Why is avogadros number used in chemistry

The current definition was adopted in November , revising its old definition based on the number of atoms in 12 grams of carbon 12 C the isotope of carbon with relative atomic mass 12 Daltons by definition. It is not obvious why eggs come in dozens rather than 10s or 14s, or why a ream of paper contains sheets rather than or The definition of a mole—that is, the decision to base it on 12 g of carbon—is also arbitrary.

The important point is that 1 mole of carbon—or of anything else, whether atoms, compact discs, or houses—always has the same number of objects: 6. We can use Avogadro's number as a conversion factor, or ratio, in dimensional analysis problems. If we are given the number of atoms of an element X, we can convert it into moles of by using the relationship. An example on the use of Avogadro's number as a conversion factor is given below for carbon.

The element carbon exists in two primary forms: graphite and diamond. Since Avogadro's number is a measured quantity with three significant figures, the result of the calculation is rounded to three significant figures. One skill needed in future chapters is the ability to determine the mass of the formula of various chemical substances.

This quantity is called the formula mass. The formula mass is obtained by adding the masses of each individual atom in the formula of the substance. Because a proper formula is electrically neutral with no net electrons gained or lost , the ions can be considered atoms for the purpose of calculating the formula mass.

Let us start by calculating the formula mass of sodium chloride NaCl. This formula mass is the sum of the atomic masses of one sodium atom and one chlorine atom, which we find from the periodic table; here, we use the masses to two decimal places:. To two decimal places, the formula mass of NaCl is For covalent substances, the formula represents the numbers and types of atoms composing a single molecule of the substance; therefore, the formula mass may be correctly referred to as a molecular mass.

The molecular formula of chloroform indicates that a single molecule contains one carbon atom, one hydrogen atom, and three chlorine atoms.

The average molecular mass of a chloroform molecule is therefore equal to the sum of the average atomic masses of these atoms. To two decimal places, the formula mass of CHCl 3 is For ionic compounds with polyatomic ions, the sum must include the number and mass of each atom in the formula for the polyatomic ion.

Aluminum sulfate, Al 2 SO 4 3 , is an ionic compound that is used in the manufacture of paper and in various water purification processes. What is the formula mass amu of this compound? For purposes of computing a formula mass, it is helpful to rewrite the formula in the simpler format, Al 2 S 3 O Following the approach outlined above, the formula mass for this compound is calculated as follows:.

The formula mass for Al 2 SO 4 3 , is Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. Continue reading with a Scientific American subscription.

They also need a way to compare these numbers and relate them to the weights of the substances, which they can measure and observe. The solution is the concept of the mole, which is very important in quantitative chemistry.

Amadeo Avogadro first proposed that the volume of a gas at a given pressure and temperature is proportional to the number of atoms or molecules, regardless of the type of gas. Although he did not determine the exact proportion, he is credited for the idea.

It is equal to 6. A dozen molecules is 12 molecules. A gross of molecules is molecules. It is defined as the amount of a substance that contains as many particles as there are atoms in 12 grams of pure carbon