Chapter 4: Compounds And Stoichiometry

Question 1

What is a molecule?

Answer 1

A molecule is a combination of two or more atoms held together by covalent bonds.

Question 2

How many moles are in 9.53 g of MgCl2?

Answer 2

Question 3

What is the molecular weight of SOCl2? How many grams of SOCl2 equals one mole?

Answer 3

Question 4

What is the weight of one carbon atom?

Answer 4

Question 5

What are the units of molarity?

Answer 5

The units of molarity are mol/L

Question 6

Calculate the molar masses of:

NaBr
SrCl2
C6H12O6

Answer 6

Question 7

What is the pattern for dividing by nines?

Answer 7

1/9=.111
2/9=.222
3/9=.333
6/9=.666

Etc.

This is important to know for MCAT, won’t have a calculator.

Question 8

What is 1/8? Quickly calculate any of the eights fractions.

Answer 8

1/8=0.125
3/8=0.375
5/8=0.625
7/8=0.875

Question 9

What is the number of moles in 100g of each of the following:

NaBr
SrCl2
C6H12O6

Answer 9

Question 10

How do the number of molecules in 18 g H2O compare to the formula units of 58.5 g NaCl?

Answer 10

Both values equal one mole of the given substance. The number of entities in a mall is always the same, equal to Avogadro’s number (NA=6.02x10-23 things)

Question 11

What is the formula unit in chemistry?

Answer 11

Formula unit simply means the smallest repeating unit of a compound.

Question 12

Determine the normality of the following solutions (note the species of interest is H+):

0.25 M H3PO4

95 g PO4 3- in 100ml of solution

Answer 12

Normality = equivalents/L

Notice how 95g/mL needed to be converted to g/L in order to get accurate units of normality.

Question 13

What is molecular weight?

Answer 13

Molecular weight is the sum of the atomic weight of all the atoms in a molecule and its units are atomic mass units per molecule.

Question 14

What is formula weight?

Answer 14

Formula weight of an ionic compound is found by adding up the atomic weights of the constituent ions, according to its empirical formula, and its units are also AMU per molecule.

Question 15

What is a mole?

Answer 15

A mole is a quantity of any substance (dollars, kittens, atoms, molecules, particles, golf balls, socks, etc) equal to the number of particles that are found in 12 g of carbon-12 (12 6 C)

This number of particles is defined as Avogadro’s number (6x10-23/mol).

Question 16

One molecule of carbonic acid has a mass of 62 AMU. What is the mass in grams per mole of carbonic acid?

Answer 16

Since one molecule of carbonic acid has a mass of 62 AMU, there are 62 g of carbonic acid per mole of carbonic acid.

Question 17

What is molar mass?

Answer 17

The mass of one mole of a compound is called molar mass and is usually expressed in g/mol.

Question 18

What are the units of molecular weight?

Answer 18

The units of molecular weight are
AMU/molecule.

Question 19

Does molecular weight equal molar mass?

Answer 19

Molecular weight is amu/molecule. Molar mass is g/mol. So molecular weight does not equal molar mass.

Question 20

What is the formula for determining the number of moles of a sample substance?

Answer 20

Moles= mass of sample (g) /
molar mass(g/mol)

Question 21

What is the concept of equivalents?

Answer 21

Equivalents asks the question: how many moles of the thing we are interested in (protons, hydroxide ions, electrons, ions) will one mole of a given compound produce?

For example. Sodium will donate one mole of electrons (one equivalent), but magnesium will donate two moles of electrons (two equivalents).

Another example. One mole of HCl will produce one mole of hydrogen ions (H+), whereas one mole of sulfuric acid (H2SO4) donate two moles of hydrogen ions and one mole of SO4 2-. Therefore to gather one mole of hydrogen ions for a particular acid-base reaction, we could use one mole of HCl, a half mole of sulfuric acid (H2SO4) or a 1/3 mole of phosphoric acid (H3PO4).

Question 22

Compare and contrast molar equivalents and gram equivalents.

Answer 22

Equivalents are equivalents. A molar equivalent would be asking how many moles of a given substance for an equivalent mole of a given substance. (One mole HCL will produce one mole of H+)

Whereas gram equivalents are mass calculations rather than mole units.

For example. Just as one mole of HCL will donate one mole of hydrogen ions, a certain mass of HCL (36.5 g) will also donate one equivalent of hydrogen ions. This gets us to the concept of gram equivalent weight.

