CHEM 111 Exam 2 Study Guidance

Q1. Determine the average atomic mass, given:

• Isotope A: 12.7396 amu & 25.000% abundance

• Isotope B: 11.3338 amu & 75.000% abundance

Background

Topic: Isotopes and Atomic Mass

This question tests your ability to calculate the average atomic mass of an element using the masses and percent abundances of its isotopes.

Key Terms and Formula:

• Isotope: Atoms of the same element with different numbers of neutrons (and thus different masses).

• Average atomic mass: The weighted average of the masses of all naturally occurring isotopes of an element.

Key formula:

$\text{Average atomic mass} = (\text{mass}_1 \times \text{fraction}_1) + (\text{mass}_2 \times \text{fraction}_2) + \ldots$

Step-by-Step Guidance

1. Convert the percent abundances to decimal fractions by dividing each by 100.

2. Multiply the mass of each isotope by its corresponding fractional abundance.

3. Add the results from each isotope together to get the average atomic mass.

Try solving on your own before revealing the answer!

Q2. Some liquid in a jar can be separated by physical means into two separate liquids, which can then be separated no further. The liquid is a(n):

Background

Topic: Classification of Matter

This question tests your understanding of the differences between mixtures, compounds, elements, and pure substances.

Key Terms:

• Mixture: A combination of two or more substances that can be separated by physical means.

• Compound: A substance composed of two or more elements chemically combined in a fixed ratio.

• Element: A substance that cannot be broken down into simpler substances by chemical means.

• Pure substance: Matter with a fixed composition; includes elements and compounds.

Step-by-Step Guidance

1. Consider what it means for a substance to be separated by physical means.

2. Think about whether the two resulting liquids can be further separated or not.

3. Recall the definitions of mixture, compound, and element to determine which fits the description.

Try solving on your own before revealing the answer!

Q3. Determine the average atomic mass, given:

• Isotope A: 10.1721 amu & 17.981% abundance

• Isotope B: 11.3963 amu & 36.489% abundance

• Isotope C: 11.2557 amu (abundance not given, but can be inferred)

Background

Topic: Isotopes and Atomic Mass

This question tests your ability to calculate the average atomic mass when given the masses and percent abundances of isotopes, including inferring the missing abundance.

Key Terms and Formula:

• Isotope

• Average atomic mass

Key formula:

$\text{Average atomic mass} = (\text{mass}_1 \times \text{fraction}_1) + (\text{mass}_2 \times \text{fraction}_2) + (\text{mass}_3 \times \text{fraction}_3)$

Step-by-Step Guidance

1. Convert the given percent abundances to decimal fractions.

2. Calculate the missing abundance for isotope C by subtracting the sum of the other two abundances from 100%.

3. Convert the missing abundance to a decimal fraction.

4. Multiply each isotope's mass by its fractional abundance.

5. Add the results to find the average atomic mass.

Try solving on your own before revealing the answer!

Q4. Indicate the number of neutrons in the following isotope: Iodine-127

Background

Topic: Isotopes, Atomic Number, and Mass Number

This question tests your ability to determine the number of neutrons in an isotope given its mass number.

Key Terms and Formula:

• Mass number ($A$): The total number of protons and neutrons in the nucleus.

• Atomic number ($Z$): The number of protons in the nucleus.

• Number of neutrons ($N$): $N = A - Z$

Step-by-Step Guidance

1. Identify the mass number ($A$) from the isotope notation (Iodine-127 means $A = 127$).

2. Look up the atomic number ($Z$) for iodine on the periodic table.

3. Subtract the atomic number from the mass number to find the number of neutrons: $N = A - Z$.

Try solving on your own before revealing the answer!

Q5. Determine the volume in mL that 53.2 grams of phosphoric acid (H3PO4) will occupy if it has a density of 1.685 g/mL

Background

Topic: Density Calculations

This question tests your ability to use the density formula to relate mass and volume.

Key Terms and Formula:

• Density ($d$): $d = \frac{m}{V}$

• Mass ($m$): The amount of matter in an object (in grams).

• Volume ($V$): The amount of space an object occupies (in mL).

Step-by-Step Guidance

1. Write the density formula: $d = \frac{m}{V}$.

2. Rearrange the formula to solve for volume: $V = \frac{m}{d}$.

3. Plug in the given mass and density values.

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

Try solving on your own before revealing the answer!

Q6. A 14.61 mL urine sample has a mass of 96.05 g. Its specific gravity is?

Background

Topic: Specific Gravity

This question tests your ability to calculate specific gravity, which is the ratio of the density of a substance to the density of water.

