General Chemistry Exam 2 Study Guidance

Q1. Balance the following chemical equations:

Background

Topic: Chemical Equations & Stoichiometry

This question tests your ability to balance chemical equations, ensuring the same number of each type of atom on both sides of the equation, in accordance with the Law of Conservation of Mass.

Key Terms and Concepts:

• Reactants: Substances present before the reaction.

• Products: Substances formed as a result of the reaction.

• Balancing: Adjusting coefficients (not subscripts) to ensure equal numbers of each atom on both sides.

Step-by-Step Guidance

1. List all elements present in the equation and count the number of atoms of each on both sides.

2. Start by balancing the atoms of elements that appear in only one reactant and one product first.

3. Balance polyatomic ions as a unit if they appear unchanged on both sides.

4. Adjust coefficients as needed, checking after each change to ensure all elements are balanced.

5. Repeat the process for each equation (a–d), but do not change any subscripts.

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Q2. Calculate the molecular weights (in grams/mole) of the following molecules:

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Topic: Molar Mass Calculations

This question tests your ability to calculate the molar mass of compounds by summing the atomic masses of all atoms in the formula.

Key Terms and Formulas:

• Molar Mass: The mass (in grams) of one mole of a substance.

• Atomic Mass: The mass of a single atom, found on the periodic table (in amu or g/mol).

Formula:

$\text{Molar Mass} = \sum (\text{number of atoms of each element} \times \text{atomic mass of element})$

Step-by-Step Guidance

1. Write the chemical formula and list the number of each type of atom in the molecule.

2. Look up the atomic masses for each element (e.g., H = 1.01 g/mol, O = 16.00 g/mol, S = 32.07 g/mol, etc.).

3. Multiply the number of atoms of each element by its atomic mass.

4. Add the results for all elements to get the total molar mass.

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Q3. Perform the following conversions:

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Topic: Mole-Mass-Number Conversions

This question tests your ability to convert between grams, moles, and number of particles using molar mass and Avogadro's number.

Key Terms and Formulas:

• Mole: The SI unit for amount of substance (6.022 × 1023 particles).

• Molar Mass: Used to convert between grams and moles.

• Avogadro's Number: $6.022 \times 10^{23}$ particles/mol.

Formulas:

$\text{Moles} = \frac{\text{grams}}{\text{molar mass}}$

$\text{Grams} = \text{moles} \times \text{molar mass}$

$\text{Number of particles} = \text{moles} \times 6.022 \times 10^{23}$

Step-by-Step Guidance

1. Identify what is given and what you are solving for (grams, moles, or number of particles).

2. Write the appropriate conversion formula based on the question.

3. Plug in the known values (e.g., molar mass, Avogadro's number).

4. Set up the calculation, but stop before performing the final multiplication or division.

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Q4. Human blood is 0.154 M NaCl. If there are 5.42 liters of blood in the average person, how many grams of salt are in the average person?

Background

Topic: Solution Concentration (Molarity)

This question tests your ability to use molarity to find the amount of solute (in grams) in a given volume of solution.

Key Terms and Formulas:

• Molarity (M): Moles of solute per liter of solution.

• Moles of solute: $\text{Moles} = M \times V$ (where V is in liters)

• Grams of solute: $\text{Grams} = \text{moles} \times \text{molar mass}$

Step-by-Step Guidance

1. Calculate the number of moles of NaCl using the molarity and volume: $\text{moles NaCl} = 0.154 \times 5.42$

2. Multiply the moles of NaCl by its molar mass (58.44 g/mol) to find the mass in grams.

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Q5. A 0.768 M solution of NaCl is needed for a patient. If we have a stock solution of 10 M NaCl, how much of the stock solution will we need to make 800 mL of the desired solution? How much solvent will need to be added?

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Topic: Solution Dilution

This question tests your ability to use the dilution equation to prepare a solution of desired concentration from a more concentrated stock solution.

Key Terms and Formulas:

• Dilution Equation: $M_1V_1 = M_2V_2$

• $M_1$ = initial (stock) molarity, $V_1$ = volume of stock solution to use

• $M_2$ = final (desired) molarity, $V_2$ = final total volume

Step-by-Step Guidance

1. Identify the known values: $M_1 = 10$ M, $M_2 = 0.768$ M, $V_2 = 800$ mL (convert to L if needed).

2. Rearrange the dilution equation to solve for $V_1$ (volume of stock solution needed): $V_1 = \frac{M_2V_2}{M_1}$

3. Calculate $V_1$ using the values above.

4. Subtract $V_1$ from $V_2$ to find the volume of solvent to add.

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Q6. Baking soda (NaHCO3) reacts with stomach acid (HCl): NaHCO3 + HCl → CO2 + H2O + NaCl

Background

Topic: Stoichiometry

This question tests your ability to use stoichiometry to relate masses and moles of reactants and products in a chemical reaction.

