Vocabulary for Standard 3 Stoichiometry







1   6.022 x 1023    

actual yield


The quantity of a product that is obtained from a chemical reaction (as opposed to the calculated or theoretical yield).
3 R

atomic mass

atomic weight.

The average mass of an atom of an element, usually expressed in atomic mass units. The terms mass and weight are used interchangeably in this case. The atomic weight given on the periodic table is a weighted average of isotopic masses found in a typical terrestrial sample of the element.
4   Avogadros Number Avogadros constant  

balanced equation


A description of a chemical reaction that gives the chemical formulas of the reactants and the products of the reaction, with coefficients introduced so that the number of each type of atom and the total charge is unchanged by the reaction. For example, a balanced equation for the reaction of sodium metal (Na(s)) with chlorine gas (Cl2(g)) to form table salt (NaCl(s)) would be 2 Na(s) + Cl2(g) = 2 NaCl(s), NOT Na(s) + Cl2(g) = NaCl(s).



Carbon-12 is the most abundant (98.89%) of the two stable isotopes of the element carbon. It contains 6 protons, 6 neutrons and 6 electrons. Carbon-12 is of particular importance as it is used as the standard from which all other isotopes' atomic weight is measured and thus the measurement of Avogadro's number.

chemical equation


A compact notation for describing a chemical change. The formulas of the reactants are added together on the left hand side of the equation; the formulas of the products are added together on the right side. Coefficients are inserted before the formulas to ensure that the equation is balanced. The phase in which each substance is found is usually indicated in parentheses after each formula. For example, 2 H2(g) + O2(g) → 2 H2O(g) indicates that 2 moles of hydrogen gas combine with one mole of oxygen gas to produce two moles of steam.


Stoichiometric coefficients

The coefficients given before substances in a balanced chemical equation. For example, the stoichiometric coefficient of carbon dioxide in the following reaction is 4:   2 C2H6(g) + 7 O2(g) →4 CO2 + 6 H2O

composition stoichiometry


1. Ratios of atoms in a compound. 2. Ratios of moles of compounds in a reaction. 3. A branch of chemistry that quantitatively relates amounts of elements and compounds involved in chemical reactions, based on the law of conservation of mass and the law of definite proportions.
10 R

conversion factor


A conversion factor is a fraction that relates one unit to another. Multiplying a measurement by a conversion factor changes the units of the measurement. For example, since 1 in = 2.54 cm, to convert 10 inches to centimeters,
(10 in)
2.54 cm
1 in


11   equation    
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dimensional analysis


Dimensional Analysis (also called Factor-Label Method or the Unit Factor Method) is a problem-solving method that uses the fact that any number or expression can be multiplied by one without changing its value. It is a useful technique.

empirical formula

simplest formula.

Empirical formulas show which elements are present in a compound, with their mole ratios indicated as subscripts. For example, the empirical formula of glucose is CH2O, which means that for every mole of carbon in the compound, there are 2 moles of hydrogen and one mole of oxygen.

excess reactant


Unreacted reactants

formula mass

formula weight

The formula weight is the sum of the atomic weights of the atoms in an empirical formula. Formula weights are usually written in atomic mass units (u).
16 H

functional groups


A substructure that imparts characteristic chemical behaviors to a molecule, for example, a carboxylic acid group.

limiting reactant

limiting reagent.

The reactant that limits the amount of product produced in a chemical reaction. For example, mixing one mole of H2(g) with one mole of O2 produces one mole of steam (H2O(g)), with half a mole of O2(g) remaining. The hydrogen gas limits the amount of steam produced in this case.

mass-to-mass conversion


mass of given ----> moles of given ----> moles of requested ----->grams of requested
19   mole   6.02 x 1023 things

mole ratio

Mole fraction

Concentration of a substance in a mixture measured as moles of the substance per mole of mixture. For example, the mole fraction of oxygen in air is about 0.21, which means that 1 mol of air contains about 0.21 mol O2.
21 R

molecular mass/ molecular weight

Molecular weight

The mass of one mole of a material. For example, the molar mass of H2O is 18.015 g (obtained by adding twice the molar mass of hydrogen to the molar mass of oxygen).

mole-to-mass conversion


moles of given -----> moles of requested ----> grams of requested



A system for naming things. For example, "organic nomenclature" is the system used to name organic compounds.
24 H


oxidize; oxidizing; oxidized.

