Stoichiometry Formula: Definition , Solved Examples

Stoichiometry Formula: Stoichiometry, a term derived from the Greek words "stoicheion," meaning elementary constituent, and "metrin," referring to measuring, involves quantitative calculations concerning the reactants and products in chemical reactions. It is grounded in the principle of the conservation of mass, where the combined mass of reactants equals the combined mass of the products. Stoichiometry allows for the precise determination of product quantities when the quantities of the individual reactants are known.

For instance, in the chemical equation:

CH4 + 2 O2 → CO2 + 2 H2O

Here, one molecule of methane (CH4) reacts with two molecules of oxygen gas to yield two molecules of carbon dioxide (CO2) and two molecules of water (H2O). Stoichiometry quantifies this relationship and is employed to ascertain the quantities of reactants produced in a given reaction.

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What Is Stoichiometry?

In simpler terms, we can define stoichiometry as the calculation of the quantities of products and reactants involved in a chemical reaction, primarily focusing on numerical values.

Stoichiometry is a vital concept in Chemistry, enabling us to use balanced chemical equations to compute the quantities of reactants and products involved. This involves the application of ratios from the balanced equation. Essentially, all reactions depend on a fundamental factor the quantity of substance present.

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Stoichiometry Formula Solved Examples

Example 1: Ammonia reacts with solid copper oxide, resulting in the release of nitrogen gas, solid copper, and water vapor. Write the chemical reaction and balance the equation according to stoichiometry.

Solution: The chemical reaction can be expressed as:

NH3 + CuO → Cu + H2O + N2

The balanced equation, considering stoichiometry, is:

2 NH3 + 3 CuO → 3 Cu + 3 H2O + N2

Example 2: Sulfur trioxide gas is generated when iron pyrites (FeS2) undergoes combustion. Please outline the chemical reaction and balance the equation by considering the stoichiometry of each compound.

Solution: The chemical reaction for this example is:

FeS2 + O2 → Fe2O3 + SO3

Combustion compounds react with oxygen.

The balanced chemical equation, taking stoichiometry into account, is:

4 FeS2 + 15 O2 → 2 Fe2O3 + 8 SO3

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Example 3: Combustion of Hydrogen Gas

When hydrogen gas (H2) combusts in the presence of oxygen gas (O2), it forms water vapor (H2O). Let's write the chemical reaction and balance the equation using stoichiometry.

Chemical Reaction:

H2 + O2 → H2O

Stoichiometry Explanation:

In this scenario, a single molecule of hydrogen (H2) reacts with one molecule of oxygen (O2) to yield one molecule of water (H2O). To achieve a balanced equation using stoichiometry, it's essential to maintain an equal count of each type of atom on both sides. Since hydrogen is present twice in the reactants, we balance it by introducing a coefficient of 2 in front of H2O:

Balanced Equation:

2 H2 + O2 → 2 H2O

This balanced equation represents the stoichiometric proportions in the reaction, allowing us to calculate the quantities of reactants and products accurately.

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Example 4: Formation of Sodium Chloride (Table Salt)

When sodium metal (Na) reacts with chlorine gas (Cl2), sodium chloride (NaCl), commonly known as table salt, is formed. Let's write the chemical reaction and balance it using stoichiometry.

Chemical Reaction:

Na + Cl2 → NaCl

Stoichiometry Explanation:

In this reaction, one sodium atom (Na) reacts with one chlorine molecule (Cl2) to produce one molecule of sodium chloride (NaCl). The equation is already balanced, as the number of each type of atom is the same on both sides.

Example 5: Formation of Calcium Oxide

Calcium metal (Ca) reacts with oxygen gas (O2) to produce calcium oxide (CaO), commonly known as quicklime. Let's write the chemical reaction and balance it using stoichiometry.

Chemical Reaction:

Ca + O2 → CaO

Stoichiometry Explanation:

In this particular reaction, a single calcium atom (Ca) undergoes a reaction with a single molecule of oxygen (O2) to yield a solitary molecule of calcium oxide (CaO). Notably, the equation is inherently balanced, with an equal count of each type of atom on both sides.

In each of these examples, stoichiometry enables us to determine the precise quantities of reactants needed and products formed in chemical reactions by applying the principles of balanced equations. It is a fundamental concept in chemistry, providing valuable insights into the quantitative aspects of chemical transformations.