Key Terms and Formulas:

• Stoichiometry: The calculation of reactants and products in chemical reactions.

• Mole Ratio: The ratio of coefficients from the balanced equation.

• Molar Mass: Used to convert between grams and moles.

General Steps:

1. Convert grams of HCl to moles using its molar mass.

2. Use the mole ratio from the balanced equation to find moles of the desired substance (NaHCO3, NaCl, or CO2).

3. Convert moles to grams if required using molar mass.

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Q7. In the reaction 4 NH3 + 5 O2 → 4 NO + 6 H2O, if 2.10 g NH3 reacts with 2.60 g O2, how many grams of NO and H2O are formed? (Determine the limiting reactant first.)

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Topic: Limiting Reactant & Stoichiometry

This question tests your ability to identify the limiting reactant and use stoichiometry to calculate the amount of products formed.

Key Terms and Formulas:

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Stoichiometry: Use mole ratios from the balanced equation.

• Molar Mass: Used to convert between grams and moles.

Step-by-Step Guidance

1. Convert grams of NH3 and O2 to moles using their molar masses.

2. Divide the moles of each reactant by their respective coefficients in the balanced equation to determine which is limiting.

3. Use the limiting reactant to calculate the moles of NO and H2O produced (using mole ratios from the equation).

4. Convert moles of NO and H2O to grams using their molar masses.

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Q8. Write the balanced molecular equation, complete ionic equation, and net ionic equation for KOH and CuCl2.

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Topic: Chemical Equations & Solubility

This question tests your ability to write different forms of chemical equations and recognize insoluble products (precipitates).

Key Terms and Concepts:

• Molecular Equation: Shows all reactants and products as compounds.

• Complete Ionic Equation: Shows all strong electrolytes as ions.

• Net Ionic Equation: Shows only the species that actually change during the reaction.

• Solubility Rules: Used to determine which compounds are soluble or insoluble.

Step-by-Step Guidance

1. Write the balanced molecular equation for the reaction between KOH and CuCl2.

2. Identify which products are soluble and which are insoluble (precipitate).

3. Write the complete ionic equation by splitting all soluble strong electrolytes into ions.

4. Write the net ionic equation by removing spectator ions.

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Q9. Determine the oxidation number of sulfur in each of the following substances:

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Topic: Oxidation Numbers

This question tests your ability to assign oxidation numbers to elements in compounds using standard rules.

Key Terms and Rules:

• Oxidation Number: The hypothetical charge an atom would have if all bonds were ionic.

• Rules: Sum of oxidation numbers in a neutral compound is zero; in a polyatomic ion, it equals the ion's charge.

• Group 1 metals = +1, Group 2 = +2, Oxygen usually = -2, Hydrogen = +1 (with nonmetals), etc.

Step-by-Step Guidance

1. Write the chemical formula and assign known oxidation numbers to other elements.

2. Set up an equation where the sum of oxidation numbers equals the overall charge (zero for neutral compounds).

3. Solve for the oxidation number of sulfur.

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Q10. Determine the oxidation number of cobalt in each of the following substances:

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Topic: Oxidation Numbers

This question tests your ability to assign oxidation numbers to transition metals in compounds.

Key Terms and Rules:

• Oxidation Number: The hypothetical charge an atom would have if all bonds were ionic.

• Use the known oxidation states of other elements to solve for cobalt.

Step-by-Step Guidance

1. Write the chemical formula and assign oxidation numbers to all elements except cobalt.

2. Set up an equation where the sum of oxidation numbers equals the overall charge (zero for neutral compounds).

3. Solve for the oxidation number of cobalt.

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Q11. Define an acid and a base.

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Topic: Acids and Bases

This question tests your understanding of the definitions of acids and bases according to different theories (Arrhenius, Brønsted-Lowry, Lewis).

Key Terms and Concepts:

• Arrhenius Acid: Produces H+ ions in water.

• Arrhenius Base: Produces OH- ions in water.

• Brønsted-Lowry Acid: Proton (H+) donor.

• Brønsted-Lowry Base: Proton (H+) acceptor.

• Lewis Acid: Electron pair acceptor.

• Lewis Base: Electron pair donor.

Step-by-Step Guidance

1. State the definition of an acid and a base according to at least one theory (Arrhenius, Brønsted-Lowry, or Lewis).

2. Optionally, mention more than one theory for a more complete answer.

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