CHEM BLOCK 2

What is a mole?

The SI unit for the amount of a substance. One contains 6.022 × 10²³ representative particles (Avogadro's number)

What is Avogadro's number?

6.022 × 10²³ particles/mol

One mole of any substance always contains what?

6.022 × 10²³ representative particles.

Representative particles depend on the substance. What are they for elements and compounds?

• Elements → atoms

• Molecular compounds → molecules

• Ionic compounds → formula units

What is molar mass?

The mass of one mole of a substance, expressed in g/mol.

Atomic mass vs. molar mass

• Atomic mass → amu

• Molar mass → g/mol

• Same numerical value, different units.

How do you calculate moles?

Moles = grams ÷ molar mass

How do you calculate grams?

Grams = moles × molar mass.

How do you calculate particles from moles?

Multiply moles by Avogadro's number

How do you calculate moles from particles?

Divide particles by Avogadro's number

How do you calculate the molar mass of a compound?

Add the atomic masses of all atoms in the chemical formula.

What is the molar mass of CO₂?

44 g/mol.

What is the molar mass of H₂O?

18 g/mol.

What is the molar mass of O₂?

32 g/mol.

Can you convert grams directly to particles?

No. Convert grams → moles → particles

What is percent composition?

The percentage by mass that each element contributes to a compound.

Formula for percent composition

(Mass of element ÷ Molar mass of compound) × 100%

Percent composition is based on what?

Mass, not number of atoms.

First step in solving percent composition problems

Calculate the compound's molar mass.

If given percentages, what mass should you assume?

100 g

What is an empirical formula?

The simplest whole-number ratio of atoms in a compound

What is a molecular formula?

The actual number of each type of atom in a molecule.

Can empirical and molecular formulas be the same?

Yes, if the formula is already in the simplest ratio (e.g., CO₂).

How do you determine an empirical formula from percent composition?

• Assume 100 g.

• Convert grams to moles.

• Divide by the smallest number of moles.

• Multiply to obtain whole numbers if needed.

How do you determine a molecular formula from an empirical formula?

Divide the compound's molar mass by the empirical formula mass, then multiply all subscripts by that whole number.

Never round mole ratios too early.

Keep decimals until the final ratio.

What can be changed when balancing equations?

Coefficients only.

What should never be changed?

Subscripts

What does a coefficient represent?

The number of molecules (or moles) of a substance

Does a coefficient multiply every atom in a compound?

Yes

Why must equations be balanced?

To satisfy the Law of Conservation of Mass.

What is stoichiometry?

The quantitative relationship between reactants and products in a balanced chemical equation.

Where do mole ratios come from?

The coefficients of the balanced equation.

What is the general sequence for gram-to-gram problems?

Grams → Moles → Mole Ratio → Moles → Grams.

What is the general sequence for particle-to-particle problems?

Particles → Moles → Mole Ratio → Moles → Particles.

Every stoichiometry problem passes through what unit?

Moles

What is the limiting reagent?

The reactant that is completely consumed first, limiting product formation

What is the excess reagent?

The reactant left over after the reaction is complete.

How do you identify the limiting reagent?

Convert all reactants to moles and compare how much product each could produce.

Is the reactant with fewer grams always limiting?

No

Which reactant determines theoretical yield?

The limiting reagent

Density formula

Density = Mass ÷ Volume.

Rearranged density equations

• Mass = Density × Volume

• Volume = Mass ÷ Density

Density of water

Density of water

Objects denser than water will ______.

Sink

What is theoretical yield?

The maximum amount of product predicted by stoichiometry.

What is actual yield?

The amount of product actually obtained experimentally.

Formula for percent yield

(Actual Yield ÷ Theoretical Yield) × 100%

Can percent yield exceed 100%?

It usually should not; values above 100% often indicate impurities or measurement error.

Name the five major reaction types tested on the OAT.

• Synthesis

• Decomposition

• Single displacement

• Double displacement

• Combustion