Key Terms and Formula:

• Specific gravity: $\text{Specific gravity} = \frac{\text{density of sample}}{\text{density of water}}$

• Density of water at standard conditions: $1.00 \ \text{g/mL}$

Step-by-Step Guidance

1. Calculate the density of the urine sample: $\text{density} = \frac{\text{mass}}{\text{volume}}$.

2. Divide the sample's density by the density of water to get the specific gravity.

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

Try solving on your own before revealing the answer!

Q7. Convert 22.592 K to °F. Report your answer to NO decimal places, no units.

Background

Topic: Temperature Conversions

This question tests your ability to convert temperatures between Kelvin, Celsius, and Fahrenheit scales.

Key Terms and Formulas:

• Kelvin to Celsius: $^\circ C = K - 273.15$

• Celsius to Fahrenheit: $^\circ F = ( ^\circ C \times \frac{9}{5} ) + 32$

Step-by-Step Guidance

1. Convert the temperature from Kelvin to Celsius using the first formula.

2. Use the Celsius value to convert to Fahrenheit using the second formula.

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

Try solving on your own before revealing the answer!

Q8. Convert 28.563 °F to °C. Report your answer to NO decimal places, no units.

Background

Topic: Temperature Conversions

This question tests your ability to convert temperatures from Fahrenheit to Celsius.

Key Terms and Formula:

• Fahrenheit to Celsius: $^\circ C = ( ^\circ F - 32 ) \times \frac{5}{9}$

Step-by-Step Guidance

1. Subtract 32 from the Fahrenheit temperature.

2. Multiply the result by $\frac{5}{9}$ to get the Celsius temperature.

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

Try solving on your own before revealing the answer!

Q9. A combination of sand, salt, and water is an example of a __________.

Background

Topic: Classification of Matter

This question tests your understanding of mixtures and their types.

Key Terms:

• Homogeneous mixture: Uniform composition throughout.

• Heterogeneous mixture: Non-uniform composition; components are distinguishable.

• Compound: Chemically combined elements.

• Pure substance: Matter with a fixed composition.

Step-by-Step Guidance

1. Consider whether the components are uniformly mixed or can be seen as separate parts.

2. Recall the definitions of homogeneous and heterogeneous mixtures.

3. Decide which type best describes the combination of sand, salt, and water.

Try solving on your own before revealing the answer!

Q10. Which one of the following represents a chemical change?

• A. lard when heated changes to liquid

• B. water disappears from a beaker in a few days at room temperature

• C. sugar dissolving in water

• D. milk turns sour in a few days at room temperature

• E. water boils below 100 °C on a mountain

Background

Topic: Physical vs. Chemical Changes

This question tests your ability to distinguish between physical and chemical changes.

Key Terms:

• Chemical change: A process that results in the formation of new substances.

• Physical change: A change in state or appearance without forming new substances.

Step-by-Step Guidance

1. Review each option and decide if a new substance is formed.

2. Recall that changes in state (melting, boiling, dissolving) are usually physical changes.

3. Identify which option involves a chemical reaction (new substances formed).

Try solving on your own before revealing the answer!

Q11. A neutron has approximately the same mass as a _____.

Background

Topic: Subatomic Particles

This question tests your knowledge of the relative masses of subatomic particles.

Key Terms:

• Neutron: Neutral subatomic particle in the nucleus.

• Proton: Positively charged subatomic particle in the nucleus.

• Electron: Negatively charged subatomic particle outside the nucleus.

• Alpha particle: Helium nucleus (2 protons, 2 neutrons).

• Beta particle: High-energy electron or positron.

Step-by-Step Guidance

1. Recall the approximate masses of protons, neutrons, and electrons.

2. Compare the mass of a neutron to each of the options.

3. Identify which particle has a mass closest to that of a neutron.

Try solving on your own before revealing the answer!

Q12. The modern periodic table is arranged according to what property?

Background

Topic: Periodic Table Organization

This question tests your understanding of how elements are ordered in the periodic table.

Key Terms:

• Atomic number: Number of protons in the nucleus.

• Mass number: Total number of protons and neutrons.

• Atomic mass: Weighted average mass of an atom.

• Neutron number: Number of neutrons in the nucleus.

Step-by-Step Guidance

1. Recall the property that increases sequentially from left to right and top to bottom in the periodic table.

2. Consider which property is unique to each element and determines its position.

3. Eliminate options that do not fit the modern arrangement.

Try solving on your own before revealing the answer!

Q13. Identify the type of compound: AlCl3

Background

Topic: Types of Compounds

This question tests your ability to distinguish between ionic and covalent compounds.

Key Terms:

• Ionic compound: Formed from metals and nonmetals; involves transfer of electrons.

• Covalent compound: Formed from nonmetals; involves sharing of electrons.

