Mols, Mass and Molar mass

atom

A single element, like Na, H, or O.

compound

A substance made of two or more different elements chemically bonded, e.g., H₂O, NaCl.

element

A pure substance made of only one type of atom, like O, H, C, Mg, or P.

molecule

Two or more atoms chemically bonded together, which can be the same or different elements.

relationship between number of particles (N), amount in moles (n), and Avogadro's number (Na)

N = n × Na; n = N / Na; Na = N / n

relationship between mass (m), amount in moles (n), and molar mass (Mr)

m = n × Mr; n = m / Mr; Mr = m / n

relative isotopic mass

The mass of a specific isotope compared to 1/12 the mass of a carbon-12 atom; approximately equal to the isotope's mass number; unitless.

relative isotopic mass of carbon-13

13

relative atomic mass (Ar)

Ar = Σ (relative isotopic mass × percentage abundance) / 100

relative atomic mass of chlorine

Ar = (0.75 × 35) + (0.25 × 37) = 35.5

percentage composition by mass of an element in a compound

% element = (mass of element in compound / molar mass of compound) × 100

percentage composition of hydrogen in water (H₂O)

(2 × 1.0) / 18.0 × 100 = 11.1%

percentage composition of oxygen in water (H₂O)

16.0 / 18.0 × 100 = 88.9%

empirical formula

The simplest whole-number ratio of atoms in a compound.

steps to find an empirical formula from percentage composition

Convert percentage to mass (assume 100 g if needed); Calculate moles from mass; Divide all moles by the smallest mole value; Multiply to get whole numbers if necessary.

difference between empirical and molecular formulas

Empirical formula shows the simplest ratio; molecular formula shows the actual number of atoms.

molecular formula from the empirical formula

Molecular formula = empirical formula × (molar mass / empirical formula mass)

Avogadro's number (Na)

6.02 × 10²³ particles/mol, the number of particles in one mole.

mole

A unit representing 6.02 × 10²³ particles of a substance.

molar mass (Mr)

The mass of one mole of a substance in grams per mole (g/mol).

difference between molecular and empirical formulas

Because the molecular formula shows the actual number of atoms, which can be multiples of the empirical formula.

calculate moles from number of particles

n = N / Na

calculate number of particles from moles

N = n × Na

percentage abundance

The relative amount of each isotope of an element found in nature.

significance of percentage composition by mass

It tells you how much of each element is present in a compound by mass.