Q1. How many significant figures are in the measurement 0.00450 g?

Background

Topic: Significant Figures

This question tests your understanding of how to count significant figures in a measured value, which is essential for reporting scientific data correctly.

Key Terms and Rules:

• Significant Figures: The digits in a measurement that are known with certainty plus one estimated digit.

• Leading zeros (zeros before the first nonzero digit) are not significant.

• Captive zeros (zeros between nonzero digits) are significant.

• Trailing zeros (zeros to the right of the decimal point and after a nonzero digit) are significant.

Step-by-Step Guidance

1. Identify all the digits in the measurement: 0.00450 g.

2. Determine which zeros are leading, captive, or trailing.

3. Apply the rules for significant figures to each digit.

4. Count the number of significant figures based on your analysis.

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Q2. Calculate the energy (in joules) of a photon with a wavelength of 500 nm.

Background

Topic: Electromagnetic Radiation and Energy of a Photon

This question tests your ability to use the relationship between wavelength and energy for photons, a key concept in atomic structure and quantum chemistry.

Key Terms and Formulas:

• Photon: A quantum of electromagnetic radiation.

• Wavelength (): The distance between successive peaks of a wave, usually in meters (m).

• Energy of a photon:

• Where:

  • J·s (Planck's constant)

  • m/s (speed of light)

  • = wavelength in meters

Step-by-Step Guidance

1. Convert the wavelength from nanometers to meters: $500= 500 \times 10^{-9}$ m.

2. Write the formula for the energy of a photon: .

3. Plug in the values for , , and (in meters) into the formula.

4. Set up the calculation, but do not compute the final value yet.

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Q3. What is the electron configuration for a neutral atom of phosphorus (P)?

Background

Topic: Electron Configuration

This question tests your understanding of how electrons are arranged in atomic orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund's rule.

Key Terms and Concepts:

• Electron configuration: The distribution of electrons among the atomic orbitals.

• Aufbau principle: Electrons fill the lowest energy orbitals first.

• Pauli exclusion principle: Each orbital can hold a maximum of two electrons with opposite spins.

• Hund's rule: Electrons fill degenerate orbitals singly before pairing.

• Phosphorus atomic number: 15 (so 15 electrons in a neutral atom).

Step-by-Step Guidance

1. List the order of orbital filling: , , , , .

2. Assign electrons to each orbital in order, keeping track of the total number (15).

3. Remember to fill orbitals with up to 2 electrons and orbitals with up to 6 electrons.

4. Write out the configuration up to the point where all 15 electrons are assigned, but stop before the final configuration is written.

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Q4. Calculate the number of moles in 25.0 g of CO2.

Background

Topic: Moles and Molar Mass

This question tests your ability to convert between mass and moles using the molar mass of a compound.

Key Terms and Formula:

• Mole: The amount of substance containing Avogadro's number of particles.

• Molar mass: The mass of one mole of a substance (g/mol).

• Formula:

Step-by-Step Guidance

1. Calculate the molar mass of CO2: C = 12.01 g/mol, O = 16.00 g/mol.

2. Add the masses: to get the molar mass of CO2.

3. Set up the calculation: .

4. Do not perform the final division yet; leave the setup for the student to complete.

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Q5. What is the empirical formula of a compound that contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

Background

Topic: Empirical Formulas

This question tests your ability to determine the simplest whole-number ratio of atoms in a compound from percent composition data.

Key Terms and Steps:

• Empirical formula: The simplest whole-number ratio of elements in a compound.

• Percent composition: The percent by mass of each element in a compound.

• Steps:

  1. Assume 100 g of the compound (so percentages become grams).

  2. Convert grams of each element to moles.

  3. Divide each by the smallest number of moles to get the ratio.

  4. Write the empirical formula based on these ratios.

Step-by-Step Guidance

1. Assume 100 g of the compound: 40.0 g C, 6.7 g H, 53.3 g O.

2. Convert each mass to moles: for each element.

3. Calculate the number of moles for C, H, and O (do not compute the final values yet).

4. Set up the ratios by dividing each mole value by the smallest number of moles.

5. Stop before writing the final empirical formula; leave the last step for the student.

Try solving on your own before revealing the answer!