Chemistry Chapter 4: Moles and Mass

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Questions and Answers

How is the number of entities calculated from the amount of substance in moles?

Number of entities = moles / 6.022 × 10^23

Number of entities = moles × mass

Number of entities = mass / 6.022 × 10^23

Number of entities = moles × 6.022 × 10^23 (correct)

What information is needed to convert grams of an element to moles?

Density of the element

Number of entities in a compound

Molar mass of the element (correct)

Mass of the compound

If you have 0.0342 mol of silver (Ag), how would you determine its mass in grams?

Mass = moles × gas constant

Mass = moles × standard temperature

Mass = moles × molar mass of Ag (correct)

Mass = moles × 6.022 × 10^23

What is the number of gallium (Ga) atoms in 2.85 x 10-3 mol of gallium?

Number of atoms = 2.85 x 10-3 x 6.022 × 10^23 Signup and view all the answers

How can the number of molecules in a sample of nitrogen dioxide (NO2) be calculated from its mass?

Molecules = grams / molar mass × 6.022 × 10^23 Signup and view all the answers

What is the empirical formula of lactic acid given its mass percentages?

CH2O Signup and view all the answers

What is the role of a limiting reactant in a chemical reaction?

It is the reactant that is completely used up during the reaction. Signup and view all the answers

Which of the following statements about empirical formulas is correct?

It is based solely on mass analysis. Signup and view all the answers

When writing an overall balanced equation for a reaction sequence, what is the correct first step?

Write the sequence of balanced equations. Signup and view all the answers

What is the molecular formula for lactic acid?

C3H6O3 Signup and view all the answers

Which statement correctly explains isomers?

They have the same molecular formula but different properties. Signup and view all the answers

What will happen when the limiting reactant is fully consumed in a reaction?

The reaction will stop as no more products can form. Signup and view all the answers

In an overall balanced equation, how should common substances be treated?

They should be canceled out between the reacting equations. Signup and view all the answers

What is the molar mass of lactic acid?

90.08 g/mol Signup and view all the answers

What percent of mass does oxygen contribute in lactic acid?

53.3% Signup and view all the answers

What does the excess reactant refer to in a chemical reaction?

It is any reactant that is present in an amount greater than needed. Signup and view all the answers

Which compound is responsible for causing milk to sour?

Lactic acid Signup and view all the answers

How do you convert moles of a reactant to grams of a product formed?

By utilizing stoichiometric coefficients from the balanced equation. Signup and view all the answers

What is the significance of balancing chemical equations?

It follows the law of conservation of mass by keeping atom counts equal. Signup and view all the answers

Which of the following compounds shares the same empirical formula as lactic acid?

Formaldehyde Signup and view all the answers

What type of function does lactic acid serve in the human body?

A nutrient for energy. Signup and view all the answers

In the reaction sequence involving copper(I) sulfide and copper(I) oxide, what is being produced from the reactions?

Copper metal and carbon monoxide gas. Signup and view all the answers

What is the primary distinction between theoretical yield and actual yield?

Theoretical yield is always greater than actual yield. Signup and view all the answers

Which formula correctly represents percent yield?

% yield = (actual yield / theoretical yield) × 100 Signup and view all the answers

Why is percent yield typically less than 100%?

Side reactions may consume reactants. Signup and view all the answers

What must remain unchanged when balancing a chemical equation?

The chemical formulas of reactants and products Signup and view all the answers

Which of the following statements about balanced equations is correct?

The smallest whole-number coefficients should generally be used Signup and view all the answers

In the context of chemical reactions, what does dynamic equilibrium refer to?

The state where reactants and products remain constant over time. Signup and view all the answers

How are stoichiometric relationships expressed in a balanced equation?

As molar ratios that serve as conversion factors Signup and view all the answers

When calculating theoretical yield, which factors are taken into account?

The balanced equation's molar ratios. Signup and view all the answers

Given the balanced equation C3H8 + 5 O2 → 3 CO2 + 4 H2O, how many moles of water vapor are produced if 1 mole of propane is burned?

4 moles Signup and view all the answers

If the theoretical yield of a reaction is 100 grams and the actual yield is 80 grams, what is the percent yield?

80% Signup and view all the answers

What can affect the actual yield obtained from a chemical reaction?

The purity of the reactants. Signup and view all the answers

What is the total mass of reactants in the balanced equation given?

204.09 g Signup and view all the answers

Which of the following describes a valid balancing method?

Multiplying all coefficients by the same integer Signup and view all the answers

Which of the following statements about side reactions is true?

They can lead to the formation of unexpected products. Signup and view all the answers

Which equation reflects the principle of conservation of mass?

C3H8 + 5 O2 → 3 CO2 + 4 H2O Signup and view all the answers

What is one key characteristic of stoichiometric calculations?

