No single carbon atom has a mass of 12.01 amu, but in a handful of C atoms the average mass of the carbon atoms is 12.01 amu.
This is the average atomic mass of carbon. On the periodic table the mass of carbon is reported as 12.01 amu. Since many elements have a number of isotopes, and since chemistsrarely work with one atom at a time, chemists use average atomicmass. Since there are a variety of carbon isotopes we must specify which C atom defines the scale.Īll the masses of the elements are determined relative to 12C.īy the way, the mass of an element is not equal tothe sum of the masses of the subatomic particles of which the elementis made! 12C has 6 neutrons, 13C has 7 neutrons, and 14C has 8 neutrons and so on. Each carbon atom has the same number of protons and electrons, 6. Carbon exists as two major isotopes, 12C, and 13C ( 14C exists and has a half life of 5730 y, 10C and 11C also exist their half lives are 19.45 min and 20.3 days respectively). Why do we specify 12C? We do not simply state the themass of a C atom is 12 amu because elements exist as a variety ofisotopes. Since we are converting grams into moles, we need to flip this conversion factor so that grams are on the bottom in order to cancel.ħ13 g x 1 mol Al 2(SO 4) 3/342.17 g = 2.Atomic mass is based on a relative scale and the mass of 12C (carbon twelve) is defined as 12 amu. The molar mass of Al 2(SO 4) 3 was earlier calculated to be 342.17 g/mol. How many moles are in 713 grams of Al 2(SO 4) 3? We earlier calculated the molar mass of Fe 2O 3 to be 159.70 g/mol.ģ.81 mol Fe 2O 3 x 159.70 g/1 mol Fe 2O 3 = 608 gĤ. What is the mass in grams of 3.81 moles of Fe 2O 3?
PERIODIC TABLE MOLAR MASS HOW TO
The following two example problems will illustrate how to use molar mass as a conversion factor between moles and mass. This allows us to convert between grams of Al 2(SO 4) 3 and moles of Al 2(SO 4) 3. Just like there are 12 inches/1 foot, for Al 2(SO 4) 3 there are 342.17 g/mol. This looks a lot like a conversion factor that relates two units: grams to moles. Notice that the units on molar mass are grams per mole (g/mol). There are 5 Mn atoms (mass of 54.94) and 16 O atoms (mass of 16.00). Or Avogadro’s number, 6.022×10 23, of Fe 2O 3 formula units (ionic compound particles) would have a mass of 159.70 g. So one mole of Fe 2O 3 would have a mass of 159.70 g.
To calculate its molar mass we have to add up the mass of two iron atoms and three oxygen atoms. To find the mass of one mole of any substance we count the elements present and add up their atomic masses.įor example, let’s calculate the molar mass of iron(III) oxide, Fe 2O 3. Fe 2O 3 is composed of two iron atoms and three oxygen atoms. So one mole of hydrogen, or 6.022×10 23 hydrogen atoms, has a mass of 1.01 g. Remember that one mole is equal to Avogardo’s number, 6.022×10 23 particles. But one incredibly handy aspect of the periodic table is that those same masses are also the mass of one mole of the element in grams.įor example, one atom of hydrogen has a mass of 1.01 amu, but one mole of hydrogen has a mass of 1.01 g. For example, the atomic mass of hydrogen (symbol H) is 1.01 amu (atomic mass units). We saw in a previous lesson on atomic mass that the periodic table provides us with the average atomic mass of elements. Molar mass is literally the mass of one mole of a substance.