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After understanding what a mole is and knowing how to convert from moles to grams for atoms, it is time to be able to determine the mass (grams) of one mole of a substance that has more than one atom in it. Being able to calculate molar masses will enable calculations to determine the mass of reactants and products in chemical reactions.The focus here is to provide a methodology that can be used to convert from moles of a substance to grams of that substance. The mole-gram conversion factor can also be used to determine the moles from the grams (use the reciprocal of the moles to grams conversion factor, as previously shown).
The molar mass is the mass of one mole. The units are usually grams per mole (g/mole).
The molar mass is sometimes called the molecular weight, but not all substances are molecules and it isn't the weight (the force of gravity on a mass) that is being determined (it is the mass, or amount of matter present, that is being determined). It is easiest to understand for molecules because a molecule has atoms that are stuck together, stay together, and act as one unit. The molar mass for a molecule would be the mass of one mole of that molecule. The molar mass for an ionic compound is sometimes referred to as the formula weight (should be formula mass) because there are no molecules in an ionic compound. In this case the molar mass refers to the mass of one mole of a formula unit. The formula unit in sodium chloride would be NaCl (even though there are no NaCl molecules).
To determine the molar mass, find the individual molar masses of the atoms in the formula of the substance from the periodic chart and add them together.
This works for both molecules and ionic compounds. The non-integer number on the periodic chart for each element is the mass of a mole of atoms for that element. The molar mass of an element is called the atomic weight or atomic mass (remember it is the mass of one mole of that element's atoms). A diatomic molecule like H2 has two moles of atoms for each mole of molecules and so the molar mass of that molecule would be two times the molar mass of individual hydrogen atoms.
One mole of HCl molecules contains one mole of hydrogen atoms and one mole of chlorine atoms. One mole of hydrogen atoms has a molar mass of 1.01 g/mole (from the periodic table) and one mole of chlorine atoms has a molar mass of 35.45 g/mole. The molar mass of HCl would be:
1.01 g/mole H + 35.45 g/mole Cl = 36.46 g/mole HCl Methane, CH4, has one carbon and four hydrogens in its formula. The molar mass would be (notice that each hydrogen atom must be accounted for):
12.01 g/mole C + 4(1.01 g/mole H) = 16.05 g/mole CH4 Magnesium hydroxide, Mg(OH)2, has one magnesium, two oxygen, and two hydrogen atoms in its formula. The molar mass would be:
24.31 g/mole Mg + 2(16.00 g/mole O) + 2(1.01 g/mole H) = 58.33 g/mole Mg(OH)2 Here are some more examples:
Br2 159.80 g/mole Details SiO2 60.09 g/mole Details C2H6 30.08 g/mole Details NaNO3 85.00 g/mole Details K2SO4 174.26 g/mole Details The corresponding word problem for the first example in the table would be: "What is the molar mass of diatomic bromine, Br2?" Others would be written similarly. The molar mass is a conversion factor between grams and moles, just like the atomic mass that was introduced in the unit conversion section.
Two quick review examples:
How many moles of NaNO3 are in 2.3 grams of NaNO3? Use the conversion factor in the above table to get:
(2.3 g NaNO3)(
1 mole NaNO3
85 g NaNO3) = 3.53 x 10-3 mole NaNO3 How many grams corresponds to 4.2 moles of NaNO3?
(4.2 mole NaNO3)(
85 g NaNO3
1 mole NaNO3) = 357 g NaNO3