In chemistry, Avogadro’s number and the mole are important concepts that are essential to understanding chemical reactions and calculations.
Avogadro’s number, denoted as NA, is the number of particles in one mole of a substance. This number is approximately equal to 6.022 x 1023, which is a huge number.
The particles can be atoms, molecules, ions, or electrons, depending on the substance. For example, one mole of oxygen gas contains 6.022 x 1023 oxygen molecules, while one mole of sodium ions contains 6.022 x 1023 sodium ions.
Avogadro’s number is a constant and is the same for all substances. It was named after the Italian physicist and chemist Amedeo Avogadro, who proposed the concept of the mole in 1811.
The mole is a unit of measurement used to express the amount of a substance. It is defined as the amount of a substance that contains as many particles as there are in 12 grams of carbon-12. This definition is based on the relative atomic masses of elements.
The symbol for a mole is mol, and it is used to represent the quantity of a substance in chemical equations and calculations. For example, the equation for the reaction between hydrogen and oxygen to form water is:
2H₂(g) + O₂(g) → 2H₂O(l)
This equation shows that two moles of hydrogen gas react with one mole of oxygen gas to produce two moles of liquid water.
The mass of one mole of a substance is called its molar mass, and it is expressed in grams per mole. For example, the molar mass of carbon-12 is 12 grams per mole, while the molar mass of oxygen gas is 32 grams per mole.
To convert between mass and moles, you can use the molar mass of the substance. The molar mass is the mass of one mole of the substance, and it is expressed in grams per mole (g/mol).
To convert from mass to moles, you can divide the mass of the substance by its molar mass. To convert from moles to mass, you can multiply the number of moles by the molar mass.
Empirical and molecular formulae are two ways of representing the chemical composition of a compound. While empirical formula represents the simplest whole number ratio of the atoms in a compound, molecular formula represents the actual number of atoms of each element in a molecule of the compound.
The empirical formula of a compound can be determined experimentally from the percent composition of the compound. The steps involved in calculating empirical formula are:
Let’s take an example to understand this better. Suppose a compound is found to contain 40% carbon, 6.67% hydrogen, and 53.33% oxygen. We can calculate its empirical formula as follows:
Assume that we have 100 grams of the compound. This means we have 40 grams of carbon, 6.67 grams of hydrogen, and 53.33 grams of oxygen.
Convert the mass of each element to moles using the molar mass of the element. The molar mass of carbon, hydrogen, and oxygen are 12.01 g/mol, 1.01 g/mol, and 16.00 g/mol, respectively.
Moles of carbon = 40 g / 12.01 g/mol = 3.33 mol
Moles of hydrogen = 6.67 g / 1.01 g/mol = 6.60 mol
Moles of oxygen = 53.33 g / 16.00 g/mol = 3.33 mol
Divide each mole value by the smallest mole value obtained. In this case, the smallest value is 3.33 mol.
Moles of carbon = 3.33 mol / 3.33 mol = 1.00 mol
Moles of hydrogen = 6.60 mol / 3.33 mol = 1.98 mol
Moles of oxygen = 3.33 mol / 3.33 mol = 1.00 mol
The resulting values represent the empirical formula of the compound. In this case, the empirical formula is CH2O.
The molecular formula of a compound can be calculated if we know its empirical formula and its molar mass. The molecular formula is obtained by multiplying the empirical formula by a whole number that represents the ratio of the molar mass of the compound to the molar mass of the empirical formula.
Let’s take an example to understand this better. Suppose the empirical formula of a compound is CH2O and its molar mass is 180 g/mol. We can calculate its molecular formula as follows:
Calculate the molar mass of the empirical formula.
Molar mass of CH2O = 12.01 g/mol + 2(1.01 g/mol) + 16.00 g/mol = 30.03 g/mol
Calculate the ratio of the molar mass of the compound to the molar mass of the empirical formula.
Ratio = 180 g/mol / 30.03 g/mol = 5.996
Round off the ratio to the nearest whole number. In this case, the ratio is approximately 6.
Multiply the empirical formula by the ratio obtained to obtain the molecular formula.
Molecular formula = 6(CH2O) = C6H12O6
The empirical and molecular formulae can be used to determine the chemical formula of a compound, which is important for predicting its chemical and physical properties.
The chemical formula can also be used to calculate the mass percent of each element in the compound, which can help in determining the composition of the compound.
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