STOICHIOMETRY

   

 Stoichiometry is the basic chemical calculation that states quantitative relation of chemical formulas and chemical equations. Here are the materials you need to know to understand, from the concept of moles and molar masses, empirical formulas and molecular formulas, basic stoichiometry of solutions and ideal gases, and the writing and equalization of reactions, with examples of problems and discussions.

I. BASIC LAW OF CHEMICAL SCIENCE

A. The Law of Conservation of Mass (Lavoisier's Law)

      "The mass of substances before and after the reaction is the same".

Example:

S + O 2 → SO 2

2 gr 32 gr 64 gr

B. Fixed Comparative Law (Proust Law)

"The ratio of the elemental masses in each compound is fixed"

Example:

H 2 O → mass H: mass O = 2: 16 = 1: 8

C. The Law of Multiple Comparisons (Dalton's Law)

      "If two elements can form two or more compounds, and the mass of one element is equal, the ratio of the mass of the second element is proportional to the simple and integer".

II. RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS

   Having discovered highly sensitive equipment in the early twentieth century, chemists experimented on the mass of one atom. For example, experiments were performed to measure.

1. mass of one atom H = 1,66 -> 10-24 g

2. mass of one atom O = 2.70 -> 10-23 g

3. mass of one atom C = 1.99 -> 10-23 g

From the data above can be seen that the mass of one atom is very small. Experts agree to use the amount of Atomic Mass Unit (sma) or Atomic Mass Unit (amu) or commonly called also units of Dalton. In the matter of atomic structure,

You have also learned that atoms are very small, therefore it is impossible to weigh atoms using a balance sheet.

A. Relative Atomic Mass (Ar)

         Experts use the C-12 carbon isotope as standard with a relative atomic mass of 12. The relative atomic mass represents the average mass ratio of one atom of an element to 1/12 of the C-12 atomic mass. Or it can be written:

           

            1 unit of atomic mass (amu) = 1/12 mass 1 atom C-12.

The experts decided to use C-12 or 12C isotope because it has an inert core stability compared to other atoms. The isotope of C-12 atom has an atomic mass of 12 sma. One sma equals 1.6605655 x

10-24 g. With the use of 12C isotope as standard then can be determined the mass of atom of other element.

The relative atomic mass of an element (Ar) is a declaring number

The mass ratio of one atom of that element to 1/12 mass of one C-12 atom.

           

           ArX = (average atomic mass X) / (1/2 mass of carbon atom - 12)

B. Relative Molecular Mass (Mr)

       Molecule is a combination of several elements with a certain ratio. The same elements combine to form elemental molecules, while different elements form molecules of compounds. The molecular mass of an element or compound is expressed by a molecular mass (Mr). The relative molecular mass is the ratio of the molecular mass of an element or compound against 1/12 x the mass of C-12 atoms.

III. THE CONCEPT OF MOL AND FIXED AVOGADRO

       When you react one carbon atom (C) with one molecule of oxygen (O2) it will form one molecule of CO2. But actually what you react is not a single carbon atom with one molecule of oxygen, but a large number of carbon atoms and a large number of oxygen molecules. Since the number of atoms or the number of molecules reacting is so great then to say it, the chemists use "mol" as the unit of the number of particles (molecules, atoms, or ions).

One mole is defined as the number of substances containing the particles of the substance as much as the atoms present in 12,000 g of carbon atoms -12.

Thus, in one mole of a substance there are 6.022 x 1023 particles. The value of 6.022 x 1023 particles per mole is called the Avogadro constant, with the symbol L or N. In everyday life, the mole can be analogous to "dozen". If it's a dozen

States the number of 12 pieces, the mole states the amount of 6.022 x 10 23 particles of the substance. The word particles in NaCl, H2O, and N2 can be expressed with ions and molecules, whereas in elements like Zn, C, and Al can be expressed with atoms.

A. Molar Mass (Mr)

      The mass of one mole of substance is called the molar mass (the symbol of Mr). The magnitude of the material molar mass is the relative atomic mass or the relative molecular mass of a substance expressed in units of grams per mole.

           Molar mass = Mr or Ar substance (g / mol)

The mass of a substance is the multiplication of its molasses (g / mol) with the mol of the substance (n). Thus the mole relationship of a substance with its mass can be expressed as follows.

Mathematically, it can be stated as follows.

         

          Molar mass = mass: mol

          Mass = mol x Mr / Ar (molar mass)

B. Molar Volume (Vm)

       The volume of one mole of a substance in a gas form is called the molar volume, denoted by Vm.

What is the volume of gas molar? How to calculate the volume of a certain amount of gas at a given temperature and pressure?

Avogadro in his experiments concluded that 1 L of oxygen gas at 0 ° C and 1 atm pressure had a mass of 1.4286 g. 

 Thus, under Avogadro's law it can be concluded:

1 mol of gas O2 = 22.4 L

         In accordance with Avogadro's law stating that at the same temperature and pressure, the same volume of gas contains the same number of molecules or the number of moles of each gas volume the same. Under the law, a volume of 1 mole of each gas in standard conditions (0 ° C and 1 atm pressure) is applied.

Volume gas in standard state = 22.4 L

C. Gas Volume in Non-Standard State

    Calculation of gas volume is not in the standard state (non-STP) used the following two approaches.

1. The ideal gas equation

         Assuming the gas to be measured is ideal, the equation that links the number of moles (n) of gas, pressure, temperature, and volume

that is:

The ideal gas law:       P. V = n. R. T

Where:

P = pressure (atmospheric unit, atm)

V = volume (liters, L)

N = number of moles of gas (mol unit)

R = gas constant (0.08205 L atm / mol K)

T = absolute temperature (° C + 273.15 K)

2. With gas conversion at the same temperature and pressure

            According to Avogadro's law, the ratio of gases having the same number of moles has the same volume. Mathematically can be expressed as follows.

                              V1 / V2 = n1 / n2

Where:

N1 = mol gas 1 V1 = gas volume 1

N2 = mol gas 2 V2 = gas volume 2

 D. Molarity (M)

      The amount of substances present in a solution can be determined by using the concentration of the solution expressed in molarity (M). Molarity states the number of moles of substances in 1 L of solution.