Step-by-Step Guidance

1. Identify the elements in AlCl3 (aluminum and chlorine).

2. Determine if the elements are metals or nonmetals.

3. Recall the rules for classifying compounds as ionic or covalent based on the types of elements involved.

Try solving on your own before revealing the answer!

Q14. Determine the number of valence electrons the following atom has: Radon

Background

Topic: Electron Configuration and Valence Electrons

This question tests your knowledge of how to determine the number of valence electrons for a given element.

Key Terms:

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

• Noble gases: Group 18 elements, typically have full valence shells.

Step-by-Step Guidance

1. Locate radon on the periodic table (atomic number 86, group 18).

2. Recall the number of valence electrons for noble gases.

3. Apply this knowledge to determine the number for radon.

Try solving on your own before revealing the answer!

Q15. Identify the block in the periodic table where the following is found: Cadmium

Background

Topic: Periodic Table Blocks

This question tests your understanding of the s, p, d, and f blocks of the periodic table.

Key Terms:

• s-block: Groups 1 and 2, plus helium.

• p-block: Groups 13-18.

• d-block: Transition metals (groups 3-12).

• f-block: Lanthanides and actinides.

Step-by-Step Guidance

1. Locate cadmium on the periodic table (atomic number 48).

2. Determine which block (s, p, d, or f) cadmium belongs to based on its position.

3. Recall the general location of each block to confirm your answer.

Try solving on your own before revealing the answer!

Q16. Determine the charge of the following when it forms an ion: Cl

Background

Topic: Ions and Charges

This question tests your knowledge of how main group elements form ions and what charges they typically have.

Key Terms:

• Anion: Negatively charged ion.

• Cation: Positively charged ion.

• Group 17 (halogens): Typically gain one electron to form -1 ions.

Step-by-Step Guidance

1. Locate chlorine on the periodic table (group 17).

2. Recall how many electrons chlorine needs to gain to achieve a noble gas configuration.

3. Determine the resulting charge after gaining that electron.

Try solving on your own before revealing the answer!

Q17. Determine the charge of the following when it forms an ion: I

Background

Topic: Ions and Charges

This question tests your knowledge of how main group elements form ions and what charges they typically have.

Key Terms:

• Anion: Negatively charged ion.

• Cation: Positively charged ion.

• Group 17 (halogens): Typically gain one electron to form -1 ions.

Step-by-Step Guidance

1. Locate iodine on the periodic table (group 17).

2. Recall how many electrons iodine needs to gain to achieve a noble gas configuration.

3. Determine the resulting charge after gaining that electron.

Try solving on your own before revealing the answer!

Q18. If a car has an EPA mileage rating of 65.7 miles per gallon, what is this rating in kilometers per liter? Helpful conversions: 1 L = 1.06 qt, 1 mile = 1.609 km, 4 qt = 1 gallon

Background

Topic: Unit Conversions (Dimensional Analysis)

This question tests your ability to convert between different units using multiple conversion factors.

Key Terms and Conversion Factors:

• 1 mile = 1.609 km

• 1 gallon = 4 qt

• 1 L = 1.06 qt

Step-by-Step Guidance

1. Set up the initial value: 65.7 miles/gallon.

2. Convert miles to kilometers using the appropriate conversion factor.

3. Convert gallons to liters using the given relationships (gallons to quarts, then quarts to liters).

4. Set up the calculation so that all units except km/L cancel out.

Try solving on your own before revealing the answer!

Q19. Express the speed of 8.7402 mi/h in m/s if 1 mi = 1.609 km.

Background

Topic: Unit Conversions (Speed)

This question tests your ability to convert speed from miles per hour to meters per second using dimensional analysis.

Key Terms and Conversion Factors:

• 1 mile = 1.609 km

• 1 km = 1000 m

• 1 hour = 3600 seconds

Step-by-Step Guidance

1. Start with the given speed in mi/h.

2. Convert miles to kilometers, then kilometers to meters.

3. Convert hours to seconds.

4. Set up the calculation so that all units except m/s remain.

Try solving on your own before revealing the answer!

Q20. What is the value of a diffusion coefficient of 18.5 in2/min expressed in cm2/s? (2.54 cm = 1 in)

Background

Topic: Unit Conversions (Area and Time)

This question tests your ability to convert area and time units using dimensional analysis.

Key Terms and Conversion Factors:

• 1 in = 2.54 cm

• 1 min = 60 s

Step-by-Step Guidance

1. Start with the given value in in2/min.

2. Convert in2 to cm2 by squaring the conversion factor.

3. Convert minutes to seconds.

4. Set up the calculation so that all units except cm2/s remain.

Try solving on your own before revealing the answer!