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Introduction Amount of Substance: The Mole Molar Mass Avogadro's Law and Gases Chemical Equations and Stoichiometry Empirical and Molecular Formulas Limiting Reagent Theoretical, Actual, and Percentage Yield Concentration of Solutions Titration: Measuring Chemical Change Summary of Key Concepts
Let's say you are going to bake a cake. You can't have a "pinch of this" or "a sprinkle of that". You need a specific quantity of each. Chemistry works the same way. To predict the amount of product a reaction will give, they have to measure the amount of each ingredient used.
In Chemistry, we are always thinking: "How much of this reacts? How much of that is produced?"
This requires a lot of measuring, counting, and calculating.
The central unit for measuring the amount of substance is the mole.
Definition: 1 mole is exactly 6.022×10²³ particles (atoms, molecules, or ions). This number is called Avogadro's number.
Example: 1 mole of carbon is 6.022×10²³ carbon atoms.
Below are some of the reasons why we use moles:
Molar mass (M) is the mass of 1 mole of a substance in grams per mole (g/mol).
Let's take an example:
To find moles from mass:
n = mass (g) / molar mass (g/mol)
To find mass from moles:
m = n × M
Avogadro's law states that when 2 gaseous substances are at the same temperature, 2 equal volumes of the gas have an equal number of particles.
1 mole of a gas occupies a volume of 22.7 liters at STP (0 °C, pressure = 1 atm) and is known as Standard Temperature & Pressure.
Example: At 45.4 liters of space, there are 2 moles of hydrogen gas.
Owing to this law, gas volumes and moles can be related, making calculations of gas reactions relatively easy.
Referring to balanced chemical equations, we can find the ratio at which chemicals are used as reactants or produced as products.
Stoichiometry is all about how to calculate the quantities of each substance that reacts or is produced.
Here's an example:
In this, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
If you have 4 moles of hydrogen, 2 moles of oxygen are required in order to react completely.
An empirical formula is the simplest ratio of atoms in the compound.
For example hydrogen peroxide (H₂O₂) has the empirical formula of HO.
In contrast, the molecular formula shows the exact number of atoms in the structure.
For example hydrogen peroxide (H₂O₂) has the molecular formula of H₂O₂.
This is why hydrogen peroxide is said to have grass composition.
There is a formula that helps in carrying out the calculation of reactive substances and helps in predicting the exact outcome.
In most reactions, one of the reactants is fully consumed, and is termed the limiting reagent.
The limiting reagent determines the mass of the product.
Consider the example:
In order for 1 mole of nitrogen (N₂) to react fully with 3 moles of hydrogen (H₂), if there is less hydrogen, the hydrogen (H₂) gas determines the limit of the reaction, hence only part of the nitrogen is able to react.
Theoretical yield is the maximum it is possible to yield for a reaction to occur with 100 percent efficiency.
Actual yield is what you are able to physically collect in the laboratory.
Percentage yield is a measure of how efficient a reaction is.
Percentage yield = (actual yield / theoretical yield) × 100
Example: If theoretical yield = 10 g, actual yield = 8 g
Percentage yield = (8/10) × 100 = 80%
This helps chemists study how successful a reaction is and adjust conditions if needed.
Molarity (M) measures how concentrated a solution is.
C = n / V
C = concentration in mol/L, n = moles of solute, V = volume in liters.
Example: Dissolving 1 mole of NaCl in 1 L water → 1 M NaCl solution.
Knowing concentration allows calculation of how much solute reacts in a solution, which is essential for titrations.
Titration helps find the unknown concentration of a solution.
You react a known volume of a solution with a standard solution until the reaction completes.
Using stoichiometry, you calculate the moles and mass of substances involved.
Steps:
Just like predicting how many servings a recipe will make, chemists predict how much of a product forms in chemical reactions. Without this, chemical reactions would be unpredictable, inefficient, and dangerous.
Always remember: moles connect the abstract world of atoms and the measurable world of chemistry.