4B2 Stoichiometric Calculations Flashcards
Perform stoichiometric calculations involving moles, mass, volume, and percent yield.
Define:
mole ratio
A ratio that shows the relationship between the number of moles of substances in a chemical reaction.
Mole ratios are derived from the coefficients in a balanced chemical equation.
How do mole ratios help in stoichiometry problems?
They act as conversion factors to calculate quantities of reactants or products.
For example, the ratio 2H₂ / 1O₂ allows conversion from moles of H₂ to moles of O₂.
What is the mole ratio formula?
Coefficient of the desired substance divided by the Coefficient of the given substance.
For example, in 2H₂ + O₂ → 2H₂O, the ratio of H₂O to O₂ is 2/1.
What is the mole ratio of Ca to Ca₃N₂ in the reaction 3Ca + N₂ → Ca₃N₂?
3 moles Ca / 1 mole Ca₃N₂
The coefficients in the balanced equation (3 and 1) give the ratio.
How much calcium is required to produce 5.4 moles of Ca₃N₂?
The molar ratio is 3 moles Ca / 1 mole Ca₃N₂.
16.2 moles Ca
The mole ratio used is 3 moles Ca / 1 mole Ca₃N₂.
Which unit must be included in a mole ratio?
Moles
Mole ratios always refer to quantities of substances measured in moles.
How many moles of NH₃ are produced from 5 moles of H₂ in the reaction N₂ + 3H₂ → 2NH₃?
3.33 moles NH₃
Using the mole ratio 2NH₃ / 3H₂, multiply 5 moles H₂ by the ratio (2/3).
True or False:
Mole ratios can be used in reverse.
True
The reciprocal of a mole ratio is also valid (e.g., 2H₂ / 1O₂ = 1O₂ / 2H₂).
Fill in the blanks:
In stoichiometry, mole ratios are used to convert moles of a ______ substance to moles of a ______ substance.
given; desired
This conversion allows solving for unknown quantities in a reaction.
How many moles of O₂ are needed to react with 2 moles of CH₄ in the equation CH₄ + 2O₂ → CO₂ + 2H₂O?
4 moles O₂
The mole ratio O₂ to CH₄ is 2:1, so multiply 2 moles CH₄ by 2.
List one example where mole ratios are used in real-world applications.
Calculating the amount of reactants needed for industrial chemical production.
Mole ratios are essential in designing reactions for efficiency and cost-effectiveness.
Define:
stoichiometry
The study of the quantitative relationships between substances in a chemical reaction.
It uses mole ratios to calculate quantities of reactants or products.
It is based on balanced chemical equations.
Which law underpins stoichiometric calculations?
The law of conservation of mass.
This law states that mass is neither created nor destroyed in a chemical reaction.
Fill in the blank:
The periodic table provides the _______ of elements, which is used to calculate molar mass.
atomic mass
The atomic mass allows conversion between grams and moles for stoichiometric calculations.
Define:
molar mass
The mass of one mole of a substance, measured in grams per mole (g/mol).
Molar mass is calculated by summing the atomic masses of all atoms in the molecule.
What information is needed to find the mass of a substance from its moles?
The molar mass of the substance.
How is the number of moles calculated from mass?
Divide the mass of the substance by its molar mass.
The formula is: moles=mass/molar mass.
What is the purpose of using mole ratios in stoichiometry?
To relate the amounts of reactants and products based on the balanced chemical equation.
Mole ratios are derived from the coefficients in the balanced equation.
List the three steps in solving a mass-to-mass stoichiometry problem.
- Convert mass of A to moles of A.
- Use the mole ratio to find moles of B.
- Convert moles of B to mass of B.
massA → molesA → molesB → massB.
These steps ensure proper use of mole ratios and molar masses.
What is the mass of water formed when 74 moles of H₂ react completely?
74×18=1332 grams
The calculation uses the stoichiometric ratio 2H₂:2H₂O and the molar mass of water.
The molar mass of H₂O = 18 g/mol.
Define:
molarity
The number of moles of a solute per liter of solution.
Molarity quantifies the concentration of a solute in a solution, expressed in moles per liter.
How do you calculate molarity?
Divide the moles of solute by the volume of solution in liters.
The formula is: Molarity=moles of solute/liters of solution.
How do you calculate the number of moles from molarity and volume?
Multiply the molarity by the volume in liters.
The formula is Moles=Molarity×Volume (L).
