A chemist cannot count atoms one by one, because there are too many of them. In this course we find the mass of a molecule, and we learn the mole, which lets us count a very large number of particles by weighing.
3 lessons, about 1 h 15 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabThe molecular mass of a substance is the sum of the atomic masses of all the atoms in one molecule, in u. For an ionic compound, which has no separate molecules, the same sum for the smallest group shown by the formula is called the formula unit mass. To find it, write the formula, count the atoms of each element using the small numbers and the brackets, multiply each count by the atomic mass, and add the results. Water is 18 u, carbon dioxide 44 u, and sodium chloride 58.5 u.
One mole is the amount of a substance that has 6.022 times ten to the power of 23 particles. This number is the Avogadro constant, the number of atoms in exactly 12 grams of carbon-12. A mole can be of atoms, molecules or ions. The molar mass is the mass of one mole in grams per mole, and its number is equal to the atomic or molecular mass in u. The number of moles is the mass divided by the molar mass, and the number of particles is the number of moles times the Avogadro constant.
Mole problems use three steps: find the molar mass from the formula, change the mass to moles, and change the moles to particles or atoms. We can also find the percentage of each element in a compound.
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