Species

Also: calcite / limestone (mineral forms)

An ionic compound of the calcium ion and the carbonate ion. It is insoluble, so it drops out as the precipitate in the flagship precipitation lesson. Its formula follows from charge balance — $\mathrm{Ca^{2+}}$ with $\mathrm{CO_3^{2-}}$ in a 1:1 ratio — and the molar mass below sums the three sourced weights.

• Which pairing is insoluble is a sourced solubility-rule call, not derived from the atomic weights.

A soluble calcium salt — the calcium source in the precipitation lesson. The 1:2 subscript is charge balance: one $\mathrm{Ca^{2+}}$ needs two $\mathrm{Cl^-}$ to cancel. The molar mass counts the two chlorines, so it is not just calcium plus chlorine.

• The subscript 2 comes from crossing the charges (+2 and -1), a machine-verified step in the Valence Table formula builder.

An insoluble salt with a non-1:1 formula — the precipitate in the calcium-phosphate lesson. Three $\mathrm{Ca^{2+}}$ (total +6) balance two $\mathrm{PO_4^{3-}}$ (total -6). The parentheses mean the whole phosphate group is taken twice, so the molar mass counts two phosphorus and eight oxygen atoms.

• The 3:2 subscripts are charge crossover: cross +2 and -3 to get 3 and 2, then confirm +6 and -6 cancel.

The gas released when a carbonate meets an acid, and a product of every complete combustion. One carbon, two oxygens. Its molar mass sums the sourced weights; the two oxygens make it heavier than it looks at a glance.

• A covalent molecular compound, not ionic — named with the count prefix 'di-' rather than by charge crossover (a Phase-2 covalent-naming topic).

Also: hydrochloric acid (aqueous solution)

One hydrogen and one chlorine. As a gas it is hydrogen chloride; dissolved in water it is hydrochloric acid, a strong acid that donates its $\mathrm{H^+}$ completely — the acid in the neutralization lesson. The name changing with phase is a convention, not a change in the molar mass, which sums the two sourced weights either way.

• 'Hydrogen chloride' (the molecule) and 'hydrochloric acid' (its water solution) are the same formula named two ways — a Phase-2 acid-naming topic.

Also: soda ash / washing soda

A soluble carbonate — the carbonate source in the precipitation lesson. Two $\mathrm{Na^+}$ balance one $\mathrm{CO_3^{2-}}$, so the formula carries two sodiums and one carbonate group. The molar mass sums both sodiums with the carbonate's carbon and three oxygens.

Also: table salt / halite

The textbook soluble salt: sodium and chloride in a 1:1 ratio. It stays fully dissolved as spectator ions through a precipitation or neutralization — the sodium and chloride never appear in the net-ionic equation. The molar mass sums the two sourced weights.

Also: lye / caustic soda

A strong base: sodium ion with hydroxide ion. In water it supplies $\mathrm{OH^-}$ completely — the base in the neutralization lesson, where its hydroxide combines with the acid's $\mathrm{H^+}$ to make water. The molar mass sums sodium, oxygen, and hydrogen.

Also: dihydrogen monoxide (systematic)

The product of every acid-base neutralization and the solvent of the whole solution course. Its molar mass is the sum of two hydrogen weights and one oxygen — the machine re-parses $\mathrm{H_2O}$ and adds the sourced atomic weights, so the 18.015 g/mol below is derived, not looked up.

• Bent, not linear — but shape is a Phase-2 (VSEPR) topic; here water is a mass on the balance and the net-ionic product $\mathrm{H^+ + OH^- \rightarrow H_2O}$.

Also: smithsonite (mineral)

An insoluble zinc salt — the precipitate whose collected mass drives the percent-yield lesson. $\mathrm{Zn^{2+}}$ pairs 1:1 with $\mathrm{CO_3^{2-}}$. The theoretical yield is this molar mass times the ledger's moles at maximum extent.

Also: dihydrogen · molecular hydrogen

Elemental hydrogen is a two-atom molecule, $\mathrm{H_2}$ — the gas that bubbles off when an active metal displaces the hydrogen from an acid. As a free element it starts at oxidation state 0, which is why single replacement is a redox reaction. Its molar mass is twice the atomic weight.

Also: dioxygen · molecular oxygen

An element, but not a lone atom — oxygen travels as a two-atom molecule. That distinction matters for balancing: the reactant is $\mathrm{O_2}$, so its molar mass is twice the atomic weight, and combustion counts it as a free element (oxidation state 0) that becomes combined.

• The Valence Table lists the oxygen atom (weight 15.999); this entry is the molecule that actually shows up in reactions, at twice that mass.

Also: carbonate ion

A polyatomic ion: one carbon and three oxygens carrying a 2- charge as a single unit. It stays intact through a reaction — you carry $\mathrm{CO_3^{2-}}$ around whole, never as loose C and O. Its molar mass sums the four atoms; the tiny mass of the two extra electrons is conventionally ignored.

• The 2- charge is the sourced ionic charge (see the Valence Table); the composition and molar mass here are the machine-checked part.

Also: hydroxide ion

One oxygen and one hydrogen carrying a 1- charge — the ion a base supplies to solution. In a neutralization it combines with the acid's $\mathrm{H^+}$ to make water: $\mathrm{H^+ + OH^- \rightarrow H_2O}$. Its molar mass sums the oxygen and hydrogen weights.

• The 1- charge is the sourced ionic charge; the composition and molar mass are the machine-checked part.