Stoichiometry and Mole Calculations – Exam 5 Practice Guidance

Q1. How many N atoms are in 2.50 mol of RDX, C3H6N6O6?

Background

Topic: Mole-to-atom conversions in compounds

This question tests your understanding of how to convert moles of a compound to moles (or atoms) of a specific element within that compound.

Key Terms and Formulas

• Mole: A unit representing $6.022 \times 10^{23}$ particles (atoms, molecules, etc.)

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

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

Step-by-Step Guidance

1. Identify the number of nitrogen atoms per molecule of RDX ($C_3H_6N_6O_6$). There are 6 N atoms per molecule.

2. Set up the conversion: Start with the given moles of RDX and use the ratio of N atoms per molecule to find moles of N atoms.

3. Multiply the moles of RDX by the number of N atoms per molecule to get moles of N atoms.

4. Remember, you can convert moles of N atoms to number of N atoms using Avogadro's number if needed.

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Q2. How many moles are in 97.0 g of C8H10N4O2 (caffeine)?

Background

Topic: Mass-to-mole conversion using molar mass

This question tests your ability to convert grams of a substance to moles using its molar mass.

Key Terms and Formulas

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

• Formula: $\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

Step-by-Step Guidance

1. Calculate the molar mass of caffeine ($C_8H_{10}N_4O_2$) by adding the atomic masses of each element.

2. Set up the conversion: Divide the given mass (97.0 g) by the molar mass of caffeine.

3. Ensure your units cancel properly to give moles.

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Q3. What mass of O2 is needed to react completely with 30.0 g of C6H12O6 to form carbon dioxide and water?

Background

Topic: Stoichiometry – mass-to-mass conversion in a chemical reaction

This question tests your ability to use a balanced chemical equation to determine the mass of a reactant needed for a reaction.

Key Terms and Formulas

• Stoichiometry: Relating quantities of reactants and products using a balanced equation

• Balanced Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

• Mass-to-mole conversion: $\text{moles} = \frac{\text{mass}}{\text{molar mass}}$

• Mole ratio: Use coefficients from the balanced equation

Step-by-Step Guidance

1. Calculate the moles of $C_6H_{12}O_6$ from the given mass using its molar mass.

2. Use the mole ratio from the balanced equation to find moles of $O_2$ needed.

3. Convert moles of $O_2$ to grams using the molar mass of $O_2$.

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Q4. Aluminum reacts with oxygen to form aluminum oxide. If 27.0 g Al reacts with excess oxygen, what is the percent yield if 40.8 g Al2O3 are actually produced?

Background

Topic: Percent yield in chemical reactions

This question tests your ability to calculate percent yield based on actual and theoretical yields.

Key Terms and Formulas

• Percent Yield: $\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$

• Theoretical Yield: Maximum amount of product possible based on stoichiometry

• Actual Yield: Amount of product actually obtained

Step-by-Step Guidance

1. Write the balanced equation: $4Al + 3O_2 \rightarrow 2Al_2O_3$

2. Convert grams of Al to moles using its molar mass.

3. Use the mole ratio to find moles of $Al_2O_3$ produced.

4. Convert moles of $Al_2O_3$ to grams (theoretical yield).

5. Set up the percent yield formula using actual and theoretical yields.

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Q5. What mass of water (H2O) can be formed when 10.0 g of H2 reacts with 80.0 g of O2?

Background

Topic: Limiting reactant and mass-to-mass conversion

This question tests your ability to identify the limiting reactant and calculate the mass of product formed.

Key Terms and Formulas

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

• Balanced Equation: $2H_2 + O_2 \rightarrow 2H_2O$

• Mass-to-mole conversion: $\text{moles} = \frac{\text{mass}}{\text{molar mass}}$

• Mole ratio: Use coefficients from the balanced equation

Step-by-Step Guidance

1. Convert the mass of $H_2$ and $O_2$ to moles using their molar masses.

2. Use the balanced equation to determine which reactant is limiting.

3. Use the limiting reactant to calculate moles of $H_2O$ produced.

4. Convert moles of $H_2O$ to grams using its molar mass.

Try solving on your own before revealing the answer!