True or False:
Molarity is the ratio of moles of solute to moles of solvent.
False
Molarity is defined as the moles of solute per liter of solution, not per mole of solvent.
What is the molarity of a solution with 48 grams of sucrose in 0.2 liters?
0.7 M
Use the formula M=moles of sucrose/volume (L).
Calculate the moles using the formula: moles=mass/molar mass.
The molar mass of sucrose is 342.2965 grams per mole.
What is the resulting molarity of ammonia when 4.46 moles are dissolved in 5 liters of water?
0.89 M
Calculation: 4.46 moles / 5 L = 0.89 moles/L.
Fill in the blank:
The _______ are listed on the left side of a chemical equation.
reactants
Reactants undergo transformation to produce products, which are on the right side of the equation.
In the reaction 2HCl + Zn → ZnCl₂ + H₂, what are the reactants?
HCl and Zn
Reactants are the starting substances in a chemical reaction.
Define:
limiting reactant
The reactant in a chemical reaction that is completely consumed.
Limiting reactants set the maximum yield of a reaction by limiting the amount of other reactants that can combine.
Define:
excess reactant
A reactant that is not completely consumed after a chemical reaction is complete.
It is present in greater quantities than required by the stoichiometric ratio.
True or False:
The product formed depends only on the excess reactant.
False
The amount of product formed is determined by the limiting reactant.
What is the purpose of an excess reactant?
To ensure that the limiting reactant is fully used during the reaction.
Excess reactants remain unreacted because they are in quantities greater than needed.
List the steps to find the limiting and excess reactants.
- Write a balanced equation.
- Convert quantities to moles.
- Compare mole ratios.
- Identify the limiting reactant and excess reactant.
Each step ensures accuracy in determining which reactants are limiting or in excess.
The reactant with a lower quantity relative to its stoichiometric ratio is the limiting reactant.
What analogy is used to explain limiting and excess reactants?
Assembling bicycles:
- Two wheels and one frame that make a bicycle represent the limiting reactants.
- Leftover frames represent the excess reactant.
The analogy simplifies the concept by comparing chemical reactions to tangible objects.
How do you find the excess reactant?
- Determine the limiting reactant.
- Calculate how much of the excess reactant is consumed.
- Subtract it from the initial quantity.
Identifying the excess reactant requires knowing the limiting reactant first.
What are the steps to calculate the amount of product formed?
- Identify the limiting reactant.
- Use the balanced equation to determine the product’s stoichiometric ratio.
- Convert moles of the limiting reactant to grams of product.
Accurate calculations depend on these systematic steps.
Fill in the blank:
In the reaction 2H₂ + O₂ → 2H₂O, oxygen is the ______ reactant when 10 moles of H₂ react with 7 moles of O₂.
excess
Based on stoichiometry, only 5 moles of O₂ are needed, leaving 2 moles unreacted.
Define:
theoretical yield
The maximum amount of product that could be formed in a reaction under perfect conditions.
Theoretical yield is determined using stoichiometry and a balanced chemical equation.
Define:
percent yield
The ratio of the actual yield to the theoretical yield, multiplied by 100.
Percent yield = (actual yield / theoretical yield) × 100.
The result is expressed as a percentage.
What does percent yield tell us?
The efficiency of a chemical reaction by comparing actual and theoretical yields.
It helps identify how much product was lost due to errors or inefficiencies.
A good percent yield, typically above 80%, indicates efficient recovery of product.
List three reasons why a percent yield might exceed 100%.
- Contamination
- Incorrect measurements
- Competing reactions
A percent yield over 100% often indicates errors in experimental procedures or unintended reactions.
List two reasons why percent yield might be less than 100%.
- Human error
- Product loss during recovery
Other reasons include incomplete reactions, competing side reactions, or contamination.
How do you calculate theoretical yield?
Theoretical yield = (given mass of reactant × molar mass of product) / molar mass of reactant.
Stoichiometric ratios from the balanced equation are used in the calculation.
Fill in the blank:
Theoretical yield is calculated based on the _______ reactant.
limiting
The limiting reactant determines the maximum amount of product that can be formed.
True or False:
Theoretical yield can be greater than the actual yield.
True
Theoretical yield represents the maximum possible product, while actual yield is typically less due to inefficiencies or errors.
What is the first step in calculating theoretical yield?
Balance the chemical equation.
A balanced equation ensures the law of conservation of mass is upheld during stoichiometric calculations.
