Step-by-Step Guidance for Sample Midterm 2 (GOB Chemistry)

Q1. Bismuth-213 decays through alpha emission with a half-life of 46 minutes. Determine the number of half-lives in each case as whole number values.

Background

Topic: Radioactive Decay and Half-Life

This question tests your understanding of half-life, which is the time required for half of a radioactive substance to decay. You are asked to determine how many half-lives have passed in a given time period.

Key Terms and Formulas

• Half-life ($t_{1/2}$): The time it takes for half of a sample to decay.

• Elapsed time ($t$): The total time that has passed.

• Number of half-lives ($n$): $n = \frac{t}{t_{1/2}}$

Step-by-Step Guidance

1. Identify the half-life of Bismuth-213 ($t_{1/2} = 46$ minutes).

2. For each case, write down the elapsed time ($t$). For example, $t = 92$ minutes for part (a).

3. Use the formula $n = \frac{t}{t_{1/2}}$ to set up the calculation for the number of half-lives.

4. Divide the elapsed time by the half-life to find the number of half-lives. Remember to round to the nearest whole number if needed.

Try solving on your own before revealing the answer!

Q2. Identify each compound as covalent or ionic.

Background

Topic: Types of Chemical Bonds

This question tests your ability to distinguish between covalent and ionic compounds based on their composition.

Key Terms

• Covalent compound: Formed when two nonmetals share electrons.

• Ionic compound: Formed when a metal transfers electrons to a nonmetal, resulting in positive and negative ions.

Step-by-Step Guidance

1. For each compound, identify the elements present and determine if they are metals or nonmetals.

2. Recall that compounds made of only nonmetals are usually covalent, while those containing a metal and a nonmetal are ionic.

3. Apply this rule to each compound in the list.

Try classifying each compound before checking the answer!

Q3. Enter the element symbol and electric charge on each ion described here.

Background

Topic: Ions and Their Charges

This question tests your understanding of how to determine the symbol and charge of an ion based on the number of protons and electrons.

Key Terms and Formulas

• Protons: Determine the element's identity (atomic number).

• Electrons: The charge is determined by the difference between protons and electrons.

• Ion charge: $\text{charge} = \text{number of protons} - \text{number of electrons}$

Step-by-Step Guidance

1. For each ion, identify the atomic number (element symbol) based on the number of protons.

2. Calculate the charge by subtracting the number of electrons from the number of protons.

3. Write the element symbol followed by the charge (e.g., $\mathrm{Na^+}$).

Try determining the symbol and charge for each ion before checking the answer!

Q4. Write formulas for the ionic compound formed from the following pairs of elements.

Background

Topic: Writing Ionic Compound Formulas

This question tests your ability to write the correct chemical formula for ionic compounds formed from given elements.

Key Terms and Concepts

• Ionic compounds are formed from cations (metals) and anions (nonmetals).

• The total positive and negative charges must balance.

Step-by-Step Guidance

1. Determine the charge of each ion formed by the elements (use the periodic table for common charges).

2. Balance the charges so the total positive and negative charges are equal.

3. Write the formula using subscripts to indicate the number of each ion needed.

Try writing each formula before checking the answer!

Q5. Write the correct name for each of the ionic compounds.

Background

Topic: Naming Ionic Compounds

This question tests your ability to name ionic compounds, including those with transition metals (which may require Roman numerals).

Key Terms and Rules

• Name the cation (metal) first, then the anion (nonmetal).

• For transition metals, indicate the charge with Roman numerals in parentheses.

• Anion names typically end in "-ide" unless they are polyatomic ions.

Step-by-Step Guidance

1. Identify the cation and its charge (especially for transition metals).

2. Name the anion, changing the ending to "-ide" if it is a single element.

3. Combine the names, using Roman numerals if needed.

Try naming each compound before checking the answer!

Q6. Write formulas for these ionic compounds.

Background

Topic: Writing Formulas for Ionic Compounds with Polyatomic Ions

This question tests your ability to write correct formulas for ionic compounds, including those containing polyatomic ions.

