Stoichiometry and General Concepts

moles

SI unit for the amount of any substance

Avogadro’s number: 6.022 × 10²³ molecules

molar mass

sum of the mass of all atoms found in one mole’s worth of the substance

Units: grams per mole

Multiply the atomic mass by its coefficients and add them together

how do you calculate number of moles in a sample?

n= m/M

m= mass in grams of sample (g)

M= molar mass (g/mol)

mole ratio

ratio between the amount of moles of any two molecules involved in a chemical reaction

comparison between the coefficients of any two molecules in a chemical reaction

percent composition

the percent of the compound’s mass that is made up by that element

molecular formula

tells you the exact number of atoms of each element in a compound

empirical formula

simplest/most reduced ratio of atoms in a compound (simplified version of molecular formula)

how to determine the molecular formula from the empirical formula?

1. calculate the molar mass of the empirical formula

2. divide the molar mass of the molecular formula by the molar mass of the empirical formula

3. multiply each subscript by the whole number that resulted from step 2

how to determine the empirical formula from the percent composition?

1. assume 100 grams of total sample, coverting percentages into grams

2. divide each number of grams by the element’s atomic mass

3. divide the numbers from step 2 by whichever number is smaller

4. if necessary, muiltply all subscripts by an integer to ensure that the smallest whole number ratio is obtained

steps to balance a chemical reaction

1. identify each element and its quantity for both reactants and products

2. choose an element on each side to compare the quantities

3. add a coefficient to balance the elements

4. repeat until all elements have been balanced

5. now compare the quanitites of hydrogen on each side

6. check to see if the chemical equation is balanced by checking if the number of atoms of each element type is the same on each side of the equation

limiting reagent

reagent that determines how much product is formed and is completely used up in a reaction → 0 g at end of reaction

excess reagent

leftover reactant that is not completely used up

how to determine the limiting reagent

1. make sure chemical equation is balanced

2. convert all given information into moles

3. pick one of the reactants and determine how much of the other reactant is required to fully react with the reactant initially chosen

4. if there is excess of the other reactant, then the reaction you chose is the limiting reagent. if there is a shortage, then the other reactant is the limiting reagent.

density

mass per unit volume

density of water

1 g/mL

*rule of thumb: if an object has a density greater than water, it will sink if placed in it. if it is less, it will float.

under constant temperature and pressure…

density of a pure substance remains constant

actual yield

the real quantity of producr resulting from a chemical reaction

theoretical yield

maximum number of products that you can expect from an ideal chemical reaction

percent yield

measures the efficiency of a chemical reaction

percent yield important notes

• full yield is unrealistic → why percent yield is typically < 100%

• theoretical yield is calculated using the liniting reagent