key Terms

  • VSEPR theory: Predicts the geometry of molecules based on electron pair repulsion.

  • Consider both bonding pairs and lone pairs on the central atom.

Step-by-Step Guidance

  1. Draw the Lewis structure for O3 and identify the central atom.

  2. Count the number of bonding pairs and lone pairs on the central atom.

  3. Use the VSEPR chart to determine the geometry based on the number of electron domains.

Try solving on your own before revealing the answer!

Q6. Identify the VSEPR geometry of boron trihydride (BH3).

Background

Topic: VSEPR Theory and Molecular Geometry

This question tests your ability to predict the shape of a molecule using VSEPR theory.

Key Terms

  • Count the number of electron domains (bonding and lone pairs) around the central atom.

  • Use the VSEPR chart to match the electron domain count to a geometry.

Step-by-Step Guidance

  1. Draw the Lewis structure for BH3.

  2. Count the number of bonding pairs and lone pairs on boron.

  3. Determine the electron domain geometry and the molecular geometry.

Try solving on your own before revealing the answer!

Q7. Which one of the following is the correct formula for ammonium chloride?

Background

Topic: Ionic Compounds and Polyatomic Ions

This question tests your knowledge of the formulas for common polyatomic ions and how to write ionic compound formulas.

Key Terms

  • Ammonium ion:

  • Chloride ion:

  • Combine ions in a ratio that results in a neutral compound.

Step-by-Step Guidance

  1. Write the formulas for the ammonium and chloride ions.

  2. Combine the ions in the correct ratio to form a neutral compound.

  3. Write the final formula, ensuring the subscripts and charges are correct.

Try solving on your own before revealing the answer!

Q8. Determine the bond angle for NCl3.

Background

Topic: Molecular Geometry and Bond Angles

This question tests your understanding of how lone pairs affect bond angles in molecules.

Key Terms

  • Tetrahedral electron geometry: Ideal bond angle is 109.5°.

  • Lone pairs on the central atom decrease the bond angle.

Step-by-Step Guidance

  1. Draw the Lewis structure for NCl3 and identify the number of lone pairs on nitrogen.

  2. Determine the electron domain geometry around nitrogen.

  3. Recall how lone pairs affect the bond angle compared to the ideal tetrahedral angle.

Try solving on your own before revealing the answer!

Q9. Determine the polarity of H3O+ (hydronium ion).

Background

Topic: Molecular Polarity

This question tests your ability to determine if a molecule is polar or non-polar based on its shape and the presence of lone pairs.

Key Terms

  • Polarity: A molecule is polar if it has a net dipole moment.

  • Check for lone pairs on the central atom and differences in surrounding atoms.

Step-by-Step Guidance

  1. Draw the Lewis structure for H3O+.

  2. Identify the central atom and any lone pairs present.

  3. Determine if the molecule has a net dipole moment.

Try solving on your own before revealing the answer!

Q10. Determine the number of valence electrons in a hydrogen atom.

Background

Topic: Valence Electrons

This question tests your knowledge of how to determine the number of valence electrons for main group elements.

Key Terms

  • Valence electrons: Electrons in the outermost shell of an atom, important for bonding.

  • Hydrogen is in Group 1 of the periodic table.

Step-by-Step Guidance

  1. Locate hydrogen on the periodic table and identify its group number.

  2. Recall the rule for determining valence electrons for main group elements.

  3. Assign the correct number of valence electrons to hydrogen.

Try solving on your own before revealing the answer!

Q11. Draw the Lewis structure for CCl22- and identify the number of lone pairs on the central atom.

Background

Topic: Lewis Structures and Lone Pairs

This question tests your ability to draw Lewis structures for polyatomic ions and count lone pairs on the central atom.

Key Terms

  • Lewis structure: A diagram showing the arrangement of valence electrons among atoms in a molecule or ion.

  • Lone pairs: Pairs of valence electrons not involved in bonding.

Step-by-Step Guidance

  1. Count the total number of valence electrons, including the extra electrons from the 2- charge.

  2. Draw the skeleton structure with carbon as the central atom and chlorine atoms attached.

  3. Distribute electrons to satisfy the octet rule and count the lone pairs on the central atom.

Try solving on your own before revealing the answer!

