Molar mass

Three very important concepts - the mole, Avogadro’s number and molar mass - provide the basis for relating masses in grams to numbers of atoms. The first, the mole, is the SI unit for amount of substance. The mole is the amount of a substance that contains the same number of particles as the number of atoms in exactly 12 g of ¹²C. The abbreviation for the mole is mol. The mole is a counting unit, like one dozen. In writing, a chemist might refer to 1 mol of Carbon, or 2 mol of Iron, or 2,567 mol of calcium. Amount of substance (in mol) is indicated as Greek letter υ.

The number of particles in a mole has been experimentally determinates in a number of ways. The best modern value is 6,022137·10²³ particles. This number is of such importance to chemistry that it is named in honor of the Italian scientist Amedeo Avogadro (1776-1856), whose ideas were crucial in the early understanding of chemistry. Avogadro’s number N_A- 6,022137·10²³ - is the number of particles in exactly one mole of each pure substance. In most cases, Avogadro’s number is rounded to 6,02·10²³ (or mol^-1).

Molar mass (M_m) is the mass in grams of one mole of pure substance (g/mol). Molar masses of elements contain equal numbers of atoms. One mole of a substance is one molar mass of this substance. An alternative definition of the mole is the amount of a substance that contains an Avogadro’s number of particles of chemical units.

The molar mass is used as the conversion factor in chemical calculations. For a specific substance, a known number of grams can be converted into moles or a known number of moles can be converted into grams. Figure 1 shows the conversions of molar mass, moles, and Avogadro’s number.

In general, mass of any chemical substance (in grams) is connected with amount of this substance by the next ratio:

ν , (1)

where ν - amount of any pure substance (mol or kmol);

m - mass of any pure substance (g or kg);

M_m - molar mass (g/mol or kg/kmol).

		Mass of an element (grams)

	=

	Number of atoms of an element

Figure 1. Relationships between mass, the number of moles, and the number of atoms of an element in the sample

Date: 2015-01-12; view: 990