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Questions and Answers
Which of the following statements best describes the law of conservation of mass?
Which of the following statements best describes the law of conservation of mass?
- In a chemical reaction, mass is neither created nor destroyed. (correct)
- The total number of atoms remains constant during a chemical reaction.
- Mass can be created during a chemical reaction if energy is supplied.
- The mass of the reactants is always less than the mass of the products.
Two compounds are formed from element X and element Y. Compound 1 contains 1.0 g of X for every 2.0 g of Y. Compound 2 contains 2.0 g of X for every 4.0 g of Y. Which law does this observation directly support?
Two compounds are formed from element X and element Y. Compound 1 contains 1.0 g of X for every 2.0 g of Y. Compound 2 contains 2.0 g of X for every 4.0 g of Y. Which law does this observation directly support?
- Law of definite proportions
- Law of multiple proportions (correct)
- Law of conservation of energy
- Law of conservation of mass
A sample of pure water from a river and a sample of pure water from the ocean, when decomposed, both yield hydrogen and oxygen in the same mass ratio. This observation supports which of the following laws?
A sample of pure water from a river and a sample of pure water from the ocean, when decomposed, both yield hydrogen and oxygen in the same mass ratio. This observation supports which of the following laws?
- Law of multiple proportions
- Law of conservation of mass
- Law of conservation of energy
- Law of constant composition (correct)
Which statement correctly describes the relationship between reaction stoichiometry and compositional stoichiometry?
Which statement correctly describes the relationship between reaction stoichiometry and compositional stoichiometry?
What information is needed to convert grams of a substance to moles?
What information is needed to convert grams of a substance to moles?
Which of the following statements correctly relates the atomic mass unit (amu) to grams?
Which of the following statements correctly relates the atomic mass unit (amu) to grams?
A gas occupies 22.4 liters at standard temperature and pressure (STP). Which of the following statement correctly describes this scenario?
A gas occupies 22.4 liters at standard temperature and pressure (STP). Which of the following statement correctly describes this scenario?
What does the mole fraction represent in a solution?
What does the mole fraction represent in a solution?
In a mixture of gases, how is the mole fraction of a gas related to its partial pressure?
In a mixture of gases, how is the mole fraction of a gas related to its partial pressure?
Consider a solution containing two components, A and B. If the mole fraction of A is 0.6, what is the mole fraction of B?
Consider a solution containing two components, A and B. If the mole fraction of A is 0.6, what is the mole fraction of B?
A compound is found to contain 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the strategy to determine the empirical formula of this compound?
A compound is found to contain 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the strategy to determine the empirical formula of this compound?
Which of the following represents the correct order of steps in balancing a chemical equation?
Which of the following represents the correct order of steps in balancing a chemical equation?
Why is it important to not alter the subscripts of the chemical formula when balancing a chemical equation?
Why is it important to not alter the subscripts of the chemical formula when balancing a chemical equation?
For chemical equations that cannot be balanced by inspection, which method can be used?
For chemical equations that cannot be balanced by inspection, which method can be used?
In a chemical reaction, what is the limiting reactant?
In a chemical reaction, what is the limiting reactant?
How does the limiting reactant affect the amount of product formed in a chemical reaction?
How does the limiting reactant affect the amount of product formed in a chemical reaction?
What is the term for the maximum amount of product that can be obtained from a chemical reaction based on the amount of limiting reactant?
What is the term for the maximum amount of product that can be obtained from a chemical reaction based on the amount of limiting reactant?
If a reaction's actual yield is lower than its theoretical yield, what does this indicate?
If a reaction's actual yield is lower than its theoretical yield, what does this indicate?
Which calculation determines the percentage yield of a chemical reaction?
Which calculation determines the percentage yield of a chemical reaction?
A reaction between sodium bicarbonate and citric acid produces carbon dioxide, sodium citrate, and water. If 1.0 g of each reactant is used, and the reaction goes to completion, which statement regarding the remaining reactants is correct?
A reaction between sodium bicarbonate and citric acid produces carbon dioxide, sodium citrate, and water. If 1.0 g of each reactant is used, and the reaction goes to completion, which statement regarding the remaining reactants is correct?
How is the limiting reactant used to determine the amount of product formed?
How is the limiting reactant used to determine the amount of product formed?
In the reaction: $C_6H_6 + Br_2 \rightarrow C_6H_5Br + HBr$, 30 g of benzene reacts with 65 g of bromine, producing 56.7 g of bromobenzene. What affects the percentage yield of bromobenzene?
In the reaction: $C_6H_6 + Br_2 \rightarrow C_6H_5Br + HBr$, 30 g of benzene reacts with 65 g of bromine, producing 56.7 g of bromobenzene. What affects the percentage yield of bromobenzene?
14.00 grams of molybdenum metal react with sulfur producing 21.02g of a compound containing only molybdenum and sulfur.
14.00 grams of molybdenum metal react with sulfur producing 21.02g of a compound containing only molybdenum and sulfur.