Q12. Draw the Lewis structure for carbon tetrahydride (CH4) and identify the number of lone pairs on the central atom.

Background

Topic: Lewis Structures and Lone Pairs

This question tests your ability to draw Lewis structures and identify lone pairs on the central atom.

Key Terms

  • Carbon is the central atom, surrounded by four hydrogen atoms.

  • Each hydrogen forms a single bond with carbon.

Step-by-Step Guidance

  1. Count the total number of valence electrons for carbon and hydrogen.

  2. Draw the Lewis structure with carbon in the center and hydrogens around it.

  3. Check for any lone pairs on the central carbon atom.

Try solving on your own before revealing the answer!

Q13. Determine the charge of cesium (Cs) when it forms an ion.

Background

Topic: Ion Formation and Periodic Trends

This question tests your knowledge of how main group elements form ions based on their group number.

Key Terms

  • Cesium is an alkali metal (Group 1).

  • Group 1 elements lose one electron to form +1 ions.

Step-by-Step Guidance

  1. Locate cesium on the periodic table and identify its group.

  2. Recall the typical charge for Group 1 elements when they form ions.

  3. Assign the correct charge to the cesium ion.

Try solving on your own before revealing the answer!

Q14. Determine the charge of barium (Ba) when it forms an ion.

Background

Topic: Ion Formation and Periodic Trends

This question tests your knowledge of how alkaline earth metals form ions.

Key Terms

  • Barium is in Group 2 of the periodic table.

  • Group 2 elements lose two electrons to form +2 ions.

Step-by-Step Guidance

  1. Locate barium on the periodic table and identify its group.

  2. Recall the typical charge for Group 2 elements when they form ions.

  3. Assign the correct charge to the barium ion.

Try solving on your own before revealing the answer!

Q15. Determine the total number of resonance structures for SO2.

Background

Topic: Resonance Structures

This question tests your ability to draw and count resonance structures for a molecule.

Key Terms

  • Resonance structures: Different Lewis structures for the same molecule that show the same arrangement of atoms but different arrangements of electrons.

Step-by-Step Guidance

  1. Draw all possible valid Lewis structures for SO2 that satisfy the octet rule.

  2. Count the number of unique resonance structures you can draw.

Try solving on your own before revealing the answer!

Q16. Identify the name of the polyatomic ion PO43-.

Background

Topic: Polyatomic Ions

This question tests your ability to recognize and name common polyatomic ions.

Key Terms

  • PO43- is a common polyatomic ion found in many compounds.

Step-by-Step Guidance

  1. Recall the names and formulas of common polyatomic ions.

  2. Match the given formula to its correct name.

Try solving on your own before revealing the answer!

Q17. Identify the name of the polyatomic ion NH4+.

Background

Topic: Polyatomic Ions

This question tests your ability to recognize and name common polyatomic ions.

Key Terms

  • NH4+ is a common polyatomic cation.

Step-by-Step Guidance

  1. Recall the names and formulas of common polyatomic ions.

  2. Match the given formula to its correct name.

Try solving on your own before revealing the answer!

Q18. What is the correct formula for sodium acetate?

Background

Topic: Writing Ionic Compound Formulas

This question tests your ability to write the correct formula for an ionic compound containing a polyatomic ion.

Key Terms

  • Sodium ion:

  • Acetate ion: or

Step-by-Step Guidance

  1. Write the formulas for the sodium and acetate ions.

  2. Combine the ions in a 1:1 ratio to form a neutral compound.

  3. Write the final formula, ensuring the subscripts and charges are correct.

Try solving on your own before revealing the answer!

Q19. Indicate the total number of sigma and pi bonds in carbon tetrabromide (CBr4).

Background

Topic: Bond Types in Molecules

This question tests your ability to identify and count sigma (σ) and pi (π) bonds in a molecule.

Key Terms

  • Sigma bond (σ): A single covalent bond formed by the direct overlap of orbitals.

  • Pi bond (π): A bond formed by the sideways overlap of p orbitals, present in double and triple bonds.

Step-by-Step Guidance

  1. Draw the Lewis structure for CBr4.

  2. Identify the number of single bonds (sigma bonds) between carbon and bromine.

  3. Check for any double or triple bonds (which would contain pi bonds).

Try solving on your own before revealing the answer!

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