They involve conversions based on molar ratios Signup and view all the answers

In the combustion of octane (C8H18), what is one of the main products formed?

Carbon dioxide Signup and view all the answers

How many molecules of oxygen are required to completely react with 1 molecule of propane in the given balanced equation?

5 molecules Signup and view all the answers

What is the primary definition of a mole in terms of substance amount?

The amount of a substance corresponding to 6.022x10^23 particles Signup and view all the answers

Which of the following correctly describes Avogadro’s number?

It represents the number of particles in one mole of a substance Signup and view all the answers

What is the mass of one mole of carbon-12?

12 g Signup and view all the answers

If 1 atom of copper (Cu) has a mass of 63.55 amu, what would be the mass of 1 mole of copper?

63.55 g Signup and view all the answers

Which of the following states the relationship between atomic mass unit and grams for a substance?

The mass in grams of 1 mole equals its mass in amu Signup and view all the answers

What is the mass of 1 mole of water (H2O) when considering its molecular mass?

18.02 g Signup and view all the answers

Which of the following correctly defines 'entities' in the context of a mole?

Any type of particle, including atoms and ions Signup and view all the answers

How many entities are contained in 1 mole?

6.022x10^23 Signup and view all the answers

Which substance has a mass of 58.44 g for 1 mole?

NaCl Signup and view all the answers

If 1 mole contains 6.022x10^23 particles, what can you say about the amount in different substances?

1 mole contains the same number of entities regardless of the substance Signup and view all the answers

Study Notes

The Mole and Stoichiometry of Formulas and Equations

The mole (mol) is the amount of substance containing the same number of entities as the number of atoms in exactly 12 g of carbon-12
'Entities' refer to atoms, ions, molecules, formula units or electrons—any particle type
One mole (1 mol) contains 6.022x10²³ entities (to four significant figures)
This number is called Avogadro's number (N_A)
1 mol of carbon-12 contains 6.022x10²³ carbon-12 atoms and has a mass of 12 g
Mass in atomic mass units (amu) of one atom/molecule/formula unit is numerically the same as the mass in grams of 1 mole of atoms/compound

Photosynthesis

Photosynthesis is a process where plants use light energy to convert carbon dioxide and water into sugar
Chemical formula for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Stoichiometry of Formulas and Equations (Page 3)

The mole
Formula of compounds
Writing and balancing chemical equations
Calculating quantities of reactants and products
Reversibility of reactions and equilibrium state

Molar Mass

The molar mass (M) of a substance is the mass per mole of its entities (atoms, molecules, or formula units).
Unit is g/mol
For monatomic elements, the molar mass is the same as its atomic mass in grams per mole. Atomic mass is from the Periodic Table.
The molar mass of Neon (Ne) = 20.18 g/mol
To find molar mass of diatomic molecules and compounds, add molar masses of atoms in the formula

Example: Chemical Formula of Glucose

Subscripts in a formula refer to both the atoms and moles of atoms
Glucose: C₆H₁₂O₆
6 atoms of Carbon, 12 atoms of Hydrogen, 6 atoms of Oxygen
Moles of atoms per mole of compound (glucose): 6 mol C, 12 mol H, 6 mol O
Atoms/mole of compound: 6x(6.022x10²³) atoms C, 12x(6.022x10²³) atoms H, 6x(6.022x10²³) atoms O
Mass/molecule of compound (glucose): 72.06 amu C, 12.10 amu H, 96.00 amu O ; Total = 180.16 amu
Mass/mole of compound (glucose): 72.06 g C, 12.10 g H, 96.00 g O; Total = 180.16 g

Moles, Mass, and Number of Chemical Entities

Mass (g) = no. of moles x molar mass (g/mol)
No. of moles = mass (g) / molar mass (g/mol)
No. of entities = no. of moles x Avogadro's number (6.022 x 10²³ entities/mol)
No. of moles = no. of entities / Avogadro's number

Amount-Mass-Number Relationship

Relationship between mass (g), amount (mol), and number of atoms/molecules of elements
Mass (g) of element is related to molar mass (g/mol) and Amount (mol)
Amount (mol) of element is related to Avogardro's number and number of atoms/molecules

Sample Problems (Page 11, 12, 13, 14, and 17)

Students will find examples or problems calculating between mass and amount of an element or compound, relating amount/number of entities to amount of an element, converting between number of entities and mass of compound

Mass Percent

Mass % of an element = (atoms of X in formula x atomic mass of X (amu)) / molecular mass of compound (amu) x 100%
or, Mass % of element X = (moles of X in formula x molar mass of element X (g/mol)) / mass (g) of 1 mol of compound x 100%

Mass of an Element

Mass of any element in sample = mass of compound x (mass of element in 1 mol of compound / mass of 1 mol of compound)

