Stoichiometry

Stoichiometry is a branch of chemistry that studies the quantitative relationship of the composition of chemicals and their reactions.

EQUATION REACTIONS
EQUAL REACTIONS HAVE THE PROPERTY

1.	The types of elements before and after the reaction are always the same
2.	The number of each atom before and after the reaction is always the same
3.	The comparison of the reaction coefficients expresses the mole ratio (specifically the gaseous coefficient comparison also denotes the volume ratio provided the temperature den pressure is the same)

 

Basic Law of Chemistry

1). The Law of Conservation of Mass (Lavoisier Law).
Namely: "In a closed system, the mass of substances before and after the reaction is the same."
Examples:
40 grams Ca + 16 gram O2 ® 56 gram CaO
12 grams C + 32 grams O2 ® 44 grams CO2

Problems example :
In a closed container, 4 grams of calcium metal is burned with oxygen, producing calcium oxide. If the mass of calcium oxide produced is 5.6 grams, then how much oxygen mass is required?
Answer:
M Ca = 4 grams
M CaO = 5.6 grams
M O2 = ..?
Under the law of conservation of mass:
Mass before reaction = mass after reaction
Û m Ca + m O2 = m CaO
Û m O2 = m CaO - m Ca
= (5,6 - 4,0) gram
= 1.6 grams
So the required oxygen mass is 1.6 grams.

2). Fixed Comparative Law (Proust Law).
Namely: "The ratio of the mass of the elements in a compound is certain and fixed."
Example: consider example 5.1 of Package 1A book 151-152!
Another example :
Water is composed of elements of hydrogen (H2) and oxygen (O2) with a constant ratio of:
11.91%: 88.81% = 1: 8

Example: Find the reaction coefficient of
S (g) 2O (l)®HNO3 (aq) + H2 NO (g) + S (s) + H
The easiest way to determine the coefficient of reactions is to assume the coefficients of each a, b, c, d and e so that:
S 2O®  A HNO3 + b H2 c NO + d S + e H
Based on the above reaction then
Atom N: a = c (before and after reaction)
  atom O: 3a = c + e®  3a = a + e ® E = 2a
  atom H: a + 2b = 2e = 2 (2a) = 4a® 2b = 3a ® B = 3/2 a
Atom S: b = d = 3/2 a
So in order to solve we take any price eg a = 2 means: b = d = 3, and e = 4 so the equation of the reaction:
S 2O® 2 HNO3 + 3 H2 2 NO + 3 S + 4 H

3). Multiple Comparative Laws / Laws of Multiple Comparisons (Dalton Law).
Namely: "If two types of elements can form more than one kind of compound, then the mass ratio of one element bound to the mass of another element of the same, is an integer and simple."
Examples:
C and O can form two types of compounds, namely CO and CO2. If the mass of C in both compounds is equal (meaning the same amount of C), then:
Mass O in CO: O mass in CO2 will be a simple integer (ie = 1: 2).
Problems example :
Carbon can combine with hydrogen by a ratio of 3: 1, forming methane gas. What hydrogen mass is required to react with 900 grams of C in methane?
Answer:
C: H = 3: 1 so that:
Û 900: m H = 3: 1
Û m H =; So, the required H mass is 300 grams.

4). Legal Comparative Volum (Gay Lussac Law).
That is: "At the same temperature and pressure, the ratio of the volumes of the reacting gases and the reaction products is a simple integer."
Examples:
Two volumes of hydrogen gas react with one volume of oxygen gas to form two volumes of water vapor.
Hydrogen gas + oxygen gas ® water vapor
        2 V 1 V 2 V
Volume comparison = 2: 1: 2

5). Avogadro's Law.
Namely: "At the same temperature and pressure, the same volumes of gases contain the same number of particles."
Examples:
On the formation of H2O molecules
2L H2 (g) + 1L O2 (g) ® 2L H2O (g)


2 molecule H2 1 molecule O2 2 molecule H2O