USING ENGLISH TO CALCULATE

STOIKIOMETRI

1.    Understanding Stokiometri

            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.

2.    The concept of Moles and Molar Mass

            In SI systems, one mole is defined as the sum of the material composed of entities (atoms, molecules, or other particles) a sum of the atoms in 12 grams of carbon-12. The value of the number of atoms is 6.022 × 1023 called the Avogadro number, NA.
The molar mass is defined as the mass of 1 mol entity (atom, ion, molecule, unit of formula) of matter. The unit of the molar mass is grams / mol.

 

Stoichiometric Material: Mass relation, number of moles, and number of atoms of the element

(Source: Chang, Raymond, 2010. Chemistry (10th edition) New York: McGraw Hill)

3.    Empirical Formulas and Molecular Formulas

            The empirical formula is the simplest integer ratio of the number of moles of each element in a compound. The molecular formula represents the true number of moles of each element in 1 mole of the compound. The molecular formula may be identical to the empirical formula or an integer multiple of the empirical formula. For example, phosphoric acid (H3PO4) has a molecular formula and an identical empirical formula. Glucose has a molecular formula C6H12O6 which is a folding of 6 times its empirical formula, CH2O.

Molecular Formula ≡ (Empirical Formula) n

Molecular formula = n × empirical formula, n = 1, 2, 3, ...

4.    Basic Stoichiometric Solution

            The term "concentration" of the solution expresses the amount of solute dissolved in a certain amount of solvent or a certain amount of solution. The concentration of the solution can be expressed in molarity. Molarity (M) is defined as the number of moles of solute per liter of solution.

5.    Basic Ideal Gas Stoichiometry

            The molar volume, Vm, is defined as the volume of 1 mol entity (atom, ion, molecule, unit of formula) of matter. The unit of the molar volume (Vm) is L / mol.

            Avogadro's law states that at certain pressures and temperatures and fixed, the volume of gas is directly proportional to the amount of gas.

In the STP state (P = 1 atm, T = 273 K), ideal gas Vm = 22.414 L / mol

In the case of RTP / ATP (P = 1 atm, T = 298 K), ideal gas Vm = 24 L / mol

In certain circumstances, an ideal gas law applies:

Where R is the gas constant, R = 0.08206 L ∙ atm / mol ∙ K = 8.314 J / mol ∙ K

6.    Writing and Equalizing Chemical Reaction Equations

            The chemical reaction equation is a statement written with a molecular formula that provides information on the identity and quantity of substances involved in a chemical or physical change. All reacting substances, called reactants, are placed to the left of the arrow, whose direction of the arrow to the right refers to the product, ie all the substances produced from the reaction.

            In the equation of the reaction, the reaction coefficient is known, which is the number on the left of the molecular formula for multiplying all the atoms in the molecular formula. The comparison of the reaction coefficients can be interpreted as the mole ratio of the substances in the reaction. In each of the reactants and products, it is written in the form of the substance (s (solid), l (liquid), g (gas), or aq (solution with water solvent)) in brackets to the right of each molecular formula.

Example of equivalent chemical reaction equation:

 Example of steps equalizing the reaction equation:
1.
2.
3.
4.

7.    Stoichiometry Reaction

            In chemical reactions, the amount of reacting reactants is sometimes incompatible with the stoichiometric amount of the reaction (not in accordance with the coefficient ratio of the equivalent equation). Therefore, there will be reactants that have reacted first compared to other reactants. The reactants that remain after reacting are called excess reagents. The out-of-date reactant is called a limiting reagent. After the barrier reagents are exhausted, no more reaction products are formed. Thus, the number of limiting reagents determines the amount of product produced.

Sample Stoichiometric Problem

Consider the following reactions:

What volume of hydrogen gas (STP) is formed when it is used 5.4 grams Al? (Ar Al = 27)

Answer :

a.    Apply the chemical reaction that occurs if the equation of the reaction is not equal. In the above equation the equation is equal so it does not need to be equalized.

b.  Declare the number of moles of known substance, ie aluminum

.
 c. Determining the number of moles of substance in question ie gas H2.

 d. Determine the volume of H2 gas produced

V  = n x Vm

     = 0.3 mol x 22.4 L mol ~ 1 = 6.72 L.

Stoichiometry – References

- Brown, Theodore L. et al. 2015. Chemistry: The Central Science (13^th edition). New Jersey: Pearson Education, Inc.
- Chang, Raymond. 2010. Chemistry (10^th edition). New York: McGraw Hill
- Earl, Bryan & Wilford, Doug. 2014. Cambridge IGCSE^® Chemistry (3^rd edition). London: Hodder Education.
- Gilbert, Thomas N. et al. 2012. Chemistry: The Science in Context (3^rd edition). New York: W. W. Norton & Company, Inc.
- Jespersen, Neil D., Brady, James E., & Hyslop, Allison. Chemistry: The Molecular Nature of Matter (6^th edition). New Jersey: John Wiley & Sons, Inc.
- Petrucci, Ralph H. et al. 2011. General Chemistry: Principles and Modern Applications (10^th edition). Toronto: Pearson Canada Inc.
- Purba, Michael. 2006. Kimia 1A untuk SMA Kelas X. Jakarta: Erlangga.
- Silberberg, Martin S. 2009. Chemistry: The Molecular Nature of Matter and Change (5^th edition). New York: McGraw Hill
- Stacy, Angelica M. 2015. Living by Chemistry (2^nd edition). New York: W.H. Freeman and Company
- Tro, Nivaldo J. 2011. Introductory Chemistry (4^th edition). Illinois: Pearson Prentice Hall