Chapter 4

Question 1

Define molecular weight (a.k.a. molecular mass).

Answer 1

The sum of the atomic weights of all atoms in a single molecule, expressed in amu per molecule.

Question 2

Define formula weight.

Answer 2

The sum of the atomic weights of the constituent ions of an ionic compound’s empirical formula (units = amu per formula unit).

Question 3

Avogadro’s number (Nₐ) = ?

Answer 3

6.022 × 10²³ particles mol⁻¹

Question 4

General equation to convert grams → moles.

Answer 4

moles = mass (g) / molar mass (g mol⁻¹)

Question 5

[Mnemonic] “G → mol → particles” shortcut phrase.

Answer 5

“Grams down, Avogadro up” — divide by molar mass to go down to moles, multiply by Nₐ to go up to particles.

Question 6

Calculate the molecular weight of SOCl₂.

Answer 6

32.1 + 16.0 + (2 × 35.5) = 119.1 amu

Question 7

9.53 g of MgCl₂ correspond to how many moles?

Answer 7

Molar mass ≈ 95.3 g mol⁻¹ ⇒ 0.10 mol.

Question 8

Define equivalent (eq).

Answer 8

The amount of a species that donates or accepts 1 mole of the specified particle (H⁺, e⁻, etc.).

Question 9

Equation for gram equivalent weight (GEW).

Answer 9

GEW = molar mass / n where n = # of particles of interest per molecule.

Question 10

GEW of H₂SO₄ for acid–base problems.

Answer 10

98.1 / 2 ≈ 49.0 g eq⁻¹ (donates 2 H⁺).

Question 11

Define normality (N).

Answer 11

Moles of equivalents per liter of solution: N = M × n.

Question 12

Normality of 2 M Mg(OH)₂ (base chemistry).

Answer 12

N = 2 M × 2 = 4 N (2 OH⁻ per formula unit).

Question 13

Empirical vs molecular formula.

Answer 13

Empirical = smallest whole‑number ratio; Molecular = actual number of each atom (multiple of empirical).

Question 14

Percent composition formula (by mass).

Answer 14

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

Question 15

Law of constant composition statement.

Answer 15

Any pure sample of a compound contains the same elements in identical mass ratios.

Question 16

Empirical formula of a carb w/ 40.9 % C, 4.58 % H, 54.5 % O.

Answer 16

C₃H₄O₃

Question 17

[MCAT‑Check] If NaBr = 102.9 g mol⁻¹, how many moles in 205.8 g?

Answer 17

2.00 mol

Question 18

Define a combination (synthesis) reaction.

Answer 18

Two or more reactants → one product (A + B → C).

Question 19

Define a decomposition reaction.

Answer 19

One reactant → two or more products (A → B + C).

Question 20

Combustion reaction hallmarks.

Answer 20

Fuel (often hydrocarbon) + O₂ → CO₂ + H₂O; exothermic oxidation.

Question 21

Single‑displacement vs double‑displacement (metathesis).

Answer 21

Single: A + BC → B + AC. Double: AB + CD → AD + CB (often precipitate, gas, or weak electrolyte).

Question 22

Neutralization reaction definition.

Answer 22

Acid + base → salt (and often water); subtype of double‑displacement.

Question 23

Laws obeyed when balancing.

Answer 23

Conservation of mass and charge.

Question 24

Heuristic order to balance.

Answer 24

1) Cations/least abundant element, 2) anions, 3) oxygen, 4) hydrogen.

Question 25

[Mnemonic] “CH O H” — Carbon first, Hydrogen last for combustion equations.

Question 26

Three‑step stoichiometry road map.

Answer 26

Convert to moles → mole ratio → convert to desired units.

Question 27

Define limiting reagent.

Answer 27

Reactant that is consumed first, limiting amount of product.

Question 28

Two principles for finding limiting reagent.

Answer 28

(i) Convert all reactant amounts to moles. (ii) Compare mole ratios to coefficients, not masses.

Question 29

Equation for percent yield.

Answer 29

(actual yield / theoretical yield) × 100

Question 30

[Practice] 28 g Cu produced from 32.7 g Zn + excess CuSO₄. Percent yield?

