4. Compounds and Stoichiometry

Question 1

What is a molecule and what type of bond must it have?

Answer 1

A combination of two or more atoms held together by a covalent bond.

Question 2

What is the molar mass?

Answer 2

It is the mass of one mole of a compound, usually grams per mole.

This is NOT the atomic number.

Question 3

How would you determine the number of moles in a given amount (g) of H₂0?

Answer 3

1. Calculate the molar mass of one H₂O atom ( 1+1+16 g/mol)
2. Divide the grams given by the molar mass (xxx g) * (mol/16g)
3. Answer in the number of moles

Pay attention to dimensional analysis.

Question 4

What are equivalents and how are they calculated? How are they related to molarity?

Answer 4

They are moles of a species of interest ( in essence).
For example, when thinking about equivalents, don’t think too hard.

For example, to create a 1 N solution of HCl, we would need 1 mole of HCl, because HCl can donate 1 hydrogen.

However, to create a 1 N solution of H2CO3, we would need .5 mole of HCl, because H2CO3 can donate 2 hydrogens.

This is more important for redox reactions.

Question 5

What is the molar mass of NaBr?

Answer 5

113 g/mol

Question 6

What is the molar mass of SrCl₂?

Answer 6

157 g/mol

Question 7

What is the molar mass of C₆H₁₂O₆?

Answer 7

180g/mol

Question 8

What is the number of moles in 100 g of NaBr?

Answer 8

~.9 mol

Question 9

What is the number of moles in 100 g of SrCl₂?

Answer 9

~.6 mol

Question 10

What is the number of moles in 100 g of C₆H₁₂O₆?

Answer 10

~.55 mol

Question 11

How do the number of molecules in 18g of H₂0 compare to the number of formula units in 58.5 g of NaCl?

Answer 11

They are the same. Each one is the atomic mass (g/mol). So each one represents one mole.

Question 12

What is the difference between the empirical formula or the molecular formula?

Answer 12

Empirical Formula: gives the simplest whole-number ratio of the elements of the compound

Molecular formula: gives the exact number of atoms of each element in the compound and isa multiple of the empirical formula.

It’s important to note that the molecular formula is either the same as the empirical formula or a multiple of it.

Empirical Formula: HO
Molecular Formula: H₂O₂

Question 13

What is the percent composition? What is the formula?

Answer 13

Percent Composition: the percent of specific compound (mass) that is made up of a given element.

Question 14

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

Answer 14

Similarities: Contain the same elements in the same ratios
Differences: Molecular formula gives the actual number of atoms in each element in the compound; but the empirical formulas only give the ratio and therefore may or may not give the actual number of atoms.

Question 15

(Chem 4.2) Find the percent composition (by mass) of sodium, carbon, and oxygen, in sodium carbonate (Na₂CO₃)

Answer 15

Sodium: 46/106= 45%
Carbon: 12/106= 9%
Oxygen: 48/106= 46%

1. Find the atomic mass of each one.
2. Find the total atomic mass of the compound
3. Divide the atomic mass of each element by the total atomic mass

Question 16

(Chem 4.2) 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 the empirical formula?

Answer 16

Fe₂S₃O₁₂

Question 17

What is a combination reaction?

Answer 17

occur when two or more reactants combine to form one product

2H₂ + O₂ -> 2 H₂O

Question 18

What is a decomposition reaction?

Answer 18

Occur when one reactant is chemically broken down into two or more products

This typically occurs as a result of heating, high-frequency radiation, or electrolysis. (you need energy to break a bond)

2 H₂O –> 2H₂ + O₂

Question 19

What is a combustion reaction? What is the result?

Answer 19

occur when a fuel and an oxidant (typically oxygen) react, forming the products of water and CO₂ (if the fuel is a hydrocarbon)

Question 20

What is a single or double displacement reaction?

Answer 20

occurs when an ion of one compound is displaced with another element

Question 21

What is a neutralization reaction?
What type of compounds are the two reagents?
What subtype of reaction is it categorized under.

Answer 21

specific types of double displacement reaction are those in which an acid reacts with a base to form a salt (and usually water)

So if you see a salt, it’s probably a neutralization reaction.

Question 22

What type of reaction is a redox reaction?

Answer 22

Single displacement reaction.

Question 23

Describe in word what occurs when Zn(NO₃)₂ is dissolved in (NH₄)₂S

Answer 23

The ammonium cations swap places with (or displace) the zinc cations yielding ammonium nitrate and zinc (II) sulfide.

Zinc (II) sulfide then precipitates out of solution as a solid salt. (Neutralization reaction)

Question 24

What is the most practical way to approach stoichiometry problems?

Answer 24

1. Convert from the given units to moles
2. Use the mole ratio
3. Convert moles to the desired units

Question 25

What is the volume of one mole of a gas at Standard Temperature and Pressure?

Answer 25

22.4L

Question 26

What is the best way to approach or solve limiting reagent/excess reagent problems?

Answer 26

1. All comparisons of reactants must be done in units of moles. Gram-to-gram comparisons will be useless and may even be misleading
2. It is not the absolute mole quantities of the reactants that determine which reactant is the limiting reagent. Rather, the rate at which the reactants are consumed (the stoichiometric ratios of the reactants) combined with the absolute mole quantities determines which reactant is the limiting reactant.

Question 27

What is the difference between the theoretical yield and the actual yield?

Answer 27

Theoretical Yield: the maximum amount of product that can be generated as predicted from the balanced equation, assuming that all of the limiting reactant is consumed, no side reactions have occurred, and the entire product has been collected. This is rarely ever accomplished
Actual Yield: The amount of product one actually obtains during the reaction

Question 28

What is the equation to calculate the percent yield of the a substance?

Answer 28

Question 29

How do you denote ionic charge in non-representative elements?

Answer 29

You use roman numerals

Fe²⁺= Iron (II)
Fe³⁺= Iron (III)
Cu⁺= Copper (1)
Cu²⁺= Copper (II)

Question 30

What nomenclature do you use to describe a monoatomic anion?

Answer 30

You add (-ide) to the end

H^- = Hydride
F^-= Fluoride
O^2-= Oxide
S ^2-= Sulfide
N^3-= Nitride
P^3-= Phosphide

A trend here is that all of these are just one atom, that have taken up the right number of hydrogens to fill their outer shell.

Question 31

What nomenclature do you use to describe oxyanion series? (x2)

Answer 31

Hypo-= Fewest oxygens
-ite = few oxygens
-ate = more oxygens
Per- =most oxygens

ClO^-= hypochlorite
ClO₂^-= chlorite
ClO₃^-= Chlorate
ClO₄^-= Perchlorate

Question 32

What nomenclature do you use to denote the number of hydrogen ions?

Answer 32

One= bi- or just hydrogen
Two= dihydrogen

HCO₃^- = hydrogen carbonate or bicarbonate
HSO₄^- = Hydrogen Sulfate or bisulfate
H₂PO₄^-= dihydrogen phosphate

Question 33

What are electrolytes?

Answer 33

contain equivalents of ions from molecules that dissociate in solution.

The strength of an electrolyte depends on its degree of dissociation or solvation

Question 34

Do strong electrolytes dissolve completely?
Do weak electrolytes dissolve completely?

Answer 34

Strong Electrolytes: dissolve completely
Weak Electrolytes: hydrolyze incompletely