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 ($t$): (a) $t = 92$ minutes (b) $t = 276$ minutes

3. Set up the formula to calculate the number of half-lives: $n = \frac{t}{t_{1/2}}$

4. Plug in the values for each case, but do not calculate the final result yet.

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 containing only nonmetals are usually covalent, while those containing a metal and a nonmetal are ionic.

3. Classify each compound accordingly, but do not write the final classification yet.

Try solving on your own before revealing 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).

• Charge = (number of protons) - (number of electrons)

• Element symbol: Use the periodic table to match the atomic number to the element.

Step-by-Step Guidance

1. For each ion, identify the number of protons to determine the element.

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 solving on your own before revealing 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

• Determine the charge of each ion based on its group in the periodic table.

• Balance the charges so the compound is neutral.

• Write the formula with the cation first, followed by the anion, using subscripts as needed.

Step-by-Step Guidance

1. Identify the symbol and charge for each ion using the periodic table.

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

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

Try solving on your own before revealing 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 first, then the anion.

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

• Use the root of the anion's name with the suffix "-ide" for simple anions.

Step-by-Step Guidance

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

2. Identify the anion and its name.

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

Try solving on your own before revealing 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 chemical 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 $\mathrm{SO_4^{2-}}$).

• Balance the charges to write a neutral formula.

• Use parentheses around polyatomic ions if more than one is needed.

Step-by-Step Guidance

1. Identify the formula and charge for 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 solving on your own before revealing 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.

Try solving on your own before revealing 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 (O) and bromine (Br) are both nonmetals with specific bonding patterns.

• Use the octet rule and periodic table trends to determine typical bonding and lone pairs.

Step-by-Step Guidance

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

2. Apply the octet rule to find out how many bonds and lone pairs are typical.

3. Write the number of bonds and lone pairs for each element, but do not fill in the final numbers yet.

Try solving on your own before revealing 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 bonding pairs 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 each 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 a full octet.

Try drawing the structures on your own before revealing 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 using the Valence Shell Electron Pair Repulsion (VSEPR) theory.

Key Concepts

• Count the number of electron groups (bonds and lone pairs) around the central atom.

• Use VSEPR theory to determine the molecular shape (e.g., linear, bent, tetrahedral).

Step-by-Step Guidance

1. Draw the Lewis structure for each molecule.

2. Count the number of electron groups around the central atom.

3. Match the number of groups to the correct molecular geometry.

Try matching the shapes before revealing 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 atoms.

• Name the more metallic element first, then the second element with the suffix "-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.

3. Write the name, but do not fill in the final answer yet.

Try naming the compounds before revealing 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 write chemical formulas from the names of covalent compounds, using prefixes to determine the number of each atom.

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.

Step-by-Step Guidance

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

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

Try writing the formulas before revealing 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 comparing the bonds before revealing 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 polar bonds.

• If the molecule has polar bonds and an asymmetric shape, it is likely polar.

• If the molecule is symmetric and all bonds are identical, it is likely nonpolar.

Step-by-Step Guidance

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

2. Identify the shape of the molecule.

3. Determine if the polar bonds are arranged symmetrically or asymmetrically.

4. Decide if the molecule is polar or nonpolar based on the above.

Try drawing and analyzing the molecules before revealing 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 bonded to N, O, or F).

4. Assign the strongest type of intermolecular force present.

Try identifying the IMFs before revealing 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.

• Balance one element at a time, starting with the most complex molecule.

Step-by-Step Guidance

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

2. Start by balancing the metal (Zn), then the nonmetal (Cl), and finally hydrogen (H).

3. Adjust coefficients as needed to balance each atom.

Try balancing the equation before revealing the answer!

Q17. Identify reaction type

Background

Topic: Types of Chemical Reactions

This question tests your ability to classify chemical 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 the correct reaction type.

Try classifying the reactions before revealing 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): Forms $1^+$ ions.

• Group 2A (2): Forms $2^+$ ions.

• Group 6A (16): Forms $2^-$ ions.

• Group 7A (17): Forms $1^-$ ions.

Step-by-Step Guidance

1. Recall the group number and typical charge for each group.

2. Write the charge as a number followed by a plus or minus sign.

Try predicting the charges before revealing the answer!