Atoms are too small to be measured in a chemical substance. To work with specific amounts of substances, scientists group them into units called moles. One mole is defined as the number of carbon atoms in 12 grams of the carbon-12 isotope, which is approximately 6.022 x 1023 atom. This number is called Avogadro's number or Avogadro's constant. The mole is used as the number of atoms for any substance and the mass of 1 mole of a substance is the molar mass.
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Method 1 of 2: Calculating the Elemental Molar Mass
Step 1. Understand the molar mass
Molar mass is the mass (in grams) of one mole of a substance. By using the atomic mass of an element and multiplying it by the conversion factor of grams per mole (g/mol), you can calculate the molar mass of the element.
Step 2. Find the relative atomic mass of the element
The relative atomic mass of an element is the average mass of a sample of all its isotopes in atomic units. This information can be found in the periodic table of the elements. Find the location of the element and look for the number under the element symbol. The number is not a whole number, but a decimal.
For example, for hydrogen, the relative atomic mass is 1.007; for carbon is 12.0107; for oxygen is 15,9994; and for chlorine it is 35, 453
Step 3. Multiply the relative atomic mass by the molar mass constant
The product is defined as 0.001 kilogram per mole or 1 gram per mole. This converts atomic units to grams per mole and makes the molar mass of hydrogen 1.007 grams per mole, carbon 12.0107 grams per mole, oxygen 15,9994 grams per mole, and chlorine 35,453 grams per mole.
- Some elements are only found in molecules of 2 or more atoms. This means that if you want to find the molar mass of an element consisting of 2 atoms, such as hydrogen, oxygen, and chlorine, you must find the relative atomic mass, multiply it by its molar mass constant, and multiply the product by 2.
- For H2: 1.007 x 2 = 2.014 grams per mole; for O2: 15,9994 x 2 = 31,9988 grams per mole; and for Cl2: 35,453 x 2 = 70.096 grams per mole.
Method 2 of 2: Calculating the Molar Mass of the Compound
Step 1. Find the chemical formula for the compound
This formula is the number of atoms in each element that makes up the compound. (This information is given in any chemistry reference book.) For example, the chemical formula for hydrogen chloride (hydrochloric acid) is HCl; the chemical formula for glucose is C6H12O6. Using this formula, you can identify the number of atoms of each element that makes up the compound.
- For HCl, there is one hydrogen atom and one chlorine atom.
- For C6H12O6, there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.
Step 2. Find the relative atomic mass of each element in the compound
Using the periodic table, find the location of the relative atomic masses for each element. This mass is the number below the element symbol. As we did in the first way to calculate the molar mass of an element, we also multiply these masses by 1 gram/mol.
- The relative atomic masses of the elements in hydrochloric acid are: hydrogen, 1.007 g/mol and chlorine, 35, 453 g/mol.
- The relative atomic masses of the elements in glucose are: carbon, 12.0107 g/mol; hydrogen, 1.007 g/mol, and oxygen, 15,9994 g/mol.
Step 3. Calculate the molar mass of each element in the compound
Multiply the atomic mass of an element by the number of atoms of that element in the compound. The product of this product will give you the relative amount that each element contributes to the compound.
- For hydrogen chloride, HCl, the molar mass of each element is 1.007 grams per mole for hydrogen and 35.453 grams per mole for chlorine.
- For glucose, C6H12O6, the molar mass of each element is: carbon, 12.0107 x 6 = 72.0642 g/mol; hydrogen, 1.007 x 12 = 12,084 g/mol; and oxygen, 15.9994 x 6 = 95.9964 g/mol.
Step 4. Add up the molar masses of each element in the compound
This sum determines the molar mass for the entire compound. Take the product you got from the next step and add up the product to calculate the molar mass of the compound.
- For hydrogen chloride, the molar mass is 1.007 + 35, 453 = 36, 460 g/mol. The mass of one mole of hydrogen chloride is 36.46 grams.
- For glucose, the molar mass is 72.0642 + 12.084 + 95.9964 = 180.1446 g/mol. The mass of one mole of glucose is 180.14 grams.
