Regents Chemistry the mole and stoichiometry: a complete skills guide to gram-formula mass, formulas, balancing and mole calculations

Why stoichiometry is the spine of Regents Chemistry

The mole and stoichiometry are the quantitative core of the Regents course, and the skills here recur in solutions, thermochemistry, titration and electrochemistry. The Regents examines them in Part B-2 and Part C, where markers reward a clear, mole-based setup as much as the right number. This guide ties together the matching dot-point pages, each with its own practice: the mole and molar mass, chemical formulas and percent composition, balancing equations and conservation of mass, types of chemical reactions, and stoichiometric calculations.

The mole and gram-formula mass

A mole is $6.02 \times 10^{23}$ particles. The gram-formula mass (molar mass) is the sum of the atomic masses in a formula, read from the periodic table, in g/mol. Table T gives the link:

Treat mass, moles and particles as a chain you can traverse either way. The commonest slip is confusing molecules with atoms: one mole of $\text{O}_2$ has $6.02 \times 10^{23}$ molecules but twice as many atoms.

Formulas and percent composition

A correct formula has zero net charge: balance cation and anion charges with subscripts (the crossover method), using Table E for polyatomic ions and parentheses when more than one polyatomic ion is needed ($\text{Ca}(\text{NO}_3)_2$). Percent composition by mass comes from Table T:

Use the element's total mass (atomic mass times subscript) if it appears more than once.

Balancing and reaction types

Balance by adjusting coefficients only (never subscripts) so atoms and charge are conserved, leaving H and O until last and reducing to smallest whole numbers. The balanced coefficients are the mole ratios for stoichiometry. The reaction types are synthesis, decomposition, single replacement, double replacement and combustion. Use Table J (activity series) to decide whether a single replacement occurs (a metal displaces one less active than itself) and Table F (solubility) to decide whether a double replacement forms a precipitate.

Stoichiometric calculations

To convert between amounts of substances, use the mole ratio from the balanced equation. For mass-to-mass problems, run the chain: grams to moles (Table T), mole ratio (coefficients), moles to grams. For mole-to-mole problems, just apply the ratio.

Check your knowledge

Attempt these under timed conditions, then check the solutions.

1. Calculate the gram-formula mass of $\text{Ca}(\text{OH})_2$. (2 marks)
2. Calculate the number of moles in $9.0$ g of water (gram-formula mass $18.0$). (2 marks)
3. Write the formula of the compound formed between $\text{Mg}^{2+}$ and $\text{PO}_4^{3-}$ (phosphate, Table E). (1 mark)
4. Calculate the percent by mass of carbon in $\text{CO}_2$ (gram-formula mass $44.0$). (2 marks)
5. Balance: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$. (2 marks)
6. For $\text{N}_2 + 3\,\text{H}_2 \rightarrow 2\,\text{NH}_3$, how many moles of $\text{NH}_3$ form from $9.0$ mol of $\text{H}_2$? (2 marks)