Sample Problem 4

Determining the mass of an element in a compound (Calculating the mass of nitrogen in ammonium nitrate)

Empirical and Molecular Formulas

Empirical formula is the simplest formula for a compound that agrees with elemental analysis (shows lowest whole number ratio of atoms)
Molecular formula is the actual number of atoms of each element in the compound (actual # of atoms per molecule)
Structural formula shows the relative placement and connections of atoms in the molecule

Determining the Empirical Formula

Finding the empirical formula of a compound
Finding mass (g) of each component
Convert each mass (g) to amount (mol) to write a preliminary formula
Convert the amount (mol) into integer subscripts to get the formula

Determining the Molecular Formula

The molecular formula is a whole-number multiple of the empirical formula.
(Molar mass (g/mol)) / (empirical formula mass (g/mol)) = whole number multiple.

Sample Problem 5

Determining the empirical and molecular formulas of lactic acid (Example of compound and mass percentages of constituent elements) (mass % C, mass % H, mass % O)

Chemical Formulas and Molecular Structures

Empirical formula does not show molecular structure
Molecular formula does not show molecular structure
Isomers are compounds with the same molecular formula but different properties

Compounds with Empirical Formula CH₂O

List of compounds with the same empirical formula, their molecular formulas, whole number multiples, and uses

Chemical Equations

A chemical equation represents identities and quantities in a physical or chemical change

Balancing a Chemical Equation

Balancing equations is needed to follow the Law of Conservation of Mass
Steps to balance equations:
- Step 1: Write a skeleton equation
- Step 2: Balance the atoms (match the numbers of each type of atom on left and right sides of reaction)
- Step 3:Adjust coefficients to balance
- Step 4: Check the balance
- Step 5: Specify states of substances (solid, liquid, gas, or aqueous)

A Three-level View of a Reaction

Shows macroscopic level reaction, atomic level reaction (with diagrams of atoms rearranging), and the balanced chemical equation

Balancing Process

Chemical formulas cannot be altered, only coefficients can be altered
Balancing equations implies maintaining same number of atoms on each side of reaction
Coefficients can be modified in order to achieve a balanced equation

Sample Problem 6

Balancing a chemical equation related to combustion of octane in a car's engine

Visualizing a Reaction with a Molecular Scene

Visual representation of reactants and products

Stoichiometric Calculations

Stoichiometry is the chemical arithmetic related to mole-mass conversion
In a balanced equation, amounts in moles of substances are stoichiometrically equivalent to one another.
Quantitative relationships are expressed as molar ratios

Information Contained in a Balanced Equation

Reactants and products showing quantities in terms of molecules and amounts in moles and grams

Amount-Mass-Number Relationships

Relationship between mass, amount, and number of entities through balanced equations

Sample Problem 7

Calculating quantities using amounts in moles and relating them to mass in grams of a specific reactant or product

Reactions in Sequence

Multiple overall reaction steps in which one substance forms as a reactant is reacted or produced.
Steps to write overall equation:
- Step 1: Write the separate balanced equations for each reaction
- Step 2: Adjust the equations arithmetically to cancel common substances (from 1)
- Step 3: Add the adjusted equations together

Sample Problem 8

Example of calculating overall equation for a specific reaction sequence

Limiting Reactants

Defined as the substance in a reaction that is present in limiting amount and completely used up before the other reactants
Determines the maximum amount of product that can be formed in a chemical reaction
Excess Reactants are any other reactants that are still present after the limiting reactant is used up (and determines how much reactant is left)

Limiting Reactants - Example

Example problem of finding a limiting reactant and/or excess reactant; including calculation from a balanced chemical equation

Reaction Table

Illustrates the initial amount, change (of moles), and final amount (of moles) of Reactants and Products in a balanced reaction

Sample Problem 9

Example calculations based on amount to amount calculations in a limiting reactant reaction

Stoichiometric Relationships

Shows the relationship among various quantities in chemical reactions: mass, amount (mol), and number of atoms/molecules (of each element, from both reactants and products)

Reaction Yields

Shows the relationship between theoretical yield (calculated yield of product using the balanced reaction and given mass) vs. the actual yield (actual amount produced by performing an experiment)
Theoretical yield vs. Actual yield

Percent Reaction Yield

The percent yield of product = (actual yield/theoretical yield) x 100%

Sample Problem 10

Example of calculating the percent yield. (Example problem about determining percent yield in processing SiC from reacting sand with carbon)

The Equilibrium State

Equilibrium reaction in open and closed container
Balanced reaction (Reactants vs Products)
Equilibrium reactions in closed vs open containers

Studying That Suits You

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Description

This quiz covers the calculations related to moles, mass, and the number of entities in chemistry. Learn how to convert grams to moles and determine the mass of various elements and compounds. Test your understanding of fundamental concepts in stoichiometry.