Question 23

What is gram equivalent weight and how can it be calculated? Knowing the equation is useful, understanding what the equation means is important.

Answer 23

Gram equivalent weight is an an amount of a compound, measured in grams, that produces one mole of the particles of interest.

Gram equivalent weight= Molar mass/n

Where n is the number of particles of interest produced or consumed per molecule of the compound in the reaction.

For example. One would need 31 g of H2CO3 (molar mass = 62 grams) to produce one equivalent of hydrogen ions because each molecule of H2CO3 donate to hydrogen ions (n=2).

Molar mass HCL is 36.5 g/mol. How many grams would you need for one equivalent of hydrogen ions? You need 36.5 grams of HCL to produce one mol of H+.

Question 24

If the amount of a compound in a reaction is known, and we need to determine how many equivalents are present, how would you calculate that? Talk about it.

Answer 24

Equivalents= mass of compound (g)
gram equivalent weight (g)

If you had the mass a compound you could calculate the gram equivalent weight (molar mass/n) and come up with the equivalents.

Example that I made up:

Question 25

What is normality (N)?

Answer 25

Normality is a measure of concentration in units of equivalents/L. Normality always assumes that the reaction will proceed to completion. 1 N solution of acid contains a [H+] equal to one mol/L. 2 N solution of acid contains a [H+] equal to two mol/L.

Question 26

What is the conversion between molarity and normality?

Answer 26

Molarity = Normality / n Where n is the number of protons, hydroxide ions, electrons, or ions produced or consumed by the solute.

Question 27

Dimensional analysis of equivalent weight, equivalents, gram weight equivalents, and normality.

Answer 27

Question 28

What is the gram equivalent weight of sulfuric acid?

Answer 28

Gram equivalent weight = molar mass / n

Question 29

What is the normality (N) of a 2M Mg(OH)2 solution?

Answer 29

Couple things here to recognize: Should know equation for molarity: molarity= normality/n Also remember while you’re at it, normality = equivalents/L n is the equivalence (using H2SO4 as the example, it produced 2 thingies of interest (in this case hydroxide ions) per mole).

Question 30

What is the Colebrook titration graph of equivalents of carbonic acid?

Answer 30

Carbonic acid, H2CO3, is diprotic, meaning it has two protons to produce per molecule and therefor will take sufficient equivalent of base to dissociate the protons. I call this the Colebrook titration graph of carbonic acid because he likes it or something.

Question 31

Calculate multiples of 16 up to 9.

Answer 31

16 32 48 64 80 96 112 128 144

Question 32

Find the percent composition by mass of sodium, carbon, and oxygen in sodium carbonate Na2CO3

Answer 32

I will need to use approximations for calculations to get values that are close to the answers found in the multiple choice of the MCAT. Actual values are: Na: 43.4% C: 11.3% O: 45.3%

Question 33

Experimental data from the combustion of an unknown compound indicates that it is 28.5% iron, 24% sulfur, and 49.7% oxygen by mass. What is its empirical formula?

Answer 33

Start by assuming a 100 g sample, which represents 28.5 g iron, 24 g sulfur, and 49.7 g oxygen. Next, divide each number of grams by the atomic weight to determine the number of moles. After determining the number of moles, find the multiplier that gives all three compounds into your values of moles. These integer values will give you an empirical formula.

Question 34

What are some similarities and differences between molecular and empirical formulas?

Answer 34

Both molecular and empirical formulas contain the same elements in the same ratios. They differ in that molecular formulas give the actual number of atoms of each element in the compound, empirical formulas give only the ratio and therefor may or may not give the actual number of atoms.

Question 35

What are structural formulas?

Answer 35

Structural formulas are skeletal representations of compounds that show the various bonds between the constituent atoms of a compound.

Question 36

What is the law of constant composition? Bonus calculation, calculate percent composition of hydrogen and oxygen in water.

Answer 36

The law of constant composition states that any pure sample of a given compound will contain the same elements in an identical mass ratio. For example, every sample of water will contain two hydrogen atoms for one oxygen atom. In terms of mass, for every one gram of hydrogen there will be 8 g of oxygen.

