Chem 1120 Final Exam Study Guide – Step-by-Step Guidance

Q1. How many valence electrons are in the nitrite ion?

Background

Topic: Lewis Structures and Valence Electrons

This question tests your understanding of how to count valence electrons for polyatomic ions, specifically the nitrite ion (NO2-).

Key Terms and Formulas

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

• Polyatomic ion: An ion composed of more than one atom.

• To find total valence electrons: Add the valence electrons for each atom, then add or subtract electrons for the ion's charge.

Step-by-Step Guidance

1. Identify the number of valence electrons for each atom in the nitrite ion (N and O).

2. Multiply the number of valence electrons by the number of each atom present (NO2- has 1 N and 2 O).

3. Add the electrons together.

4. Since the ion has a negative charge, add one more electron to the total.

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Q2. Choose the pair of names and formulas that do NOT match.

Background

Topic: Naming Ionic Compounds

This question tests your ability to match chemical names with their correct formulas, especially for compounds containing transition metals and polyatomic ions.

Key Terms and Formulas

• Copper(II): Indicates Cu2+

• Nitrite: NO2-

• Nitride: N3-

• Nitrate: NO3-

Step-by-Step Guidance

1. Review the charge of copper(II) and the formulas for nitrite, nitride, and nitrate.

2. Write the correct formula for each compound name.

3. Compare the given formulas to the correct ones you wrote.

4. Identify which pair does not match.

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Q3. How many significant figures are in the number 9.80 x 104?

Background

Topic: Significant Figures

This question tests your understanding of how to count significant figures in numbers written in scientific notation.

Key Terms and Formulas

• Significant figures: All nonzero digits, zeros between nonzero digits, and trailing zeros in the decimal part are significant.

• Scientific notation: Only the digits in the coefficient are counted for significant figures.

Step-by-Step Guidance

1. Look at the coefficient (the number before the x 104).

2. Count all digits in the coefficient, including zeros between nonzero digits and after the decimal point.

3. Ignore the exponent when counting significant figures.

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Q4. Iodine-131 has a half-life of 8 days. How much of a 1000 mg sample would be left after 24 days?

Background

Topic: Radioactive Decay and Half-Life

This question tests your ability to use the concept of half-life to determine how much of a radioactive sample remains after a certain period.

Key Terms and Formulas

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

• Number of half-lives: $\text{Number of half-lives} = \frac{\text{Elapsed time}}{t_{1/2}}$

• Remaining amount: $\text{Final amount} = \text{Initial amount} \times \left(\frac{1}{2}\right)^{\text{number of half-lives}}$

Step-by-Step Guidance

1. Calculate how many half-lives have passed by dividing the total time (24 days) by the half-life (8 days).

2. Use the formula for remaining amount to set up the calculation.

3. Plug in the initial amount (1000 mg) and the number of half-lives.

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Q5. Which of the following measurements has the correct significant figures for a graduated cylinder with marks to the mL?

Background

Topic: Measurement and Significant Figures

This question tests your understanding of how to properly record measurements using the correct number of significant figures based on the precision of the measuring instrument.

Key Terms and Formulas

• Graduated cylinder: Typically, you record one digit beyond the smallest marking.

• Significant figures: The digits that are known with certainty plus one estimated digit.

Step-by-Step Guidance

1. Determine the smallest increment on the graduated cylinder (to the mL means 1 mL increments).

2. Recall that you should estimate one digit beyond the smallest marking.

3. Identify which measurement includes one estimated digit beyond the marked value.

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Q6. What is the density of an object that has a mass of 15.1 g and, when placed into a graduated cylinder, causes the water level to rise from 25.2 mL to 33.6 mL?

Background

Topic: Density Calculations

This question tests your ability to calculate density using mass and volume (determined by water displacement).

Key Terms and Formulas

• Density ($d$): $d = \frac{\text{mass}}{\text{volume}}$

• Volume by displacement: $\text{Volume} = \text{Final volume} - \text{Initial volume}$

Step-by-Step Guidance

1. Calculate the volume of the object by subtracting the initial water level from the final water level.

2. Write the formula for density.

3. Plug in the mass and the calculated volume into the density formula.

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Q7. The initial volume of a gas cylinder is 750.0 mL. If the pressure of a gas inside the cylinder changes from 360.0 mm Hg to 840.0 mm Hg, what is the final volume the gas occupies?

Background

Topic: Gas Laws (Boyle's Law)

This question tests your ability to use Boyle's Law to relate the pressure and volume of a gas at constant temperature.

Key Terms and Formulas

• Boyle's Law: $P_1V_1 = P_2V_2$

• Pressure and volume must be in consistent units.

Step-by-Step Guidance

1. Identify the initial and final pressures and volumes ($P_1$, $V_1$, $P_2$).

2. Write Boyle's Law equation.

3. Rearrange the equation to solve for the unknown final volume ($V_2$).

4. Plug in the known values and set up the calculation.

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Q8. What is the molecular geometry of SO2?

Background

Topic: VSEPR Theory and Molecular Geometry

This question tests your ability to determine the molecular geometry of a molecule using the Valence Shell Electron Pair Repulsion (VSEPR) model.

Key Terms and Formulas

• VSEPR Theory: Predicts the shape of molecules based on electron pair repulsion.

• Electron domains: Bonding pairs and lone pairs around the central atom.

Step-by-Step Guidance

1. Draw the Lewis structure for SO2.

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

3. Use VSEPR theory to determine the molecular geometry based on the number of domains and lone pairs.

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Q9. Which molecule below has hydrogen bonding?

Background

Topic: Intermolecular Forces

This question tests your understanding of hydrogen bonding and which molecules are capable of this type of intermolecular force.

Key Terms and Formulas

• Hydrogen bonding: A strong intermolecular force between hydrogen and N, O, or F atoms.

• Look for molecules where H is directly bonded to N, O, or F.

Step-by-Step Guidance

1. Examine each molecule to see if hydrogen is bonded to N, O, or F.

2. Identify which molecule(s) meet this criterion.

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Q10. Which of the following statements is TRUE for gases?

Background

Topic: Properties of Gases

This question tests your understanding of the basic properties and behaviors of gases as described by the kinetic molecular theory and gas laws.

Key Terms and Formulas

• Kinetic molecular theory: Explains the behavior of gases in terms of particle motion and collisions.

• Direct and inverse relationships: Know how temperature, pressure, and volume relate for gases.

Step-by-Step Guidance

1. Review each statement and compare it to what you know about gas behavior.

2. Recall that pressure is due to collisions of gas molecules with container walls.

3. Consider the relationships between temperature, pressure, and volume.

Try solving on your own before revealing the answer!