Atoms are too small units to allow accurate measurements of chemicals. When it comes to working with exact quantities, scientists prefer to group atoms into units called moles. One mole is equal to the number of atoms present in 12 g of the isotope-12 of carbon and is equivalent to approximately 6.022 x 1023 atoms. This value is called Avogadro's number, or Avogadro's constant, and is used as the number of atoms of each substance; the mass of a mole of substance represents the molar mass.
Steps
Method 1 of 2: Calculate the Molar Mass of an Element
Step 1. Understand the concept of molar mass
This is the mass expressed in grams of one mole of substance. Starting from the atomic mass of an element and multiplying it by the conversion factor "grams to mole" (g / mol) you can calculate its molar mass.
Step 2. Find the relative atomic mass of the element
It represents the average value of the mass of the specific atom, expressed in atomic units, calculated considering a sample of all the existing isotopes of that element. You can find this data on the periodic table of elements. First, locate the box corresponding to the item in question and find the number under its symbol; it will not be an integer but a decimal number.
For example, the relative atomic mass of hydrogen is 1.007; that of carbon is 12, 0107; that of oxygen is equal to 15, 9994 and finally that of chlorine is 35, 453
Step 3. Multiply the relative atomic mass by the molar mass constant
This is defined as 0.001 kilograms per mole, i.e. 1 gram per mole. This will convert the atomic units into the correct unit of measurement (g / mol). Consequently, the molar mass of hydrogen is 1.007 g / mol; that of carbon 12, 0107 g / mol, that of oxygen is 15, 9994 g / mol and that of chlorine is 35, 453 g / mol.
- Some elements are found only in molecules composed of two or more atoms. This means that if you want to find the molar mass of an element made up of two atoms, such as hydrogen, oxygen and chlorine, then you need to find their relative atomic masses, multiply them by the molar mass constant, and finally multiply the result. for two.
- For H2: 1, 007 x 2 = 2, 014 grams per mole; However2: 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: Calculate the Molar Mass of a Compound
Step 1. Find the chemical formula of the compound
This represents the number of atoms of each element that makes up the substance; this is generally information that is provided by the textbook. For example, the formula of hydrogen chloride is HCl; for glucose it is C.6H.12OR6. Thanks to this formula you can identify the number of atoms of each element that contributes to the compound.
- In HCl there is a hydrogen atom and a chlorine atom.
- In C6H.12OR6 there are six carbon atoms, twelve hydrogen atoms and six oxygen atoms.
Step 2. Find the relative atomic mass of each element that makes up the compound
You can find this data thanks to the periodic table. This is the number that is written under the atomic symbol of the element. Just as described in the first method, to find the molar mass you just need to multiply this number by 1 g / mol.
- The relative atomic masses of the elements that make up hydrogen chloride are: hydrogen = 1, 007 g / mol and chlorine = 35, 453 g / mol.
- The relative atomic masses of the elements that make up 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 of the compound
Multiply the atomic mass of each element by the number of atoms involved. This way you find the quantity of each element within the compound.
- For hydrogen chloride, HCl, the molar mass of each element is 1.007 g / mol for hydrogen and 35.453 g / mol for chlorine.
- For glucose, on the other hand, C.6H.12OR6, 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 together the molar masses of each element of the compound
This way you find the molar mass of the whole substance. Take the various products obtained in the previous step and add them together to find the molar mass of the compound.
- For hydrogen chloride the molar mass is 1.007 + 35.453 = 36.460 g / mol. This value represents the mass of one mole of hydrogen chloride.
- For glucose the molar mass is 72, 0642 + 12, 084 + 95, 9964 = 180, 1446 g / mol. This value represents the mass of one mole of glucose.
