Week 2: Chemistry

What are the four common compound units?

Speed (mi/hr), Energy (kg m², sec²(joule)), Pressure (N/m²(pascal), Density (mass(g)/volume(m))

Intenive property

A property that does not depemd oon the amount of matter presentDensity

• Temperature

• Boiling point

• Color

• Electrical conductivity

• Specific heat

• Pressure

Extensive Property

A property that does depend on the amount of matter present.Mass

• Volume

• Length

• Total charge

• Total energy

• Heat capacity

• Number of moles

Mass/Volume/Density/ Triange

What is a mole?

A mole is the amount of material containing 6.022×1023 particles.

What is Avogadro’s number?

: 6.022×1023, the number of particles in 1 mol.

What does “1 mol of anything” mean?

it means 6.022×1023 units of that thing (atoms, molecules, ions, marbles, sand grains). Cited: “One mole of anything is 6.022 × 10^23 units of that thing.”

Why do chemists use the mole?

Because atoms are too small to count individually, so chemists count them by weighing.
Cited: “Atoms are far too small to count… therefore… we count them by weighing.”

How is the mole defined using carbon‑12?

1 mol is the number of atoms in exactly 12 g of carbon‑12.

What conversion factor relates moles and atoms? A:

1 mol atoms=6.022×1023 atoms

How do you convert moles → atoms?

Multiply moles by Avogadro’s number.

How do you convert atoms → moles?

Divide atoms by Avogadro’s number.

What is molar mass?

The mass of 1 mol of atoms of an element, in g/mol.

How is molar mass related to atomic mass?

They are numerically equal (amu ↔ g/mol).

How do you convert grams → moles?

moles=grams/molar mass

How do you convert moles → grams?

grams=moles×molar mass

What is density?

density=mass/volume

volume

mass/density

density

mass *volume

: Why does 1 mol of different elements have different masses?

because different atoms have different masses.

Why are intensive properties important?

The intensive properties of a substance are characteristic of that substance and help distinguish it from other substances. (kind of like identifiers)

Significant Figure Rule for Addition and Subtraction

answer must have the same number of decimal places as the value with the fewest decimal

Sig Figs Rule for Multiplication/Division

The answer must have the same number of significant figures as the factor with the fewest sig figs

Exact Numbers (Sig Figs)

Exact numbers have infinite significant figures because they have no measurement uncertainty (counted or defined values), so they never limit the sig figs in a calculation.

Natural abundance

The percentage of each isotope of an element found in nature.

Dominant isotope

The isotope with the highest natural abundance; it pulls the average atomic mass closest to its mass.

Low‑abundance isotope

An isotope that contributes very little to the average atomic mass.

If one isotope has a much higher abundance, what happens to the average atomic mass

It becomes closer to the mass of the more abundant isotope.

Why does chlorine have an average atomic mass of about 35.5 amu

Because Cl‑35 is more abundant than Cl‑37, pulling the average toward 35.

What information is needed to calculate average atomic mass

Each isotope’s mass and its natural abundance.

What does a very low natural abundance mean for an isotope

It barely affects the average atomic mass.

Why do periodic table atomic masses have decimals

They reflect weighted averages, not single isotope masses.

atomic mass formula

What is the key idea that connects mass, moles, and number of atoms?

Mass tells you how many moles you have, and moles tell you how many atoms you have.

Why can different elements have the same number of atoms but different masses?

Because atoms of different elements have different masses, so equal counts don’t mean equal weight.

What does it mean if a sample contains exactly 6.022×1023 atoms?

It means the sample contains one mole of that element, no matter which element it is.

What does the atomic mass on the periodic table actually represent?

It represents the mass of one mole of that element in grams (g/mol), because the periodic table is defined so that the mass of one atom in amu becomes the mass of one mole in grams.

Why does 9.0 g of aluminum contain fewer atoms than a full mole?

Because aluminum atoms are relatively heavy, so 9.0 g is only part of a mole, meaning it contains only part of Avogadro’s number of atoms.

Why is “There are about 2×1023 atoms in 9.0 g of Al” reasonable?

Because 9.0 g is roughly one‑third of a mole, so it should have roughly one‑third of Avogadro’s number of atoms.

Why does one mole of beryllium contain the same number of atoms as 22.99 g of sodium?

Because 22.99 g of sodium is one mole of sodium, and a mole of any element always contains the same number of atoms.