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Step-by-Step Guidance for General Chemistry Exam 2 Practice (Chapters 3–5)

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Q1. When the following equation is balanced, what are the coefficients? C8H18 + O2 → CO2 + H2O

Background

Topic: Balancing Chemical Equations (Combustion Reactions)

This question tests your ability to balance combustion reactions, which involve a hydrocarbon reacting with oxygen to produce carbon dioxide and water.

Key Terms and Formulas:

  • Combustion Reaction: Hydrocarbon + O2 → CO2 + H2O

  • Balancing Order: C first, then H, then O

Step-by-Step Guidance

  1. Count the number of C, H, and O atoms on both sides of the equation. Start with the hydrocarbon (C8H18).

  2. Balance the carbon atoms by adjusting the coefficient in front of CO2.

  3. Balance the hydrogen atoms by adjusting the coefficient in front of H2O.

  4. Balance the oxygen atoms last by adjusting the coefficient in front of O2. If you get a fraction, multiply all coefficients to clear it.

Try solving on your own before revealing the answer!

Q2. Calculate the percentage by mass of ammonia in cisplatin, PtCl2(NH3)2.

Background

Topic: Percent Composition by Mass

This question tests your ability to calculate the percent by mass of a component in a compound using molar masses.

Key Terms and Formulas:

  • Percent by mass = (mass of part / mass of whole) × 100%

  • Molar mass: sum of atomic masses for all atoms in the formula

Step-by-Step Guidance

  1. Calculate the molar mass of cisplatin, PtCl2(NH3)2, by adding the atomic masses of all atoms present.

  2. Calculate the total mass of ammonia (NH3) units in one formula unit (there are two NH3 groups).

  3. Divide the total mass of ammonia by the molar mass of cisplatin and multiply by 100% to set up the percent by mass calculation.

Try solving on your own before revealing the answer!

Q3. What is the total number of atoms in 0.139 mol of Fe(OH2)63+?

Background

Topic: Mole-to-Atoms Conversion

This question tests your ability to use Avogadro's number and the formula unit to find the total number of atoms in a given amount of substance.

Key Terms and Formulas:

  • Avogadro's number: particles/mol

  • Count the number of atoms per formula unit: Fe(OH2)63+ has 1 Fe, 12 H, and 6 O atoms per unit.

Step-by-Step Guidance

  1. Calculate the total number of atoms in one formula unit of Fe(OH2)63+ by adding up all atoms present.

  2. Multiply the number of atoms per formula unit by Avogadro's number to get the number of atoms in one mole.

  3. Multiply this value by the number of moles given (0.139 mol) to set up the calculation for the total number of atoms.

Try solving on your own before revealing the answer!

Q4. A nitrogen oxide is 63.65% by mass nitrogen. The molecular formula could be ______.

Background

Topic: Empirical and Molecular Formulas from Percent Composition

This question tests your ability to use percent composition to deduce possible molecular formulas.

Key Terms and Formulas:

  • Percent composition: (mass of element / mass of compound) × 100%

  • Empirical formula calculation: convert % to grams, then to moles, then find the simplest ratio.

Step-by-Step Guidance

  1. Assume a 100 g sample so that the mass of N is 63.65 g and O is 36.35 g.

  2. Convert these masses to moles using the atomic masses of N and O.

  3. Divide by the smallest number of moles to get the simplest whole-number ratio (empirical formula).

  4. Compare the empirical formula to the molecular formulas given to see which matches the percent composition.

Try solving on your own before revealing the answer!

Q5. Which hydrocarbon pair below has identical mass percentage of C?

Background

Topic: Percent Composition Comparison

This question tests your ability to compare the percent composition of carbon in different hydrocarbons.

Key Terms and Formulas:

  • Percent by mass of C = (mass of C in formula / molar mass of compound) × 100%

  • Hydrocarbon: compound containing only C and H

Step-by-Step Guidance

  1. For each hydrocarbon, calculate the mass of carbon and the total molar mass.

  2. Compute the percent by mass of carbon for each compound.

  3. Compare the values for each pair to determine which pair has identical percent C.

Try solving on your own before revealing the answer!

Q6. Propane (C3H8) reacts with oxygen to produce CO2 and H2O. In a particular experiment, 38.0 g of CO2 are produced from 22.05 g of propane with excess oxygen. What is the percent yield?

Background

Topic: Percent Yield and Stoichiometry

This question tests your ability to calculate percent yield using stoichiometry and the percent yield formula.

Key Terms and Formulas:

  • Percent yield = (actual yield / theoretical yield) × 100%

  • Stoichiometry: use balanced equation to relate moles of reactants and products

Step-by-Step Guidance

  1. Write the balanced equation for the combustion of propane.

  2. Convert the mass of propane to moles using its molar mass.

  3. Use the mole ratio from the balanced equation to find the theoretical moles of CO2 produced.

  4. Convert the theoretical moles of CO2 to grams using the molar mass of CO2.

  5. Set up the percent yield calculation using the actual yield (38.0 g) and the theoretical yield you just calculated.

Try solving on your own before revealing the answer!

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