Unit 3, Stoichiometry

Question 1

Stoichiometry

Answer 1

the study of the quantitative aspects of chemical reactions

Question 2

The Mole (4)

Answer 2

• the mole: (n)
• Avogardo’s number (n)
• one mole = 6.022x10^23 particles
• The mole amount of substance containing 6.022x10^23 of anything… just as a dozen is the amount of substance containing 12 of anything

Question 3

Molecular Mass

Answer 3

mass of one molecule of a covalent compound. Simply add the masses of each atom from the periodic table

Question 4

Atomic Mass

Answer 4

mass of a single atom (units: u)

Question 5

Molar mass

Answer 5

add the unit g/mol (or g*mol^-1) to the molecular mass

Question 6

Formula Mass

Answer 6

the mass of a formula unit(the smallest usit of an ionic compound)
to calculate, ass the mass of each atom from the periodic table

Question 7

Mass spectrometer

Answer 7

measures the molar mass of a new substance

Question 8

Combustion analyzer

Answer 8

determines the percentages of C, H, O, and sometimes N

Question 9

Empirical formula

Answer 9

the simplest whole-number ratio of the atoms (or ions) in the compound

Question 10

Molecular formula

Answer 10

gives the actual number of atoms of each element in one molecule of the compound

Question 11

3 ways to determine the empirical/molecular formula

Answer 11

1. percent composition data
2. combustion analysis (AP)
3. formula of a hydrate (AP)

Question 12

How to find empirical formula (then molecular)

Answer 12

assume 100g
columns: the different elements
rows: mass (g), Molar mass (g/mol), Moles (mol), Mole ratio (in this case it’s the emperical formula)
after, to find the molecular formula divide the mass of sample by the mass of emperical formula to get the number of times the molecular formula is bigger than the empirical formula

Question 13

Limiting reactant/reagent

Answer 13

the reactant that reacts completely and is therefore not present when the reaction is complete
since the limiting reactant reacts completely, its amount determines the amount of product that can form

Question 14

Excess reagent

Answer 14

any reactant that is left over at the conclusion of a reaction

Question 15

Example of limiting/excess reactants

Answer 15

When making burgers you need all of them to have exactly the same things, whichever ingredient runs out first is the limiting one. Because you can only make that many burgers, whatever is leftover is in excess

Question 16

What do the coefficients in a balanced chemical equation mean?

Answer 16

they tell us about the relative numbers of moles of reactants and products

Question 17

Mole ratio

Answer 17

used to relate the number of moles of one reactant or product to another
mole ratios are obtained from the coefficients in the balanced equation

Question 18

Law of conservation of mass

Answer 18

states that the masses of the reactants that are consumed must equal the masses of the products that are formed
mass consumed reactants = mass formed products

Question 19

Procudure for solving limiting reagent problems (5)

Answer 19

1. balance equation
2. convert into units of moles
3. (mole amount of each reactant) / (coefficient from equation)
4. the reactant with the smallest quotient from step 3 will be the limiting reagent
5. work as typical stoichiometry problem using the limiting reagent

Question 20

Actual yield

Answer 20

the amount of product actually obtained in the lab (usually less that theoretical yield)

Question 21

Theoretical Yield

Answer 21

the amount predicted by calculations
only attained if 100% of the limiting reagent is converted to the desired product

Question 22

Yields

Answer 22

describe the amount of product, and can be in mass units, moles, or number of molecules

Question 23

Percent yield

Answer 23

describes how much of a product is actually obtained relative to the amount that should form assuming complete reaction of the limiting reactant
Percent yield = (actual yield) / (theoretical yield) x 100 –> to find percentage

Question 24

How to calculate percent yield

Answer 24

1. balance
2. determine limiting reagent
3. use mole ratios to determine the mass (theoretical yield) of the product under analysis
4. use the actual yield given, to calculate percent yield
   Percent yield = (actual yield) / (theoretical yield) x 100%

Question 25

# Factors affecting the reaction yield The nature of the reaction

Answer 25

In some reactions the product will form back into reactant (reverse reaction) (use the half and half arrow)

Question 26

# Factors affecting the reaction yield The experimental procedure

Answer 26

materials used during the experiment, not all product can be transferred/poured out, or through isolation and purification of the product

Question 27

# Factors affecting the reaction yield Impurities

Answer 27

- chemicals are rarely 100% pure especially if not stored properly - sodium hydroxide readily absorbs water - magnesium strips form an oxide layer when exposed to air. This increases the mass of these chemicals

Question 28

# Factors affecting the reaction yield Competing side reactions

Answer 28

another, more unfavorable product may form

Question 29

Factors affecting the reaction yield

Answer 29

1. the nature of the reaction 2. experimental procedure 3. impurities 4. competing side reactions

Question 30

Percent composition

Answer 30

ex. percent composition of C in CO2: % = (mass C) / (mass CO2) x 100 % = 12.011u / 44.01u x 100 = 27.29% for O in CO2 do (mass O x2)/(mass CO2)

Question 31

Conversion Factor

Answer 31

A numerical ratio or fraction used to express a measurement where the units are changed, but not the value. exact numbers that demonstrate a relationship

Question 32

Why are conversion factors useful?

Answer 32

It allows us to find different measurements of a substance without changing anything.

Question 33

Law of definite proportions

Answer 33

Different samples of the same compound always contain its constituents’ elements in the same proportions by mass.