Introductory Chemistry Exam Review – 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.

• Nitrite ion: $\mathrm{NO_2^-}$

Step-by-Step Guidance

1. Determine the number of valence electrons for a neutral nitrogen atom ($N$) and each oxygen atom ($O$).

2. Multiply the number of valence electrons for oxygen by 2 (since there are two oxygen atoms).

3. Add the electrons from nitrogen and both oxygens together.

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

Try solving on your own before revealing the answer!

Final Answer: 18 valence electrons

$N$ has 5 valence electrons, each $O$ has 6, and the extra negative charge adds 1 more: $5 + (2 \times 6) + 1 = 18$.

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

Background

Topic: Nomenclature of Ionic Compounds

This question tests your ability to match chemical names with their correct formulas, especially for transition metal compounds.

Key Terms and Formulas:

• Copper(II): Indicates Cu2+

• Nitrite: $\mathrm{NO_2^-}$

• Nitride: $\mathrm{N^{3-}}$

• Nitrate: $\mathrm{NO_3^-}$

Step-by-Step Guidance

1. Recall the charges for copper(II), nitrite, nitride, and nitrate ions.

2. Write the correct formula for each compound by balancing the charges.

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

4. Identify which pair does not match (either the name does not fit the formula or vice versa).

Try solving on your own before revealing the answer!

Final Answer: Copper(II) nitrate: CuNO3

The correct formula for copper(II) nitrate is Cu(NO3)2, not CuNO3.

Q3. How many significant figures are in the number 9.80 × 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 × 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.

Try solving on your own before revealing the answer!

Final Answer: 3 significant figures

In 9.80 × 104, the digits 9, 8, and 0 are all significant.

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 the remaining amount of a radioactive substance after a certain period.

Key Terms and Formulas:

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

• Formula: $\text{Remaining amount} = \text{Initial amount} \times \left(\frac{1}{2}\right)^{n}$

• $n = \frac{\text{elapsed time}}{t_{1/2}}$

Step-by-Step Guidance

1. Calculate how many half-lives have passed: $n = \frac{24}{8}$.

2. Use the formula for remaining amount: $\text{Remaining} = 1000 \times \left(\frac{1}{2}\right)^{n}$.

3. Evaluate $\left(\frac{1}{2}\right)^{n}$ using your value for $n$.

4. Multiply the initial amount by this fraction to get the remaining mass.

Try solving on your own before revealing the answer!

Final Answer: 125 mg

Three half-lives pass in 24 days, so $1000 \times (1/2)^3 = 125$ mg remains.

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 instrument's precision.

Key Terms and Formulas:

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

• Significant figures: The last digit is an estimate.

Step-by-Step Guidance

1. Identify the smallest marking on the graduated cylinder (to the mL means 1 mL increments).

2. Measurements should be recorded to one decimal place beyond the smallest marking (e.g., 52.1 mL).

3. Check which answer choice reflects this rule.

Try solving on your own before revealing the answer!

Final Answer: 52.1 mL

One digit beyond the smallest marking is included, so 52.1 mL is correct.

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: $\text{Final volume} - \text{Initial volume}$

Step-by-Step Guidance

1. Calculate the volume of the object: $33.6\ \text{mL} - 25.2\ \text{mL}$.

2. Use the mass given (15.1 g).

3. Plug the mass and calculated volume into the density formula: $d = \frac{15.1}{\text{volume}}$.

4. Calculate the density, keeping the correct significant figures.

Try solving on your own before revealing the answer!

Final Answer: 1.8 g/mL

Volume is 8.4 mL, so $15.1 \div 8.4 = 1.8$ g/mL (rounded to two significant figures).

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 apply Boyle's Law, which relates pressure and volume for a fixed amount of gas at constant temperature.

Key Terms and Formulas:

• Boyle's Law: $P_1V_1 = P_2V_2$

• $P_1$ = initial pressure, $V_1$ = initial volume, $P_2$ = final pressure, $V_2$ = final volume

Step-by-Step Guidance

1. Write down the known values: $P_1 = 360.0$ mm Hg, $V_1 = 750.0$ mL, $P_2 = 840.0$ mm Hg.

2. Set up Boyle's Law: $P_1V_1 = P_2V_2$.

3. Rearrange to solve for $V_2$: $V_2 = \frac{P_1V_1}{P_2}$.

4. Plug in the values and calculate $V_2$ (be sure to keep units consistent).

Try solving on your own before revealing the answer!

Final Answer: 321.4 mL

$V_2 = \frac{360.0 \times 750.0}{840.0} = 321.4$ mL

Q8. What is the molecular geometry of SO2?

Background

Topic: VSEPR Theory and Molecular Geometry

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

Key Terms and Formulas:

• SO2: Sulfur dioxide

• VSEPR: Predicts molecular geometry based on electron 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 shape based on the number of domains and lone pairs.

Try solving on your own before revealing the answer!

Final Answer: Bent

SO2 has three electron domains (two bonds, one lone pair), resulting in a bent geometry.