Chapter 3

Question 1

What is Chemcial Stoichichiometry

Answer 1

The quantities of materials consumed and produced in chemical reactions.

Question 2

Formula for Average Mass

Answer 2

Total Mass / # of samples

Question 3

True of False

Objects need to have identical masses to be counted by weighing?

Answer 3

False
Objects do not need to have identical masses to be counted by weighting. For purposes of counting, the objects behave as though they each actually had the average mass.

Question 4

How did nineteenth-century chemists calculate relative masses?

Answer 4

By observing the proportions in which elements combine to form various compounds.

Question 5

The modern system of atomic masses (1961) is based on?

Answer 5

12 C or carbon twelve. In this system, 12 C is assigned a mass of exactly 12 atomic mass units (u) and the masses of all other atoms are given relative to this standard.

Question 6

What is the most accurate method currently available for comparing the masses of atoms

Answer 6

Mass Spectrometer - see page 83 fig 3.1

Question 7

Atomic mass is sometimes called?

Answer 7

Atomic Weight

Question 8

What is Atomic Mass?

Answer 8

Atomic mass is an average value reflecting the average of the isotopes composing it.

Question 9

What is the Atomic Mass of Carbon

Answer 9

12.01 g Even though natural carbon does not contain a single atom with mass 12.01, for stoichiometric purposes, we can consider carbon to be composed of only one type of atom with a mass of 12.01.

Question 10

How is the mass for each element determined?

Answer 10

It is an average value based on the isotopic composition of the naturally occurring element.

Question 11

How is isotopic composition determined

Answer 11

Mass spectrometer - useful for determining accurate mass values for individual atoms - is used to determine the isotopic composition of a natural element.

Question 12

What is a Mole

Answer 12

The number equal to the number of carbon atoms in exactly 12g of pure 12C. Because samples of matter typically contain so many atoms, a unit of measure called the mole has been established for use in counting atoms.

mole = 6.02214X10 23

Question 13

A mole of something consists of how many units of that substance.

Answer 13

6.022 x 10^23

Question 14

How many eggs in a mole.

Answer 14

6.022 x 10^23

Question 15

What is Avogadro’s number

Answer 15

The number of atoms in exactly 12g of 12C or 12g of 12C contains 6.022 x 10^23 atoms.

Question 16

Definition of Mole?

Answer 16

The mole is defined such that a sample of a natural element with a mass equal to the element’s atomic mass expressed in grams contains 1 mole of atoms.

Question 17

What is a chemical compound?

Answer 17

A collection of atoms.

Question 18

What is the molar mass of a substance?

Answer 18

The mass in grams of 1 mole of the compound.

Question 19

Define Formula Unit

Answer 19

Many substances are ionic - they contain simple ions or polyatomic ions. Because ionic compounds (i.e. NaCl Na+ and Cl- CaCO3 Ca 2+ and CO3 2- ) do not contain molecules, we use the term formula unit.

Question 20

What is Conceptual Problem Solving

Answer 20

Solve problems in a flexible, creative way based on understanding the fundamental ideas of chemistry.

Question 21

What are 2 fundamentally different ways you might use to approach a problem?

Answer 21

Pigeonholing method - emphasizes memorization. Label the problem to decide in which pigeonhole it fits.

Conceptual problem solving - big picture - a real understanding of the situation.

Question 22

What are the 3 steps to solving a problem.

Answer 22

1 - Decide the final goal - where are we going.
2- How do we get there?
3- Reality check

Question 23

What are the two common ways of describing the composition of a compound?

Answer 23

1- Terms of the numbers of its constituent atoms

2- Terms of the percentages (by mass) of its elements.

Question 24

What is another name for Mass Percent

Answer 24

Weight Percent

Question 25

Give an example of Mass Percent

Answer 25

Mass percent of C in C2H5OH mass of C in 1 mol C2H5OH/mass of 1 mol C2H5OH x 100%

Question 26

A sample burned in excess oxygen yields what?

Answer 26

Converts all its carbon to CO2 and all its hydrogen to H2O. Other substances as measured by absorption - measure the increase in masses of the absorbents.

Question 27

Empirical Formula vs. Molecular Formula

Answer 27

Any molecule that can be represented as (CH5N)n where n is an integer, have the empirical formula CH5N. To be able to specify the exact formula of the molecule involved, the molecular formula, we must know the molar mass.

Question 28

What is the Molecular Formula

Answer 28

(Empirical Formula)n where n is an integer.

Question 29

Problem Solving Strategy: Empirical Formula Determination

Answer 29

1- Since mass percentage gives the number of grams of a particular element per 100g of compound, base the calculation on 100g of compound. Each percent will then represent the mas in grams of the element. 2- Determine the number of moles of each clement present in 100 g of compound using the atomic masses of the elements present. 3- Divide each value of the number of moles by the smallest of the values. If each resulting number is a whole number (after appropriate rounding), these numbers represent the subscripts of the elements in the empirical formula. 4- If the numbers obtained in the previous steps are not whole numbers, multiply each number by an integer so that the results are all whole numbers.