Key Terms and Concepts

• Polyatomic ions: Ions made up of more than one atom (e.g., sulfate $\mathrm{SO_4^{2-}}$).

• Use parentheses if more than one polyatomic ion is needed.

• Balance the charges to write the correct formula.

Step-by-Step Guidance

1. Identify the formula and charge of each ion (use a polyatomic ion table if needed).

2. Determine the ratio of ions needed to balance the charges.

3. Write the formula, using parentheses for multiple polyatomic ions.

Try writing each formula before checking the answer!

Q7. In a molecule composed of covalently bonded atoms, ________.

Background

Topic: Covalent Bonding

This question tests your understanding of how covalent bonds form and which types of atoms are involved.

Key Concepts

• Covalent bonds involve the sharing of electrons between nonmetal atoms.

• Metals typically do not form covalent bonds with nonmetals or noble gases.

Step-by-Step Guidance

1. Review the definitions of covalent and ionic bonds.

2. Consider which types of atoms are involved in covalent bonding.

3. Identify the correct statement about covalent bonding from the options provided.

Try selecting the correct statement before checking the answer!

Q8. How many covalent bonds and lone pairs does each element typically have?

Background

Topic: Lewis Structures and Valence Electrons

This question tests your knowledge of how many bonds and lone pairs are typical for certain elements based on their position in the periodic table.

Key Concepts

• Oxygen typically forms 2 bonds and has 2 lone pairs.

• Bromine typically forms 1 bond and has 3 lone pairs.

Step-by-Step Guidance

1. Determine the number of valence electrons for each element.

2. Recall the octet rule and how many electrons are needed to complete the octet.

3. Calculate the typical number of bonds and lone pairs for each element.

Try filling in the numbers before checking the answer!

Q9. Draw the Lewis structure for each molecule, including lone pairs of electrons.

Background

Topic: Lewis Structures

This question tests your ability to draw Lewis structures, showing all valence electrons as either bonds or lone pairs.

Key Concepts

• Count the total number of valence electrons for the molecule.

• Arrange atoms to satisfy the octet rule (or duet for hydrogen).

• Assign lone pairs to complete octets where needed.

Step-by-Step Guidance

1. Count the total number of valence electrons for the molecule.

2. Draw a skeleton structure, connecting atoms with single bonds.

3. Distribute remaining electrons as lone pairs to complete octets.

4. Check that all atoms (except hydrogen) have 8 electrons around them.

Try drawing the structures before checking the answer!

Q10. Predict the shape of the following molecules.

Background

Topic: Molecular Geometry (VSEPR Theory)

This question tests your ability to predict the 3D shape of molecules based on the number of bonding and lone pairs around the central atom.

Key Concepts

• VSEPR theory: Electron pairs around a central atom arrange themselves to minimize repulsion.

• Common shapes: linear, trigonal planar, tetrahedral, bent, pyramidal.

Step-by-Step Guidance

1. Draw the Lewis structure for each molecule.

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

3. Use VSEPR theory to determine the molecular shape based on these counts.

Try predicting the shapes before checking the answer!

Q11. Name each of the following molecular compounds.

Background

Topic: Naming Molecular (Covalent) Compounds

This question tests your ability to name covalent compounds using prefixes to indicate the number of each atom.

Key Terms and Rules

• Use prefixes (mono-, di-, tri-, etc.) for the number of each atom.

• The second element's name ends in "-ide".

Step-by-Step Guidance

1. Identify the number of each type of atom in the formula.

2. Apply the appropriate prefix to each element (except "mono-" is usually omitted for the first element).

3. Write the name, ending the second element with "-ide".

Try naming each compound before checking the answer!

Q12. Write the formula for each of the following molecular compounds.

Background

Topic: Writing Formulas for Molecular Compounds

This question tests your ability to translate the name of a molecular compound into its chemical formula using prefixes.

Key Terms and Rules

• Prefixes indicate the number of each atom (e.g., di- = 2, tetra- = 4).

• Write the element symbols in the order given in the name, using subscripts for numbers greater than one.