Question 37

What is empirical formula? What is molecular formula? Provide an example for benzene (C6H6) and glucose (C6H12O6)

Answer 37

The empirical formula gives the simplest whole number ratio of the elements in the compound. The molecular formula gives the exact number of atoms of each element in the compound and is a multiple of the empirical formula. Empirical formula for benzene will be CH Empirical formula for glucose will be CH2O

Question 38

What is percent composition and how do you calculate it?

Answer 38

The percent composition of an element, mass, is the percent of a specific compound that is made up of a given element. The formula is as follows: %comp=mass of element/molar mass x 100

Question 39

What is the percent composition of chromium in K2Cr2O7?

Answer 39

Question 40

What are the empirical and molecular formulas of a carbohydrate that contains 40.9% carbon, 4.58% hydrogen, and 54.52% oxygen and has a molar mass of 264 g/mol?

Answer 40

How to calculate. Gather molar masses of constituents. Assume 100g sample to calculate ratios based on molar masses of constituents. Extrapolate empirical formula by finding the simplest whole number among the ratios. Find the molar mass of the empirical formula. Take the given molar mass and divide it by empirical molar mass to get a multiplication factor for the empirical formula. Multiply the factor by the empirical formula to calculate the molecular formula.

Question 41

What is a combination reaction?

Answer 41

A combination reaction has two or more reactants forming one product.

Question 42

What is a decomposition reaction?

Answer 42

Decomposition reaction is the opposite of a combination reaction: a single reactant breaks down into two or more products, usually as a result of heating, high frequency, radiation, or electrolysis. The example equation is silver chloride being exposed to sunlight and producing silver crystals and chlorine gas.

Question 43

What is a combustion reaction?

Answer 43

A combustion reaction is a special type of reaction that involves fuel, usually a hydrocarbon, and an oxidant, normally oxygen. The example equation is a simple hydrocarbon of methane reacting with oxygen to produce carbon dioxide and water.

Question 44

How would you go about recognizing a combustion reaction?

Answer 44

Combustion involves oxidation (using oxygen or similar) of a fuel (typically a hydrocarbon, but could be other fuels such as sulfur or sugars). The products can differ, but carbon dioxide and water are almost always present. Therefore, it is important to recognize the reactants and products of this reaction type because you may see it in various contexts.

Question 45

What is a single displacement reaction?

Answer 45

A single displacement reaction occurs when an atom or ion in a compound is replaced by an atom or ion of another element. The example uses solid copper metal displacing silver ions in a clear solution of silver nitrate to form a blue copper nitrate solution and solid silver metal.

Question 46

What is a double displacement reaction (another name for double displacement reactions is metathesis reactions)? What are the products of a double displacement reaction (not molecular products, types of products)?

Answer 46

In double displacement reactions elements from two different compound swap places with each other to form two new compounds. This type of reaction occurs when one of the products is removed from the solution as a precipitate or gas or two of the original species combined to form a weak electrolyte that remains dissociated in solution. For example, when solutions of calcium chloride and silver nitrate are combined, insoluble, silver chloride forms in a solution of calcium nitrate.

Question 47

What are the products of the reaction of zinc nitrate and ammonium sulfide? Zn(NO3)2 + (NH4)2S

Answer 47

This is a double displacement reaction and produces zinc sulfide and ammonium nitrate.

Question 48

What are the products of the reaction of zinc nitrate and sodium hydroxide? Zn(NO3)2 + NaOH

Answer 48

This is a double displacement reaction and produce zinc hydroxide and sodium nitrate solution.

Question 49

What are the products of the reaction of zinc nitrate and sodium carbonate? Zn(NO3)2 + Na2CO3

Answer 49

This is a double displacement reaction, producing zinc carbonate, and sodium nitrate.

Question 50

What is a neutralization reaction?

Answer 50

Neutralization reactions or a specific type of double displacement reaction in which an acid reacts with the base to produce a salt and usually water. An example would be the reaction of hydrochloric acid with sodium hydroxide.

Question 51

Complete and classify the most likely reactions in the table

Answer 51

Question 52

Exercise in balancing an equation.

Answer 52

Question 53

Balance the following:

Answer 53

Question 54

Stoichiometry, an application of dimensional analysis, is often simplified to a series of three fractions. These fractions demonstrate an underlying three step process. What is that process?

Answer 54

Convert from units to moles Use the mole ratio in the balance equation Convert from moles to the desired units

Question 55

How many grams of calcium chloride are needed to prepare 71.7 g of silver chloride according to the following equation?