Question 30

Problem Solving Strategy Determining Molecular Formula from Empirical Formula

Answer 30

1- Obtain the empirical formula 2- Compute the mass corresponding to the empirical formula 3- Calculate the ratio: Molar Mass/Empirical Formula Mass 4- The integer from the previous step represents the number of empirical formula units in one molecule. When the empirical formula subscripts are multiplied by this integer, the molecular formula results. This procedure is summarized by the equation: Molecular formula = empirical formula x Molar Mass / empirical formula mass

Question 31

True or False For each compound there is only one form

Answer 31

False

Question 32

Problem Solving Strategy Determining Molecular Formula from Mass Percent and Molar Mass

Answer 32

1- Using hte mass percentages and the molar mass, determine the mass of each element present in 1 mole of compound. 2- Determine the number of moles of each element present in 1 mole of compound. 3- The integers from the previous step represent the subscripts in the molecular formula.

Question 33

What is a chemical change?

Answer 33

A chemical change involves a reorganization of the atoms in one or more substances.

Question 34

What is a chemical equation

Answer 34

Chemical Equation - Reactants on the left side of an arrow and the products on the right side. CH4 + 202 --> CO2 = 2H20 reactants products

Question 35

True of False | In a chemical reaction - bonds have been broken and new ones have been formed.

Answer 35

True

Question 36

True of False | In a chemical reactions, atoms are neither created nor destroyed.

Answer 36

True

Question 37

The chemical equation for a reaction gives what two important types of information.

Answer 37

1- The nature of the reatants and the products and the relative numbers of each.

Question 38

True of False The reactants and products in a specific reaction must be identified by experiment.

Answer 38

True

Question 39

Besides specifying the compounds involved in the reaction, the equation gives the physical states of the reactants and products. What are the four states and symbols

Answer 39

1- Solid s 2- Liquid l 3- Gas g 4- Dissolved in water (in aqueous solution) aq

Question 40

True of False The formulas of the compounds must never be changes in balancing a chemical equation.

Answer 40

True The subscripts in a formula cannot be changed, nor can atoms be added or subtracted from a formula.

Question 41

True of False It is always best to start with the most complicated molecules when balancing chemical equations.

Answer 41

True | Start with those containing the greatest number of atoms.

Question 42

Problem solving: What are the steps for Writing and Balancing the Equation for a Chemical Reaction.

Answer 42

1- Determine what reactions is occurring. What are the reactants, the products, and the physical states involved. 2- Write the unbalanced equations that summaries the reaction described in step 1. 3- Balance the equation by inspection, starting with the most complicated molecules. Determine what coefficients are necessary so that the same number of each type of atom appears on both reactant and product sides. Do on change the identities (formulas) of any of the reactants or products.

Question 43

Problem Solving Calculating Masses of Reactants and Products in Chemical Reactions

Answer 43

1- Balance the equation for the reaction. 2- Convert the known mass of the reactant or products to moles of that substance. 3- Use the balanced equation to set up the appropriate mole rations. 4- Use the appropriate mole ratios to calculate the numbers of moles of he desired reactant or product. 5- Convert from oles back to grams if required. Review diagram

Question 44

What is: Stoichiometric Mixture

Answer 44

One that contains the relative amounts of reactants that match the numbers in the balanced equation. In this case all reactants will be consumed to form products.

Question 45

What is: Limiting Reactant

Answer 45

The reactant that runs out first and thus limits the amounts of products that can form.

Question 46

What are the 2 ways to determine the limiting reactant.

Answer 46

1- Determination of the Limiting Reactant Using Reactant Quantities 2- Determination of Limiting Reactant Using Quantities of Productions Formed.

Question 47

What is Theoretical Yield

Answer 47

The amount of a product formed when the limiting reactant is completely consumed.

Question 48

What is Percent Yield

Answer 48

The actual yield of product is often given as a percentage of the theoretical yield.

Question 49

Problem Solving Solving a stoichemetry Problem Involving Masses of Reactants and Products.

Answer 49

1- Write and balance the equation for the reaction. 2- Convert the known masses of substances to moles. 3- Determine which reactant is limiting 4- Using the amount of the limiting reactant and the appropriate mole ratios, compute the number of moles of the desired product. 5- Convert from moles to grams, using the molar mass. See diagram

Question 50

Single Replacement Reaction

Answer 50

Sometimes called a single displacement reaction, is a reaction in which one element is substituted for another element in a compound.

Question 51

Double Replacement Reaction

Answer 51

Also called double displacement, exchange, or metathesis reactions - occurs when parts of two ionic compounds are exchanged, making two new compounds.

Question 52

Decomposition Reaction

Answer 52

Occurs when one reaction breaks down into two or more products. General equation: AB -> A + B

Question 53

Synthesis Reaction

Answer 53

Occurs when two or more reactants combine to form a single product. General equation: A + B -> AB

Question 54

Combustion Reaction

Answer 54

A combustion reaction is a reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat. Combustion reactions must involve O2 as one reactant. The combustion of hydrogen gas produces water vapor.

Question 55

What is the Net Ionic Equation

Answer 55

The net ionic equation is the chemical equation that shows only those elements, compounds, and ions that are directly involved in the chemical reaction. ... Net ionic equations must be balanced by both mass and charge. Balancing by mass means making sure that there are equal numbers of each element.