NCERT Solutions for Class 11 Chemistry Stoichiometry Notes

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STOICHIOMETRY

What is stoichiometry?

The study of the quantitative aspects of the reactants and products involved in a chemical reaction is known as stoichiometry.

The relationship between the relative quantities of substances taking part in a chemical reaction or forming compounds, typically a ratio of whole integers.

Stoichiometry is based on the law of conservation of mass i.e. the total mass of the reactants is equal to the total mass of the products.

To understand stoichiometry, we should have the knowledge of mole concept.

Mole: The mole is the SI unit for the amount of substance. In short, it is written as “mol”. Mole is defined as the amount of a substance that contains the same number of entities (electrons, atoms, ions, or molecules) as the number of atoms in 12 grams of carbon-12 (¹²C). This is given by Avogadro’s number, in honor of an Italian physicist Amedeo Avogadro.

Avogadro number gives the number of units in one mole of any substance, which is equal to 6.0221×10²³.The units may be electrons, atoms, ions or molecules depending on the nature of the substance and the reaction it undergoes. This means that one mole contains 6.023×10²³ entities/units.

Watch animated videos on the same for a detailed explanation by clicking Chemistry notes for Class 11.

The importance of the mole unit is:

The mole unit allows us to relate the number of entities to the mass of a sample of those entities. For example; 1mol of ¹²C contains 6.022×10²³ carbon-12 atoms and has a mass of 12 grams. This means that by the use of mole unit, we can count the number of entities by weighing the sample. If we find the weight of the sample of ¹²C to be 6 grams (g), so it means the sample of carbon-12 is 0.5 mol and it contains 3.011×10²³
The mole maintains the same numerical relationship between mass on the atomic scale (atomic mass units, amu) and mass on a macroscopic scale (grams, g). This relationship is same for the elements as well as for the compounds. As explained here:

Elements: The mass in the atomic mass unit (amu) of one atom of an element is numerically same as a mass in grams (g) of 1 mole of atoms of the element. Asit is stated that each atom of an element is considered to have the atomic mass given in the periodic table; thus,

1 atom of sulphur (S) has a mass of 32.07 amu and 1 mol (6.022×10²³ atoms) of S has a mass of 32.07g.

1 atom of iron (Fe) has a mass of 55.85 amu and 1 mol (6.022×10²³ atoms) of Fe has a mass of 55.85g.

Compounds: The mass in the atomic mass unit (amu) of one molecule or formula unit of a compound numerically same as the mass in grams (g) of 1 mole of the compound. For example;

1 molecule of H₂O has a mass of 18.02 and 1 mol (6.022×10²³molecules) of H₂O has a mass of 18.02g

1 formula unit^# of NaCl has a mass of 58.44 amu and 1 mol (6.022×10²³ formula units) of NaCl has a mass of 58.44g.

^# The chemical formula with the least number of elements out of the set of empirical formulas having the same proportion of ions as elements. It is used for ionic compound that does not form molecules, as most salts; NaCl is the formula unit for ionic compound sodium chloride, and covalent networks like C is the formula unit for diamond or graphite.

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