Answer 55

Question 56

What is the volume of one mole of any ideal gas at STP? How many particles in one mole of any substance? What is one mole of any substance equal to?

Answer 56

22.4 L 6.02 x 10-23 particles It’s molar mass and grams

Question 57

What is a limiting reagent, what is an excess reagent?

Answer 57

In most reactions, one reactant will be used or consumed first. This reactant is known as the limiting reagent. The reactants that remain after all the limiting reagent is used or called excess reagents.

Question 58

If you are faced with a question that proposes two reactants on the MCAT, what should you do?

Answer 58

When the quantities of two reactants are given on the MCAT, expect to have to figure out which is the limiting reagent.

Question 59

When calculating limiting reagents, in what units should your reactants be? Other than absolute mole quantity, what is the other factor for limiting reagents?

Answer 59

When calculating limiting reagents, the units must always be in moles. The rate at which the reactants are consumed combined with the absolute mole quantities determines which reactant is the limiting reagent.

Question 60

Question on page 141

Answer 60

Question 61

What is yield? What is theoretical yield? What is actual yield? What is percent yield?

Answer 61

Yield of a reaction is either the amount of product predicted or actually obtained. Theoretical yield is the maximum amount of product that can be generated as predicted from the balance equation, assuming that all of the limiting reactant is consumed, no side reactions have occurred, and the product has been collected. Actual yield is the amount of product one actually obtains during reaction. Yield is the ratio of the actual yield to the theoretical yield multiplied by 100%.

Question 62

What is the percent yield for a reaction in which 28 g of copper is produced by reacting 32.7 g of zinc in excess copper sulfate solution?

Answer 62

Question 63

MCAT concept check 4.5 page 142

Answer 63

Question 64

Question 4 page 142

Answer 64

Question 65

Speak about the magnitude of the electrostatic force and an ionic bond and the distance between nuclei in ionic bonds.

Answer 65

The magnitude of the electrostatic force in an ionic bond follows the same conventions for coulombs law. The distance between nuclei in ionic bonds is inversely proportional to the force. Therefore, ionic compounds with long bond distances are much more weakly held together.

Question 66

How are ionic compounds held together?

Answer 66

Ionic compounds are held together by ionic bonds, which rely on the force of electrostatic attraction between opposite charged particles.

Question 67

For elements (usually metals) that can form more than one positive ion. What is the nomenclature for such positive ions? What are some examples? What is the older, less commonly use method for naming these ions?

Answer 67

For elements that can form more than one positive ion the charge is indicated by a Roman numeral in parentheses following the name of the element. The other method is adding -ous and -ic. For example:

Question 68

How are mono atomic anions named?

Answer 68

Monoatomic ions are named by dropping the ending of the name of the element and adding -ide. For example:

Question 69

What are polyatomic anions containing oxygen called?

Answer 69

Polyatomic anions containing oxygen are called oxyanions.

Question 70

When an element forms two oxyanions, the name of the one with less oxygen ends in (blank), and the one with more oxygen ends in (blank. Examples?

Answer 70

When an element forms two oxyanions, the name of the one with less oxygen ends in -ite, the one with more oxygen ends in -ate. For example:

Question 71

Extended series of oxyanions exist. Oxyanions could have 1, 2, 3, 4 oxygen. There is a naming system. What is that naming system?

Answer 71

Hypo- and per (for hyper) are used as suffixes for oxyanions with less oxygen or more oxygen, respectively. For example:

Question 72

Polyatomic anions often gain one or more hydrogen ions to form anions of lower charge. How are those resulting ions named? There is an older, less common method. What’s that one?

Answer 72

The resulting ions created by gaining one or more hydrogen ions to form an ion of lower charge are named by adding the word hydrogen, or dihydrogen to the front of the anions name. The older less common method uses the prefix bi- to indicate the addition of a single hydrogen ion. For examples:

Question 73

There are eight common polyatomic ions listed in the book that have unique names. What are they, what is their charge, and what is their name? This card needs to be written until memorized, and after that you can delete this sentence from your card.

Answer 73

Question 74

What is an oxidation state?

Answer 74

Oxidation states are assigned to atoms within a chemical compound, including ionic and covalent compounds. The oxidation state represents the hypothetical charge an atom would have if all bonds were completely ionic. Essentially, oxidation state describes the degree of electron loss (OXIDATION) or gain (REDUCTION) of an atom in a molecule, even if the molecule isn’t fully ionic.

