Atomic Theory and Chemical Stiochometry

Atomic theory Law of Conservation of Mass: Matter cannot be neither created nor destroyed. Instead it can only be changed from one form to another. This means that there must be the same number of each type of atom on each side of the equation. The atoms are simply being rearranged through the reaction process. In 1808, John Dalton proposed the atomic theory, which states: 1.

Matter is composed of tiny indivisible particles called atoms.

2.

All atoms of one element are identical, but different from the atoms of other elements

3.

Chemical reactions consist of combining, separating or rearranging atoms in simple whole number ratios.

This theory led to the use of symbols which represented elements, formulae for molecules and compounds and equations for chemical reactions. Using his model, Dalton reasoned that in a chemical reaction, atoms simply were rearranged into new substances, with no net loss or gain of atoms.

Atomic weight Since the atoms of one element are different from the atoms of another element, the atoms of different atoms will have different masses. However, since atoms are too small to measure individually, so as a result, only relative mass can be measured through their reactions with other elements. 

Originally hydrogen was used as the standard, being assigned a mass of 1, since it had one proton. However, since using hydrogen as a standard, meant that some weights did not result in nice whole numbers.



Carbon 12 was adopted as the standard in 1961, such that a single atom of Carbon 12 had a relative atomic mass of 12. The relative atomic mass (atomic weight) of an element is the average mass of the atoms present in the naturally occurring element relative to the mass of an atom of the carbon-12 isotope, taken as exactly 12.

However, certain elements may have atoms that contain different numbers of neutrons (called isotopes), which affects the relative atomic masses.

Relative molecular mass and formula mass 

The relative molecular mass (or molecular weight) of a compound is the mass of a molecule of the compound relative to the mass of an atom of the carbon-12 isotope, taken as exactly 12.



The relative formula mass of an ionic compound is the mass of the compound as stated in the formula, relative to the mass of an atom of the carbon-12 isotope, taken as exactly 12. Calculating relative molecular mass: The relative molecular mass of a compound is the sum of the atomic masses of the atoms as given in the molecular formula. Calculating relative formula mass: The relative formula mass (formula weight) of a compound is the sum of atomic masses of the atomic species as given in the stated formula of the compound

Percentage composition of compounds Knowing the atomic weights of elements has allowed chemists to calculate the percentage composition of compounds and structures by mass. Through a comparison with experimental data, a sample can tested also for its purity. Calculating the theoretical percentage weight of an element in a compound: molar mass of element Composition % = % molar mass of compound Calculating the percentage weight of an element in a compound given experimental data: Composition % =

mass of element mass of compound

%

The mole concept Gay-Lussac and Avogandro’s work The French chemist Joseph Gay-Lussac determined that gases reacted in whole number ratios, as demonstrated through his experiments involving the synthesis of gases, for example: 

100 ml of hydrogen reacts with 100 ml of chlorine gas to produce 200 ml of hydrogen chloride gas.



200 ml of hydrogen reacts with 100 ml of oxygen gas to produce 200 ml of water vapour Gay-Lussac’s Law of Combining Volumes: When measured at a constant temperature and pressure, the volumes of gases taking part in a chemical reaction are able to be expressed in simple whole number ratios.

It became apparent that Gay-Lussac’s experiments using Dalton’s atomic theory. Dalton reasoned that if the reacting gases were composed of single atoms, and if the compounds were composed of single aggregates of these atoms, then the reaction between hydrogen and oxygen to form water, would require the splitting of atoms. Amadeo Avogadro in 1811, proposed a solution to Gay-Lussac’s experiments, which is that elements could exist as atomic aggregates called molecules. Avogadro’s Law: At constant temperature and pressure, equal volumes of all gases contain equal numbers of molecules.

Applying the mole concept Avogadro’s Law has allowed chemists to compare atomic weights of equal number of molecules of gases at the



same temperature and pressure. Chemists have adopted a standard unit of measurement, in order to measure the amount of substance. This unit is called the mole. Using the modern mass spectrometer, chemists were able to measure the weight of a single carbon atom to be



around 1.992 × 10

–23

grams. Then they calculated the number of atoms in 12g of carbon-12 is

.

This is called Avogadro’s number (NA). This was used to define the mole. A mole is defined as the same amount of substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12. One mole of any substance contains For example:  

1 mole of Fe consists of 6.022 x 10

23

atoms

1 mole of N2 consists of 6.022 x 10

23

molecules

particles of that substance

Molar mass The mole concept can be related to the atomic weight of an element. A mole of any substance has a mass equal to its relative atomic masses, relative molecular mass or relative formula mass expressed in grams. It is called the molar mass. Calculating number of particles or moles (given either): number of particles n= NA

23

where n is the number of moles and NA is Avogadro’s constant: 6.022 × 10 . Calculating mass or number of moles: m n= M where n is the number of moles and m is the mass in grams (must be in grams), and M is molar mass which can be calculated from a Periodic table of weights.

Moles and chemical equations 

A chemical equation tells us the ratios by mass in which substances react or are formed in a reaction.



The coefficients used to balance a chemical equation represent the mole ratio of the reactants and products. Calculating mole ratios in a chemical equation: 1.

First, write a balanced equation.

2.

Use the coefficients to determine the mole ratio.

Calculating mass in equations: 1.

Write the balanced equation.

2.

Determine the mole ratio

3.

Convert any existing known information such as mass provided into moles, to find how many moles of the other substances will be required or produced.

4.

Convert back to grams.

Limiting reagent In some chemical reactions, one reactant gets depleted before the any of the other reactants. This is called the



limiting reagent because it “limits” the process of the reaction. Calculating the amount of limiting reagent: 1.

Write the balanced equation.

2.

Determine the mole ratio

3.

Convert any existing known information such as mass provided into moles, to find how many moles of the other substances will be required or produced.

4.

Calculate how many moles will be required to complete the reaction, (This is done by multiplying the lowest mole quantity through the mole ratio).

5.

Subtraction from the number of moles supplied for each element.

6.

Convert back to grams.