Oxidation is the loss of one or more electrons by an atom, molecule, or ion. Oxidation is accompanied by an increase in oxidation number on the atoms, molecules, or ions that lose electrons.
25 H

oxidation numbers

oxidation state; positive valence.

A convention for representing a charge of an atom embedded within a compound, if the compound were purely ionic. For example, H2O is a covalent compound; if it were ionic, the hydrogens would be H+ (oxidation number +1) and the oxygen would be O2- (oxidation number -2). Oxidation number rises for at least one atom in a compound that is oxidized; oxidation number becomes smaller if the compound is reduced
26 H

oxidation state

oxidation number

Oxidation state is a number assigned to an element in a compound according to some rules. This number enable us to describe oxidation-reduction reactions. For the purpose of balancing oxidation-reduction reactions, You have to assign oxidation states (or oxidation numbers) to a variety of compounds or ions.

percentage composition


Percent composition is the percent by mass of each element present in a compound.

percentage yield

percentage yield.

Percent yield equals experimental yield divided by theoretical yield times 100%.


(↓) ppt.

An insoluble substance that has been formed from substances dissolved in a solution. For example, mixing silver nitrate and sodium chloride solutions produces a precipitate, insoluble silver chloride (along with soluble sodium nitrate.
30   products   The thing you make in a chemical reaction.
31   reactamts    

reaction stoichiometry


When reactants combine to form products in a chemical reaction they do so in a very precise way. For example in the reaction 2H2 + O2 --> 2H2O, exactly two hydrogen molecules react with exactly one oxygen molecule to form exactly 2 water molecules. This information, in combination with the principle of conservation of mass should allow us to accurately calculate precisely the amounts for reactants consumed as well as the amount of products generated in a chemical reaction. Calculations of this kind are known as "stoichiometric" calculations. There are three steps to successfully do stoichiometric calculations:

I) Realize that the chemical equation is a MOLAR relation

II) Determine mole-mole conversion factors

III) Use the mole-mole conversion factors to solve the problem

33 H


electrochemical reaction; oxidation-reduction reaction;

A reaction that involves transfer of electrons from one substance to another. Redox reactions always involve a change in oxidation number for at least two elements in the reactants.
34 H



Reduction describes the uptake of an electron by a molecule or atom.

reversible reaction


(A process or reaction that can be reversed by an infinitesimally small change in conditions. For example, ice and water coexist at 1 atm and 0C; a very slight temperature increase causes the ice to melt; a tiny temperature decrease causes the water to freeze. Melting or freezing under these conditions can be considered reversible. Reversible processes are infinitesimally close to equilibrium. ) reversible process; reversible reaction. Compare with irreversible and irreversible process.
36 H

rules to assign oxidation numbers


The guidelines for assigning oxidation states (numbers) are given below:

The oxidation state of any element such as Fe, H2, O2, P4 and S8, is zero (0).

The oxidation state of oxygen in its compounds is -2, except for peroxides like H2O2, and Na2O2, in which the oxidation state for O is -1.

The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, LiAlH4, etc., in which the oxidation state for H is -1.

The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.

The following elements usually have the same oxidation states in their compounds:

+1 for alkali metals - Li, Na, K, Rb, Cs;

+2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;

-1 for halogens except when they form compounds with oxygen or one another;

37 R

significant digits

significant figures

A convention for recording measurements. Measurements are rounded so that they contain only the digits up to and including the first uncertain digit, when the number is written in scientific notation.
38   stoichiometric calculation   The art of figuring how much stuff you'll make in a chemical reaction from the amount of each reagent you start with.



In a chemical formula a subscript is used to show how many atoms of each element are present in the compound. Subscripts are the small numbers that follow behind and a little below the chemical symbols

theoretical yield

maximum yield; stoichiometric yield.

The amount of product obtained when all of the limiting reagent reacts.

word equation


An equation describing a chemical reaction using words rather than chemical symbols.