Step-by-Step Guidance

1. Identify the elements and the number of each from the name and prefixes.

2. Write the correct element symbols and use subscripts for the numbers.

Try writing each formula before checking the answer!

Q13. Select the more polar covalent bond in each pair.

Background

Topic: Bond Polarity and Electronegativity

This question tests your understanding of how differences in electronegativity between atoms affect bond polarity.

Key Concepts

• Bond polarity increases with greater difference in electronegativity between the two atoms.

• Compare the electronegativity values for each pair to determine which bond is more polar.

Step-by-Step Guidance

1. Look up or recall the electronegativity values for each element in the bond.

2. Calculate the difference in electronegativity for each bond in the pair.

3. The bond with the greater difference is more polar.

Try selecting the more polar bond before checking the answer!

Q14. Draw the Lewis dot structures of the following molecules and determine if each is polar or nonpolar.

Background

Topic: Molecular Polarity and Lewis Structures

This question tests your ability to draw Lewis structures and use them to determine molecular polarity based on shape and bond polarity.

Key Concepts

• Draw the Lewis structure to determine the shape and distribution of electrons.

• Assess whether the molecule has a net dipole moment (is polar) or not (is nonpolar).

Step-by-Step Guidance

1. Draw the Lewis structure for each molecule, showing all bonds and lone pairs.

2. Determine the molecular shape using VSEPR theory.

3. Assess the symmetry and the polarity of the bonds to decide if the molecule is polar or nonpolar.

Try drawing and analyzing each molecule before checking the answer!

Q15. For each compound shown, identify the major type of intermolecular forces experienced between molecules identical to itself.

Background

Topic: Intermolecular Forces

This question tests your understanding of the different types of intermolecular forces (IMFs) that can exist between molecules.

Key Terms

• London dispersion forces: Present in all molecules, especially nonpolar ones.

• Dipole-dipole forces: Present in polar molecules.

• Hydrogen bonding: Present when H is bonded to N, O, or F.

• Ionic forces: Present in ionic compounds.

Step-by-Step Guidance

1. Determine if the compound is ionic or molecular.

2. If molecular, decide if it is polar or nonpolar.

3. Identify if hydrogen bonding is possible (H attached to N, O, or F).

4. Assign the strongest type of intermolecular force present.

Try identifying the IMFs before checking the answer!

Q16. Balance the reaction by adding the necessary coefficients.

Background

Topic: Balancing Chemical Equations

This question tests your ability to balance a chemical equation by ensuring the same number of each type of atom on both sides.

Key Concepts

• Law of Conservation of Mass: Atoms are neither created nor destroyed in a chemical reaction.

• Coefficients are used to balance the number of atoms of each element.

Step-by-Step Guidance

1. Write down the number of atoms of each element on both sides of the equation.

2. Start by balancing the metals, then nonmetals, and finally hydrogen and oxygen.

3. Add coefficients as needed to balance each element.

4. Check your work to ensure all atoms are balanced.

Try balancing the equation before checking the answer!

Q17. Identify reaction type

Background

Topic: Types of Chemical Reactions

This question tests your ability to classify reactions as combination, decomposition, single replacement, or double replacement.

Key Concepts

• Combination: Two or more substances form one product.

• Decomposition: One substance breaks down into two or more products.

• Single replacement: One element replaces another in a compound.

• Double replacement: Two compounds exchange ions.

Step-by-Step Guidance

1. Analyze the reactants and products in each equation.

2. Match the pattern to one of the four reaction types.

Try classifying each reaction before checking the answer!

Q18. Identify the charge of the ions formed when elements from the following Groups react.

Background

Topic: Predicting Ion Charges from the Periodic Table

This question tests your ability to predict the charge of ions formed by main group elements based on their group number.

Key Concepts

• Group 1A (1): +1 charge

• Group 2A (2): +2 charge

• Group 6A (16): -2 charge

• Group 7A (17): -1 charge

Step-by-Step Guidance

1. Recall the typical charges for each group based on their position in the periodic table.

2. Write the charge as a number followed by a plus or minus sign (e.g., 2+ or 1-).

Try predicting the charges before checking the answer!