Chapter 27 - Stoichiometry Flashcards
Define a compound:
A pure substance composed of 2 or more elements in a fixed proportion
How are compounds broken down? What does this produce?
Chemically to produce their constituent elements or other compounds
What elements can react with other elements to form new compounds?
All elements except noble gases
Define a molecule:
A combination of 2 or more atoms held together by covalent bonds and is the smallest unit of a compound that displays the properties of that compound
Do ionic compounds form true molecules?
No
Define molecular mass:
The sum of the atomic masses of the atoms that make up a molecule
Define formula mass:
The sum of the atomic masses of each constituent atom according to the empire formula of the substance
Define a mole:
Amount of a substance that contains the same number of particles found in a 12g sample of carbon-12
- Avogadros Number: 6.002 x 10^23
What is the mass of one mole of a compound called? What is it usually expressed in?
Molar mass or molar weight
- g/mol
What is the formula for “number of moles”?
Mols = Weight of Sample (g) / Molar Weight (g/mol)
How many moles are in 9.52g of MgCl2 (page314)?
0.10 mol of MgCl2
Why is it important to define a measure of reactive capacity for some substances? What is this called?
Expresses the fact that some molecules are more potent than others in performing certain reactions
This difference is expressed by using the term equivalent
How do you determine equivalents? What is the equation?
By a new measure of weight called gram-equivalent weight (GEW):
Equivalents = Weight of Compound / GEW
AND
GEW = molar mass / n
*n = number of hydrogen atoms that could be donated/accepted
What is the Law of Constant Composition?
States that all samples of a given compound will contain the same elements in identical mass ratios
What are the 2 ways to express a formula for a compound?
1) Empirical Formula: Gives simplest whole number ratio of the elements in the compound
2) Molecular Formula: Gives the exact number of atoms of each element in the compound and is a multiple of the empirical formula
- Simplifying this formula will give the empirical formula
Define percent composition:
The mass percent of the element in a specific compound
How do you determine percent composition?
Percent Composition = [Mass of X in Formula / Formula Weight of Compound] x 100
Can you determine the percent composition with the empirical and molecular formulas?
Yes
- If the percent composition is known, the empirical formula can be derived
- If both percent composition and molecular mass is know, the molecular formula can be derived
What is the percent composition of chromium in K2Cr2O7 (page 316)?
- K = 39g/mol
- Cr = 52g/mol
- O = 16g/mol
35.4%
What are the empirical and molecular formulas of a compound that contains 40.9% C, 4.58% H, and 52.52% O by mass and has a molecular mass of 264g/mol (page316/317)?
Molecular formula = C9H12O9
Empirical formula = C3H4O3
What is the Law of Conservation of Mass? Why is this important? What is used to demonstrate this?
- The mass of reactants in a reaction must be equal to the mass of the products
- Chemical equations must be balance so that there are same number of atoms of each element in the product as there are in the reactants
- Stoichiometric coefficients are used to indicate number of moles of a species involved in the reaction
What steps are used in stoichiometry to achieve a balanced equation?
1) Balance carbon reactant/products
2) Balance H reactant/products
3) Balance O reactant/products
4) Achieve whole numbers by multiplying/dividing, etc
What is an application of stoichiometry?
Once an equation is balance, it can be used to solve many types of problems:
- Limiting reagent
- Yield
Application of Stoichiometry: How many grams of CaCl2 is needed to prepare 72g of AgCl according to the following equation (page 318)? CaCl2 + 2AgNO3 —-> Ca(NO3)2 + 2AgCl
- Ca = 40g/mol
- Cl = 35.5g/mol
- Ag = 108g/mol
27.5g of CaCl2 is needed to produce 72g of AgCl
What is a limiting reagent? What is an excess reactant?
In most reactions, one reactant will be confused first (limiting reagent) which limits the amount of product formed
The excess reactant is what remains after the limiting reagent is used up
If 28g of Fe are reacted with 24g of S to produce FeS, what is the limiting reactant? How many grams of excess reactant are present in the vessels at the en of the reaction (page 319)?
8g of S
What is the yield of the reaction? Name the 3 types:
The amount of product predicted or obtained when the reaction is carried out
1) Theoretical: Product predicted from a balanced equation
2) Actual: Product isolated from reaction experimentally
3) Percent Yield: Express relationship between actual and theoretical which is expressed by
Percent Yield = [Actual / Theoretical] x 100
What is the percent yield for a reaction in which 27g of Cu is produced by reacting 32.5g of Zn in excess CuSO4 solution (page 319)?
- Cu = 63.5g/mol
- Zn = 65g/mol
- S = 32g/mol
- O = 16g/mol
84%
