Q1. State whether the following properties are physical or chemical. Briefly explain your reasoning.

• a. Slicing an apple.

• b. A sliced apple turns brown.

• c. A slab of marble feels cool to the touch.

• d. A sapphire is blue.

Background

Topic: Physical vs. Chemical Properties

This question tests your understanding of the difference between physical and chemical properties, and your ability to explain the reasoning behind classifying each property.

Key Terms:

• Physical property: A characteristic that can be observed or measured without changing the substance's chemical identity (e.g., color, temperature, state).

• Chemical property: A characteristic that describes a substance's ability to undergo a specific chemical change (e.g., flammability, reactivity).

Step-by-Step Guidance

1. For each property, ask: Does observing or measuring this property change the chemical identity of the substance?

2. If the answer is no, it is a physical property. If yes, it is a chemical property.

3. For each part (a–d), briefly explain your reasoning based on whether a chemical change occurs.

4. For example, consider if slicing an apple changes its chemical composition, or if turning brown involves a new substance being formed.

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Q2. A Fahrenheit and a Celsius thermometer are immersed in the same medium.

• a. At what Celsius temperature will the numerical reading on the Fahrenheit thermometer be one-eighth that on the Celsius thermometer?

• b. What temperature does this correspond to in Kelvin?

Background

Topic: Temperature Conversions (Celsius, Fahrenheit, Kelvin)

This question tests your ability to use the relationship between Celsius and Fahrenheit temperature scales, and to convert between Celsius and Kelvin.

Key Formulas:

Step-by-Step Guidance

1. Let be the Celsius temperature and the Fahrenheit temperature. According to the problem, .

2. Set up the equation: and .

3. Set the two expressions for equal to each other: .

4. Rearrange the equation to solve for (combine like terms and isolate $C$).

5. Once you have , use the formula to find the temperature in Kelvin.

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Q3. Two students use different digital kitchen scales to measure a known 100.0-g standard mass five times and obtain the following data:

• Student A: 104.2 g, 104.1 g, 102.5 g, 104.3 g, 104.2 g

• Student B: 97.1 g, 98.4 g, 106.0 g, 99.0 g, 104.2 g

Which student's data shows the effect of a dominant systematic error, and which shows the effects of a dominant random error? Explain your reasoning and identify how these data relate the types of error to the concepts of accuracy and precision.

Background

Topic: Accuracy, Precision, Systematic and Random Error

This question tests your understanding of the difference between systematic and random errors, and how they relate to accuracy and precision in measurements.

Key Terms:

• Systematic error: Consistent, repeatable error associated with faulty equipment or bias, affecting accuracy but not precision.

• Random error: Error that varies unpredictably from one measurement to another, affecting precision.

• Accuracy: How close measurements are to the true value.

• Precision: How close repeated measurements are to each other.

Step-by-Step Guidance

1. Examine Student A's data: Are the measurements close to each other (precise)? Are they close to the true value (accurate)?

2. Examine Student B's data: Are the measurements spread out (less precise)? Are they close to the true value (accurate)?

3. Determine which student shows a consistent offset (systematic error) and which shows more variability (random error).

4. Relate your findings to the definitions of accuracy and precision.

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Q4. A solution used to chlorinate a home swimming pool contains 7.0% chlorine by mass. An ideal chlorine level for the pool is one part per million (1 ppm). You can think about ppm as being 1 g chlorine per million grams of water. If you assume densities of 1.10 g/mL for the chlorine solution and 1.00 g/mL for the swimming pool water, what volume of the chlorine solution, in liters, is required to produce a chlorine level of 1 ppm in an 18,000-gallon swimming pool? (1 gallon = 3.785 L)

Background

Topic: Solution Concentration, Unit Conversions, Mass Percent, ppm

This question tests your ability to use mass percent and ppm concepts, and to perform unit conversions involving volume, mass, and density.

Key Formulas and Concepts:

• ppm (parts per million):

• Mass percent:

• Density:

Step-by-Step Guidance

1. Convert the pool volume from gallons to liters using .

2. Calculate the mass of pool water using its density () and the volume in liters (remember ).

3. Determine the mass of chlorine needed to achieve 1 ppm in the total mass of pool water.

4. Calculate the mass of chlorine solution required to provide this mass of chlorine, using the 7.0% by mass value.

5. Convert the mass of chlorine solution to volume using its density (), and then to liters.

6. Set up the final calculation for the student to complete.

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Q5. Determine the number of neutrons, protons, and electrons in a single 123I- ion and a single 131I- ion.

Background

Topic: Atomic Structure, Isotopes, Ions

This question tests your understanding of how to determine the number of protons, neutrons, and electrons in isotopes and ions.

Key Concepts:

• Atomic number (Z): Number of protons in the nucleus (Iodine, Z = 53).

• Mass number (A): Number of protons + neutrons.

• Neutrons:

• Electrons in an ion: For I-, add one electron to the neutral atom.

Step-by-Step Guidance

1. For each isotope, identify the mass number () and atomic number () for iodine ().

2. Calculate the number of protons (equals ).

3. Calculate the number of neutrons ().

4. Determine the number of electrons: For I-, add one electron to the number of protons.

5. Set up the values for both 123I- and 131I- for the student to complete.

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Q6. A 2.500 g sample of an unknown oxygenated hydrocarbon (CxHyOz) is found to contain 1.000 g C and 0.167 g H.

• a. Calculate the mass of oxygen in the sample.

• b. Calculate the mass percent of carbon, hydrogen, and oxygen.

• c. Convert the mass of each element into moles.

• d. Determine the empirical formula of the compound.

• e. The molar mass of the compound is approximately 180 g/mol. Determine the molecular formula.

• f. Explain why the empirical formula and molecular formula need not be identical.

Background

Topic: Empirical and Molecular Formulas, Percent Composition

This question tests your ability to determine the composition of a compound, calculate percent composition, convert mass to moles, and deduce empirical and molecular formulas.

Key Formulas and Concepts:

• Mass of oxygen:

• Percent composition:

• Moles:

• Empirical formula: Simplest whole-number ratio of moles of each element.

• Molecular formula: Multiple of empirical formula, based on molar mass.

Step-by-Step Guidance

1. Calculate the mass of oxygen by subtracting the masses of C and H from the total sample mass.

2. Calculate the mass percent of each element using the formula for percent composition.

3. Convert the mass of each element to moles using their respective molar masses (C: 12.01 g/mol, H: 1.008 g/mol, O: 16.00 g/mol).

4. Divide each mole value by the smallest number of moles to get the simplest ratio for the empirical formula.

5. Determine the empirical formula, then use the given molar mass to find the molecular formula by comparing the empirical formula mass to the molar mass.

6. Explain why the empirical and molecular formulas may differ (the molecular formula can be a whole-number multiple of the empirical formula).

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