Question 75

When a substance or atom is reduced, what does that mean? When a substance or atom is oxidized, what does that mean?

Answer 75

When a substance or atom is reduced, it means it has gained electrons (and thus gained a negative charge) When a substance or atom is oxidized, it means it has lost electrons (and thus gained a positive charge)

Question 76

There are eight step wise rules for determining oxidation number, listed in priority. What are those eight steps?

Answer 76

1 in a neutral compound, all oxidation numbers add up to zero. 2 in an ion, all oxidation numbers must add up to the charge of the ion. 3 free elements have oxidation number of zero 4 fluorine is always -1 5 The group number generally dictates an oxidation number (group 1A = +1, IIA = +2, III A = +3). 6 hydrogen with nonmetals is +1, hydrogen with metals is -1. 7 Oxygen is usually -2 (except with fluorine and peroxides) 8 Group VA is -1, VI A is -2, VII A is -3

Question 77

In their natural state, what would be the oxidation number of Group IA (1) and Group IIA (2)?

Answer 77

Group one oxidation state would be +1, group 2 oxidation state would be +2.

Question 78

Nonmetals generally form anions. What would be the theoretical oxidation state for all the halogens (what Group are the halogens)?

Answer 78

Halogens are group VIIA (17) and form monoatomic anions with oxidation state of negative one. The reason they do this is they already have seven electrons and aim to fill an octet.

Question 79

What would be the oxidation state of a metal in an an ionic species (MnO4 -, CrO4 2-)? What would be the oxidation state of the halogens in an oxyanionic ion (ClO2 -, ClO -)?

Answer 79

Even though the ionic species that contain the metallic elements are involved in a polyatomic anion, the metals will still have a positive oxidation state (they give up electrons). MnO4 - (permanganate ion) oxidation state of Mn is +7 CrO4 2- (chromate ion)oxidation state of Cr is +6 In oxyanionic species, the halogen is assigned positive oxidation state. ClO2 - (chlorite ion) oxidation state of Cl is +3 ClO - (hypochlorite ion) oxidation state of Cl is +1

Question 80

How can you think of solving for oxidation states being an algebraic equation?

Answer 80

You can set up an algebraic equation to solve for oxidation states such as (image), whereas the equation equals the charge of the ionic compound, the unknown oxidation state would be X, and using the step wise rules of oxidation you can solve for X.

Question 81

What is one way to qualitatively determine the oxidation state of a given element in a solution (generally transition metals). Why? Don’t need to memorize oxidation states of transition metals, don’t need to memorize colors associated with the oxidation states. Just need to know what said in the answer.

Answer 81

The color of a solution can be indicative of the oxidation state of a given element in solution. The same element in different oxidation states can undergo different electron transitions, and therefore absorb different frequencies of light. The image shows the different colors of oxidation states of plutonium.

Question 82

Why do transition metals of oxyanions make good oxidizing agents?

Answer 82

Oxyanions of transition metals like MnO4 -2 (permanganate) and CrO4 -2 (chromate) have an inordinately high oxidation number on the metal. As such, they tend to gain electrons in order to reduce this oxidation number, and thus make good oxidizing agents.

Question 83

What are electrolytes?

Answer 83

Electrolytes enable solutes to carry electrical currents.

Question 84

What is the word for an aggregate that consists of a solute or molecule with one or more solvent molecules?

Answer 84

Solvate

Question 85

What is a strong electrolyte? Example. What is a weak electrolyte? Example. What is a non-electrolyte? Example.

Answer 85

A strong electrolyte dissociates completely into its constituent ions in aqueous solution. NaCl, HCl, for example. A weak electrolyte, ionizes or hydrolyzes incompletely in aqueous solution. Weak acids like acetic acid, and weak bases like ammonia, for example. Non-electrolytes do not ionize at all in aqueous solution, and retain their molecular structure in solution. Organic compounds such as glucose and nonpolar gases (O2, CO2), for example.

Question 86

Label the following solutions as electrolytes or non-electrolytes. Assume these compounds are all in aqueous solution.

Answer 86

Question 87

Identify the following ions as cations or anions, and then provide the formula or chemical symbol.

Answer 87

Question 88

Equations to remember for chapter 4:

Answer 88