Unit 6 Stoichiometry

volume of gas

What does STP also mean?

moles —> STP

multiply by 22.4 L / 1 mol

STP —> moles

multiply by 1 mol / 22.4 L

moles —> mass

multiply by molar mass (g) / 1 mol

mass —> moles

multiply by 1 mol / molar mass (g)

moles —> number of particles

multiply by 6.023 × 10²³ particles / 1 mol

number of particles —> moles

multiply by 1 mol / 6.023 × 10²³ particles

percent composition

percent by mass of each element in a compound

% mass of element

(mass of element / mass of compound) * 100%

72.2% Mg, 27.8% N

A compound is formed when 9.03g Mg combined with 3.48g N. What is the percent composition of this compound?

tip: add two masses for total mass compound

82.22% N, 17.88% H

Calculate the percent composition of ammonia, NH₃. Assume you have one mole, use the molar mass to solve the problem.

empirical formulas

smallest whole number ratio of different atoms in a chemical compound

Ex. C₃H₆O₃ —> CH₂O

molecular formula

actual number ratio of different atoms in a chemical reaction

Ex. C₃H₆O₃

calculating empirical formula

1. take percent comp

2. multiply by 1 mol / molar mass (g)

3. divide by smallest mole number

4. multiply by constants for whole number constants

molecular formula from empirical formula

1. molar mass / empirical formula mass

2. multiply each subscript of empirical formula by number obtained in step 1

tip: find molar mass of entire compound for empirical formula; molar mass of molecule usually given

(s)

solid

(l)

liquid

(g)

gas

(aq)

in aqueous solution (dissolved in water)

diatomic elements

Br, I, N, Cl, H, O, F

rule for polyatomic ion in balancing equation

if unchanged on the other side of the equation, count as one unit

net ionic equation

equation containing only the elements that changed states of matter

Zn²⁺ (aq) + CO₃^2- (aq) —> ZnCo₃ (s)

net ionic equation of Zn(NO₃)₂ (aq) + NaCO₃ (aq) —> ZnCO₃ (s) + 2NaNO₃ (aq)

Zn (s) + Cu²⁺ (aq) —> Zn²⁺ (aq) + Cu (s)

net ionic equation of Zn (s) + Cu(NO₃)₂ (aq) —> Zn(NO₃)₂ (aq) + Cu(s)

Zn (s) + 2HCl (aq) —> ZnCl₂ (aq) + H₂ (g)

net ionic equation of Zn (s) + 2H⁺ (aq) —> Zn²⁺ (aq) + H₂ (g)

synthesis reactions

also known as combination reactions; A + B —> AB

halide salt

reaction of metals with halogens (all pure, elemental halogens are diatomic: BrINClHOF)

decomposition reactions

opposite of synthesis reactions; AB —> A + B

rule of diatomic elements in decomposition reactions

decompose into respective elements diatomically

Ex. 2HgO (s) —> 2 Hg (l) + O₂ (g)

rule for metal carbonate in decomposition reactions

metal carbonates plus heat decompose into metal oxides plus carbon dioxide

Ex. Ca₃CO₃ (g) —> CaO (s) + CO₂ (g)

rule for carbonic acid decomposing

H₂CO₃ (aq) —> CO₂ (g) + H₂O (l)

rule for ammonium hydroxide decomposing

NH₂OH (aq) —> NH₂ (aq) + H₂O (l)

single replacement reactions

usually occur in aqueous environments and A & B independently solid; A + BX —> AX + B

rule for determining reactions of halogens and single replacement reactions

determining if it can occur or not requires activity series of metals —> higher reactivity = kicks other element out and reacts

Ex. Cu (s) + 2AgNO₃ (aq) —> 2Ag (s) + Cu(NO₃)₂ (aq)

double replacement reactions

occur in aqueous solutions w/ two ionic compounds; one product usually precipitate or liquid & gas; AX + BY —> AY + BX

basic solubility rules

all nitrates, acetates, and ammoniums soluble (with few exceptions)

rules for formation of a precipitate (DR)

occurs when positive ions of one reactant combine w/ negative ions of another reactant —> rosm insoluble / slightly soluble product

Ex. 2NaPO₄ (aq) + 3CaCl₂ (aq) —> Ca₃(PO₄)₂ (s) + 6NaCl (aq)

rules for formation of a gas (DR)

occurs when one product is a gas that bubbles out of mixture

Ex. 2HCl (aq) + K₂CO₃ (aq) —> 2KCl (aq) + H₂O (l) + CO₂ (g)

neutralization reaction

acid + base —> water + salt

Ex. HCl (aq) + NaOH (aq) —> NaCl (aq) + H₂O (l)

hydrocarbon combustion reactions

burning of hydrocarbon / alcohol in presence of oxygen

Ex. CH₄ (g) + 2O₂ —> CO₂ (g) + 2H₂O (l)

complete combusion

turns all carbon atoms into carbon dioxide and all hydrogen atoms into water; assume this is true unless told otherwise

common gases

CO₂, CO, NH₃, H₂S, SO₂, N₂O, NO₂, H₂, N₂, O₂, F₂, Cl₂