Molar Ratio
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Direct Molar Ratio Derivation from Balanced Equations
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This category focuses on the student's ability to directly read and interpret the stoichiometric coefficients of a balanced chemical equation to state the ratio between any two substances (reactants or products) in terms of moles. These problems are primarily conceptual or recall-based.
PRACTICE PROBLEM #1.1
Consider the balanced combustion reaction of propane:
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PRACTICE PROBLEM #1.2
For the synthesis reaction of ammonia:
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PRACTICE PROBLEM #1.3
The reaction used in an airbag to inflate it is the decomposition of sodium azide:
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These problems require a single-step conversion between the mass of a substance (in grams) and the amount of substance (in moles) using the calculated molar mass as the conversion factor.
PRACTICE PROBLEM #2.1
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PRACTICE PROBLEM #2.2
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PRACTICE PROBLEM #2.3
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This category involves the derivation and application of molar ratios from the subscripts within a chemical formula, rather than stoichiometric coefficients, to relate the moles of constituent elements within a compound. This is foundational for empirical and molecular formula determination.
PRACTICE PROBLEM #3.1
Dinitrogen pentoxide has the molecular formula $\text{N}_2\text{O}_5$. What is the molar ratio of nitrogen atoms to oxygen atoms in a sample of this compound? If a sample contains $0.40$ moles of $\text{N}_2\text{O}_5$ molecules, how many moles of oxygen atoms does it contain?
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PRACTICE PROBLEM #3.2
Consider the ionic compound calcium phosphate, $\text{Ca}_3(\text{PO}_4)_2$. Determine the mole ratio of calcium ions ($\text{Ca}^{2+}$) to phosphate ions ($\text{PO}_4^{3-}$) and the mole ratio of oxygen atoms to phosphorus atoms within the compound.
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PRACTICE PROBLEM #3.3
A newly synthesized hydrocarbon is found to be composed of $79.89\%$ Carbon and $20.11\%$ Hydrogen by mass. Assuming a $100.00 \text{ g}$ sample, convert the masses of C and H to moles. Then, use the moles to determine the smallest whole-number molar ratio between Carbon and Hydrogen (the empirical formula).
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This category assesses a deeper conceptual understanding of the role of molar ratio, particularly its necessity, its relationship to the Law of Conservation of Mass, and its use in proportional reasoning within stoichiometry problems before performing mass-to-mass calculations.
PRACTICE PROBLEM #4.1
Explain why the molar ratio is the only valid conversion factor between two different chemical species in a stoichiometric calculation (e.g., converting grams of A to grams of B), and why direct mass-to-mass ratios cannot be used.
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PRACTICE PROBLEM #4.2
Consider the reaction $\text{A} + 2\text{B} \rightarrow \text{C} + \text{D}$. A student claims that to produce $5.0 \text{ mol}$ of $\text{C}$, they will always need $5.0 \text{ mol}$ of $\text{A}$ and $10.0 \text{ mol}$ of $\text{B}$ if the reaction goes to completion.
Critique this claim based on the concept of molar ratio and the requirement of a balanced equation. What essential assumption is the student making?
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PRACTICE PROBLEM #4.3
A student is attempting to calculate the amount of product formed from a given amount of reactant. They correctly used the molar mass of the reactant, but they forgot to include the molar ratio from the balanced equation in their calculation. If the molar ratio of reactant to product is $3:2$, how will the student's calculated answer compare to the true theoretical yield (i.e., will the calculated answer be too high, too low, or correct)? Justify your answer using the concept of dimensional analysis and conversion factors.
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Answers & Explanations
PRACTICE PROBLEM #1.1
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PRACTICE PROBLEM #1.2
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PRACTICE PROBLEM #1.3
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PRACTICE PROBLEM #1.4
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PRACTICE PROBLEM #1.5
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PRACTICE PROBLEM #1.6
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PRACTICE PROBLEM #2.1
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PRACTICE PROBLEM #2.2
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PRACTICE PROBLEM #2.3
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PRACTICE PROBLEM #2.4
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PRACTICE PROBLEM #2.5
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PRACTICE PROBLEM #2.6
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PRACTICE PROBLEM #3.1
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PRACTICE PROBLEM #3.2
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PRACTICE PROBLEM #3.3
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PRACTICE PROBLEM #3.4
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PRACTICE PROBLEM #3.5
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PRACTICE PROBLEM #3.6
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