When copper metal is inserted into a silver nitrate solution, silver is produced: $Cu_{(s)}$ + $2AgNO_{3(aq)}$ $Cu(NO_3){2(aq)}$ + $2Ag{(s)}$. If 3.14 grams of copper react completely with excess silver nitrate, how much silver metal is produced?
When copper metal is inserted into a silver nitrate solution, silver is produced: $Cu_{(s)}$ + $2AgNO_{3(aq)}$ $Cu(NO_3){2(aq)}$ + $2Ag{(s)}$. If 3.14 grams of copper react completely with excess silver nitrate, how much silver metal is produced?
Potassium which reacts violently with water produces a reaction between 2K (s) + 2 $H_2O (l)
ightarrow H_2 (g) + 2 KOH (aq)$. What volume of hydrogen is produced at standard pressure of 1 atm at 20 °C?
Potassium which reacts violently with water produces a reaction between 2K (s) + 2 $H_2O (l) ightarrow H_2 (g) + 2 KOH (aq)$. What volume of hydrogen is produced at standard pressure of 1 atm at 20 °C?
Consider the reaction: $4HCl_{(aq)}$ + $MnO_{2(s)}
ightarrow MnCl_{2(aq)} + 2H_2O(l) + Cl_2(g)$. If 34.5 g of manganese oxide reacts with 41.5g of hydrochloric acid. With limited reagents, what is the theoretical yield of $Cl_2$?
Consider the reaction: $4HCl_{(aq)}$ + $MnO_{2(s)} ightarrow MnCl_{2(aq)} + 2H_2O(l) + Cl_2(g)$. If 34.5 g of manganese oxide reacts with 41.5g of hydrochloric acid. With limited reagents, what is the theoretical yield of $Cl_2$?
What is the mass in grams of 1.00 mole of chromium atoms?
What is the mass in grams of 1.00 mole of chromium atoms?
What is the mass in grams of 6.02 x $10^{23}$ gold atoms?
What is the mass in grams of 6.02 x $10^{23}$ gold atoms?
What is the mass in grams of one million phosphorus atoms?
What is the mass in grams of one million phosphorus atoms?
Identify element X given that one single atom of element X has a mass of 2.28 x $10^{-22}$ grams.
Identify element X given that one single atom of element X has a mass of 2.28 x $10^{-22}$ grams.
How many atoms of sulfur are there in 100.0 g of sulfur?
How many atoms of sulfur are there in 100.0 g of sulfur?
Naira coin that is 86% copper weighs 7.39g. Find the number of atoms of copper in the coin.
Naira coin that is 86% copper weighs 7.39g. Find the number of atoms of copper in the coin.
How many moles of ammonia gas are in 500 $cm^{3}$ of the gas?
How many moles of ammonia gas are in 500 $cm^{3}$ of the gas?
What is the mass in grams of a compound with 0.433 mol of Sucrose?
What is the mass in grams of a compound with 0.433 mol of Sucrose?
Which of the following describes the mass of a 1.45 X $10^{23}$ molecules of sucrose?
Which of the following describes the mass of a 1.45 X $10^{23}$ molecules of sucrose?
What is the percentage composition of sulfur in sodium sulfate?
What is the percentage composition of sulfur in sodium sulfate?
Consider the combustion of hydrogen gas in air to form water: H₂ + O₂ → H₂O. What is incorrect about the equation?
Consider the combustion of hydrogen gas in air to form water: H₂ + O₂ → H₂O. What is incorrect about the equation?
How can scientists make an unbalanced reaction equation give limited information?
How can scientists make an unbalanced reaction equation give limited information?
Flashcards
What is stoichiometry?
What is stoichiometry?
The quantitative study of the relationship between reactants and products in chemical reactions.
Law of conservation of mass
Law of conservation of mass
Mass is neither created nor destroyed in a chemical reaction
Law of multiple proportions
Law of multiple proportions
When two elements form multiple compounds, the mass ratio of one element to a fixed mass of the other is a small whole number.
Law of constant composition
Law of constant composition
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Reaction stoichiometry
Reaction stoichiometry
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Compositional stoichiometry
Compositional stoichiometry
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What is a mole?
What is a mole?
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What is molar volume?
What is molar volume?
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What is mole fraction?
What is mole fraction?
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What is a limiting reagent?
What is a limiting reagent?
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Theoretical yield
Theoretical yield
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Actual yield
Actual yield
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Percentage yield
Percentage yield
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What is a chemical equation?
What is a chemical equation?
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Balancing Chemical Equations
Balancing Chemical Equations
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Study Notes
- Chemical equations and stoichiometry are part of CHM 101.
Stoichiometry
- Stoichiometry is derived from the Greek words "stoikeion" (element) and "metron" (measurement).
- Stoichiometry studies the qualitative and quantitative relationships between substances undergoing chemical changes.
- Stoichiometry is based on the following laws:
- Law of conservation of mass
- Law of multiple proportions
- Law of constant composition
Law of Conservation of Mass
- This law states that mass is neither created nor destroyed during a chemical reaction.
- Antoine Lavoisier stated the law of conservation of energy in the 17th century.
