Molecules and Moles

Before you can balance an equation or calculate a percent yield, you need precise vocabulary for what you're counting and weighing. This subtopic covers the distinction between a molecule and a compound, why ionic substances get their own terminology (formula unit, formula weight), and three related concentration/counting terms that will resurface throughout acid-base and redox chemistry: equivalents, equivalent weight, and normality.

Key Takeaways

• A molecule is two or more atoms held together by covalent bonds (same or different elements); a compound is two or more different elements bonded together (ionic or covalent). A molecular compound is both; a diatomic element (O₂, N₂) is a molecule but not a compound; an ionic compound (NaCl) is a compound but not a molecule.

• Ionic compounds form a 3D crystal lattice, not discrete molecules — so chemists use formula unit instead of "molecule" and formula weight instead of "molecular weight."

• Equivalents = moles of a species of interest (H⁺, electrons, etc.) produced or consumed per mole of compound.

• Gram equivalent weight = molar mass ÷ n; equivalents in a sample = mass (g) ÷ gram equivalent weight.

• Normality (N) = equivalents/liter = molarity × n. Most commonly tested on the MCAT via hydrogen ion concentration in acid-base contexts.

Molecules vs. Compounds

A molecule forms when two or more atoms are held together by covalent bonds — atoms sharing electrons to create a stable, discrete unit. A molecule can be made of a single element, like nitrogen (N₂) or oxygen (O₂), or of different elements, like carbon dioxide (CO₂).

A compound is a substance made of two or more different elements chemically bonded together, whether by ionic or covalent bonding. Compounds have their own characteristic properties — color, odor, melting point, boiling point — distinct from the elements that make them up.

Putting the two definitions together: a molecular compound (like CO₂) is both a molecule and a compound. But the two categories don't fully overlap. A diatomic element like O₂ or N₂ is a molecule but not a compound, since it's made of only one element. And, as the next section covers, an ionic compound like NaCl is a compound but not a molecule at all — it doesn't exist as a discrete, bonded unit the way CO₂ does.

Ionic Compounds, Formula Units, and Formula Weight

Covalent bonding produces discrete molecules — you can point to one CO₂ molecule and say exactly which atoms belong to it. Ionic bonding doesn't work that way.

In a solid ionic compound, oppositely charged ions arrange themselves into a nearly infinite three-dimensional array called a crystal lattice, rather than pairing off into separate molecules. In solid sodium chloride (NaCl), for example, each Na⁺ ion is surrounded by Cl⁻ ions, and each Cl⁻ ion is surrounded by Na⁺ ions, throughout the entire crystal. There's no single, separable "NaCl molecule" to isolate — the whole lattice is one continuous structure.

Because ionic compounds don't form discrete molecules, chemists use different terms for them:

• Formula unit: the empirical formula of an ionic compound (e.g., NaCl), used in place of "molecule."

• Formula weight: the mass of one formula unit, used in place of "molecular weight."

So "molecular weight" applies to covalent, molecule-forming substances, while "formula weight" applies to ionic compounds — the calculation works the same way (summing atomic masses), but the terminology reflects the fact that an ionic solid isn't made of discrete molecules.

Equivalents and Equivalent Weight

Equivalents measure the moles of a specific species of interest that one mole of a compound produces or consumes — a concept that shows up constantly in acid-base and redox reactions (and will reappear when you get to those chapters).

In an acid-base reaction, an equivalent typically tracks hydrogen ions (H⁺):

• Hydrochloric acid (HCl) produces 1 mole of H⁺ per mole of HCl → 1 mole of HCl = 1 equivalent of H⁺.

• Sulfuric acid (H₂SO₄) produces 2 moles of H⁺ per mole of H₂SO₄ → 1 mole of H₂SO₄ = 2 equivalents of H⁺.

In a redox reaction, an equivalent instead tracks the moles of electrons a substance donates or accepts.

The gram equivalent weight is the mass, in grams, of a compound that supplies one equivalent of the species of interest:

Gram Equivalent Weight = Molar Mass ÷ n

where n is the number of equivalents one mole of the compound produces. For sulfuric acid (molar mass ≈ 98 g/mol, n = 2):

Gram Equivalent Weight of H₂SO₄ = 98 ÷ 2 = 49 g/equivalent

To find how many equivalents are present in a given mass of a compound:

Equivalents = Mass of Compound (g) ÷ Gram Equivalent Weight

Normality

Normality (N) is a measure of concentration expressed in equivalents per liter. On the MCAT, it's most often applied to hydrogen ion concentration: a 1 N acid solution contains 1 mole of H⁺ per liter, and a 2 N acid solution contains 2 moles of H⁺ per liter.

Normality relates to molarity (M) through the number of equivalents per mole (n):

N = M × n

So a 1 M solution of H₂SO₄ (n = 2) is a 2 N solution with respect to H⁺, while a 1 M solution of HCl (n = 1) is also a 1 N solution with respect to H⁺.

Equivalents, equivalent weight, and normality will come back in more depth when you cover acid-base chemistry and oxidation-reduction reactions — the underlying idea (moles of the species of interest per mole of compound) stays the same throughout.

Common MCAT Mistakes

• Using "molecular weight" for an ionic compound. Ionic compounds like NaCl don't form discrete molecules — the correct terms are formula unit and formula weight, not "molecule" and "molecular weight."

• Assuming equivalents always equal moles. They're only equal when n = 1 (like HCl). For H₂SO₄, n = 2, so 1 mole of acid supplies 2 equivalents of H⁺ — mixing this up leads directly to a wrong gram equivalent weight.

• Treating equivalents as an acid-base-only concept. Equivalents also track electrons transferred in redox reactions, not just H⁺ produced in acid-base reactions.

• Confusing normality with molarity. Normality (N) is molarity scaled by n (N = M × n); for any species where n ≠ 1, N and M are numerically different, even though both describe the same solution.

MCAT-Style Concept Check

Question: A 1 M solution of H₃PO₄ (n = 3 with respect to H⁺) is prepared. What is the normality of this solution with respect to H⁺?

• A) 1 N

• B) 2 N

• C) 3 N

• D) 4 N

Answer: C

Explanation: Normality equals molarity multiplied by n (N = M × n). H₃PO₄ can donate 3 moles of H⁺ per mole of acid, so n = 3. With M = 1, N = 1 × 3 = 3 N.

FAQ

What's the difference between a molecule and a compound?

A molecule is two or more atoms held together by covalent bonds — it can be a single element (O₂) or multiple elements (CO₂). A compound is two or more different elements bonded together, by either ionic or covalent bonds. A molecular compound like CO₂ is both; an ionic compound like NaCl is a compound but not a molecule.

Why do ionic compounds use "formula unit" instead of "molecule"?

Ionic compounds form a continuous three-dimensional crystal lattice rather than discrete, separable units, so there's no single "molecule" to point to. "Formula unit" refers to the empirical formula instead, and "formula weight" replaces "molecular weight" for the same reason.

What is a gram equivalent weight?

It's the mass, in grams, of a compound that supplies exactly one equivalent of the species of interest (like H⁺ or an electron). It's calculated as molar mass divided by n, the number of equivalents one mole of the compound produces.

How is normality related to molarity?

Normality (N) equals molarity (M) multiplied by n, the number of equivalents per mole. For a species where n = 1, normality and molarity are numerically identical; for n > 1 (like H₂SO₄, where n = 2 with respect to H⁺), normality is a multiple of molarity.