STOICHIOMETRY

10.1 Atomic Mass

You know the atoms of different elements have different masses. Thousands of experiments led to the organization of a scale of relative atomic masses. The scale is based on the mass of carbon-12. The mass of an atom relative to the mass of carbon-12 is the Atomic Mass of the Atom. The SI unit for atomic mass is amu.

On the scale, the atomic mass of hydrogen is 1.0079 amu, oxygen is 15.994 amu and sulfur is 32.06 amu. This means that the mass of oxygen atoms are about 16 times the mass of hydrogen atoms, while sulfur atoms are nearly 32 times as heavy as hydrogen atoms.

10.2 Formula Mass or Molecular Mass

• Molecular Mass - is used for compounds that are composed of molecules and covalent compounds like CO_{2 and sucrose or table sugar} C₁₂H₂₂O_{11. However, the term formula mass is usually used for both ionic and covalent compounds.}

• Formula Mass - is used for ionic compounds like NaCl or for compounds that ionize in aqueous solutions like H₂SO₄

- It is the sum of the atomic masses of each atom in a chemical formula. To calculate the formula mass of H₂SO₄ refer to the periodic table for the atomic mass values of each of the elements that constitute 

the compund and the compute it as follows

You can watch this for further explanation :)

10. 3 The Mole

In chemistry, particles are counted by the Mole (mol), a unit that contains 6.02 x 10^{23 species.} 

This very large number is called Avogadro's number, after Amedeo Avogadro (1776-1856), an Italian physicist who contributed to the development of the concept of the atomic nucleus.

^{Avogadro's Number}

^{602 000 000 000 000 000 000 000 =} 6.02 x 10²³

This is used to indicate very small species that make up matter like atoms, ions, or molecules. It is also the number of atoms in 12 grams of  ¹²₆C. One mole of any element always has Avogadro's Number of atoms. 

You should watch the video clearly because it shows how to calculate for the mole.

This video is very useful. So i recommend you to watch and understand it :)

10. 4 Molar Mass

It is not possible to weigh a single atom or molecule. Even the most accurate balance available cannot weigh very small masses. In the laboratory, you will use a balance that measures in grams. When you weigh one mole of a substance, you would weigh its mass in grams which is numerically equal to its mass in atomic mass unit. The mass in grams of one mole of a substance is called its Molar Mass. 

Example

What is the molar mass of one mole of ascorbic acid?

Solution

For more convient tutorial, watch this video :)

10. 5 Percentage Composition

In the laboratory, you perform experiments that will require you to prepare solutions or to verify the purity of some substances. If you are asked to prepare 10% salt solution, you dissolve 10g of salt in 90g of water to make a 100g salt solution. Thus you need to know how to calculate the percent compositions of compounds.

The percent by mass of an element in a compound is obtained by using the equation

Example

5.00 g of Calcium reacts completely with 10.0 g of Sulfur. What is the percent composition of the calcium sulfide produced?

^Solution

^{A total of 100%}

The mass of CaS is obtained by adding the masses of the components.

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10. 6 Empirical and Molecular Formulas

Empirical or simplest formula gives the smallest whole-number ratio of the elements in a formula. Using the mole, and empirical formula is the lowest whole-number ratio of moles of atoms of the elements in the compound. An empirical formula may or may not be the same as a molecular formula. For example, the simplest empirical formula and molecular formula for water are both H₂O. However, for hydrogen peroxide they are HO and H₂O_{2 respectively.}

When the molecular formula is different from the empirical formula, it is some whole number multiple of the empirical formula. Consider formaldehyde and glucose. They have the same empirical formula, CH₂O. The molecular mass of the formaldehyde and glucose are 30 g/mol and 180 g/mol,

 respectively. Thus, the empirical formula is also the molecular formula of formaldehyde, 

while the molecular formula of glucose is 6 (CH₂O). The mutiple 6 is obtained by dividing the

molecular mass of glucose (180 g/mol) by the given empirical mass (30 g/mol)

 Therefore, the molecular formula of glucose is C₆H₁₂O₆

Example 

What is the empirical formula of a compound that contains 27.27% carbon 

and 72.72% oxygen?

Solution

a. Convert the percent composition in terms of mass and express the mass of each element 

into moles.

b. Determine the smallest whole number ratio for carbon and oxygen by dividing by the

least number obtained.

c. The ratios obtained in the calculation are the subscripts of the elements in the formula

to represent their mole ratio.

The empirical formula of the compound is CO₂

Here is another example. Kindly watch the video :)

10. 7 Stoichiometry

              The focus is on the stoichiometry of individual compounds (ratios of atoms in chemical formulas) In this section, the same ideas are extended to chemical reactions. Moles, grams, and number of atoms and molecules are used to specify the amount of substances used in chemical reactions.

  

               In a balanced chemical equation, if the amount of one reactant or product is given, stoichiometry indicates the amount of every other reactant and product. This is so because a balanced equation summarizes the quantitative relationships among the amounts of reactants and products.

Reaction Stoichiometry - is the study of the quantitative relationships among reactants and products in a chemical reaction.

Here is one example. Try watching it :)

10. 8 Limiting Reagent

            In the previous problems, the reactants supplied are in the exact ratio described by the balanced equation. They are said to be in stoichiometric amounts. But in actual applications, this is hardly the case. Most often, reactants available are in other proportions. In a situation where a reactant exceeds that of the required amount, the reactant is called the excess reagent and the reactant that is completely used up is called the limiting reagent. 

View this video for further explanation on how to solve :)