- In the reaction 2H₂ + O₂ → 2H₂O, the masses are as follows:
- 2x2 of Hâ‚‚ + 16x2 of Oâ‚‚ results in 2x18 of Hâ‚‚O
- 4 + 32 equals 36
- The complete combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) demonstrates mass conservation
- 16g of CHâ‚„ + 64g of Oâ‚‚ yields 44g of COâ‚‚ + 36g of Hâ‚‚O
- 80g equals 80g
Law of Multiple Proportions
- This law states that when two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.
- For example, carbon and oxygen can form CO and COâ‚‚.
- The ratio of oxygen masses is 1.333:2.666, which simplifies to 1:2.
- Similarly, the ratio of oxygen masses is 16:32, also simplifying to 1:2.
Law of Constant Composition
- This law states that samples of a pure compound always contain the same elements in the same mass proportion.
- Example is a cupric oxide sample which weighed 1.375 g and was heated with hydrogen to yield 1.098 g of copper.
- In another example, 1.179 g of copper was dissolved in nitric acid to produce copper nitrate, which was then burned to produce 1.476 g of cupric oxide.
- Calculations:
- First sample: Copper Oxide = 1.375 g, Copper = 1.098 g, Oxygen = 1.375 - 1.098 = 0.277 g, percent oxygen in CuO = 20.15%.
- Second sample: Copper = 1.179 g, Copper Oxide = 1.476 g, Oxygen = 1.476 - 1.179 = 0.297 g, percent oxygen in CuO = 20.12%.
- Both samples follow the law of constant composition.
Types of Stoichiometry
- Reaction stoichiometry: N₂ + 3H₂ → 2NH₃
- Compositional stoichiometry: e.g., NH₃
Mole Concept
- For any element, the relative atomic mass (Ar) of that element expressed in grams contains the same number of atoms
- This number is 6.022 x 10²³.
- 6.02 x 10²³ is used for three significant figures.
- Aᵣ(C) = 12.0 implies 12.0 g of carbon contains 6.02 x 10²³ carbon atoms
- Aᵣ(Na) = 23.0 implies 23.0 g of sodium contains 6.02 x 10²³ sodium atoms
- Aᵣ(O) = 16.0 implies 16.0 g of oxygen contains 6.02 x 10²³ oxygen atoms
- A mole is the amount of substance containing as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 12 g of ¹²C
- Avogadro's hypothesis: One molecule of gas at STP occupies 22.4 dm³.
- One mole of oxygen (O₂) occupies 22.4 liters at STP, has a mass of about 32 grams, and contains about 6.022 x 10²³ molecules of oxygen.
- 6.022 x 10²³ atoms (12amu/atoms) equals 12 g; hence, 6.022 x 10²³ amu equals 1 g.
Formulae
- Mole = mass given (g)/molar mass (g/mol)
- Mole = Number of molecules, ions, atoms or electron/Avogadro's number
- Mole of gas = Volume of the gas (cm³ or dm³)/molar volume (cm³/mol or dm³/mol)
Mole Fraction
- Mole fraction is the ratio of the mole of a component to the total moles of all components in a solution.
- For components A, B, and C in a solution: XA = nA/nT, where nT is the total number of moles.
- The total mole fraction of all components must equal unity.
- The mole fraction of a gas is related to its partial pressure via the equations Pâ‚œV = nâ‚œRT and PA V = nA RT.
- Substituting, XA = PA/PT
Percent Composition
- The percent by mass of each element in a compound.
- % composition = (n x molar mass of element/mm of compd) x 100, where n is the number of moles of element in one mole of the compound.
Chemical Formula and Equations
- A chemical formula indicates the number of atoms in a molecule.
- Chemical symbols and formulas are combined to form a chemical equation.
- Chemical equations provide two key pieces of information:
- Nature of reactants and products.
- Relative numbers of each.
Balancing Chemical Equations
- Alter the coefficient but never change the subscript.
- Identify the number of each of the atoms on both sides of the equation (LHS and RHS).
- Use the simplest possible set of whole number coefficients to balance the chemical equation.
- Establish the correct state of existence of atoms and molecules.
- For complex equations, use the simultaneous equation method.
- For each element in the equation, form a mathematical relation (LHS = RHS).
- LHS represents the total number of atoms of each type in the reactants.
- RHS represents the total number of atoms of each type in the products.
- Assume the coefficient of the first compound to be unity, then find the coefficients of other compounds in terms of the first.
Limiting Reagents and Percentage Yield
- The limiting reagent is the reactant completely used when a reaction goes to completion.
- It limits the amount of product formed thus giving the smallest yield, which is the theoretical yield,
- The mole of the limiting reactant is used to determine the moles/amount of other reactants and products.
- Procedure to find the limiting reagent and theoretical yield:
- Find the moles of each reactant present.
- Calculate the moles of a product formed from each mole of reactant.
- Identify the reactant giving the smaller number of moles of product; this is the limiting reagent.
- Calculate the grams of product produced by the limiting reagent, which is the theoretical yield.
- Theoretical yield refers to the amount of a given product formed when the limiting reactant is completely consumed.
- Percentage yield is calculated as (Actual yield/Theoretical yield) X 100%.
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