Answer 30

~ 87.5 %.

Question 31

Oxidation states of hydrogen (general, cationic, anionic).

Answer 31

+1 (H⁺), 0 (H₂), –1 (H⁻).

Question 32

Roman‑numeral nomenclature example: Fe²⁺.

Answer 32

Iron(II).

Question 33

Latin suffix method for Fe²⁺ / Fe³⁺.

Answer 33

Fe²⁺ = ferrous; Fe³⁺ = ferric.

Question 34

Rule for monatomic anions.

Answer 34

End name with “‑ide”: O²⁻ = oxide, N³⁻ = nitride.

Question 35

Oxyanion endings [Mnemonic] “Lite has less, ate has more O.”

Answer 35

NO₂⁻ = nitrite; NO₃⁻ = nitrate.

Question 36

Prefixes for oxyanion series.

Answer 36

Hypo‑ (less O) < ite < ate < per‑…ate (most O).

Question 37

Polyatomic ions with hydrogen naming convention.

Answer 37

Add “hydrogen” or “dihydrogen” before anion OR use prefix bi‑ for one H⁺ (e.g., HCO₃⁻ = hydrogen carbonate / bicarbonate).

Question 38

Table of common polyatomic ions to memorize.

Answer 38

NH₄⁺ (ammonium), CN⁻ (cyanide), MnO₄⁻ (permanganate), SCN⁻ (thiocyanate), CrO₄²⁻ (chromate), Cr₂O₇²⁻ (dichromate), BO₃³⁻ (borate), C₂H₃O₂⁻ (acetate).

Question 39

Define electrolyte.

Answer 39

A solute that dissociates into ions in solution, allowing it to conduct electricity.

Question 40

Differences between strong and weak electrolytes.

Answer 40

Strong dissociate completely (e.g., NaCl, HCl); Weak ionize incompletely (e.g., CH₃COOH, NH₃).

Question 41

[Mnemonic] Strong electrolytes “NACKI BASE”

Answer 41

— NaCl, KCl, ionic BASE salts.

Question 42

Concept 4.1 #1 – Calculate molar mass: NaBr.

Answer 42

22.99 + 79.90 ≈ 102.9 g mol⁻¹

Question 43

Concept 4.2 – One similarity & one difference between empirical vs molecular formulas.

Answer 43

Similarity: both show element proportions; Difference: empirical = simplest ratio, molecular = actual numbers.

Question 44

Concept 4.3 – Classify reaction: CaCl₂(aq) + 2 AgNO₃(aq) → Ca(NO₃)₂(aq) + 2 AgCl(s).

Answer 44

Double‑displacement (precipitation)

Question 45

Balance: ___ Na + ___ Cl₂ → ___ NaCl.

Answer 45

2 Na + 1 Cl₂ → 2 NaCl.

Question 46

Identify limiting reagent: 27.9 g Fe + 24.1 g S → FeS.

Answer 46

Fe is limiting (0.5 mol Fe vs 0.75 mol S).

Question 47

Is sucrose an electrolyte in water?

Answer 47

No – covalent, does not ionize (nonelectrolyte).

Question 48

Provide formula & charge for hypochlorite.

Answer 48

ClO⁻

Question 49

Provide cation/anion & formula for chromium(II).

Answer 49

Cation; Cr²⁺

Question 50

Percent yield definition?

Answer 50

(Actual / Theoretical) × 100 %.

Question 51

Avogadro = ?

Answer 51

“Six‑OH‑Two‑Two” (6.022×10²³).

Question 52

Gram Equivalent Weight: ?

Answer 52

“Molar over n.”

Question 53

Stoichiometry Steps: ?

Answer 53

“m → mol → mol → m.”

Question 54

Reaction Types: ?

Answer 54

“Combos Decompose, Singles Date, Doubles Marry, Fires Combust, Acids Neutralize.”

Question 55

Oxyanions: ?

Answer 55

“Per ‑ate > ‑ate > ‑ite > Hypo‑ite.”

Question 56

Strong Electrolytes: ?

Answer 56

“Salts & Strong acids/bases are ‘SS’ — Super Strong.”