The formula mass value is the sum of the atomic masses of all the atoms in a chemical formula. It is expressed in atomic mass units (amu).
Here's how to calculate the formula mass:
- Identify the chemical formula of the compound. For example, the chemical formula for water is H₂O.
- Look up the atomic masses of each element in the formula. You can find these values on the periodic table. The atomic mass of hydrogen is 1.008 amu, and the atomic mass of oxygen is 15.9994 amu.
- Multiply the atomic mass of each element by the number of atoms of that element in the formula. In the case of water, there are two hydrogen atoms, so we multiply 1.008 amu by 2. There is one oxygen atom, so we multiply 15.9994 amu by 1.
- Add the results together to get the formula mass. For water, the formula mass would be (1.008 amu 2) + (15.9994 amu 1) = 18.0152 amu.
Example:
Let's calculate the formula mass of sodium chloride (NaCl):
- Atomic mass of sodium (Na) = 22.98977 amu
- Atomic mass of chlorine (Cl) = 35.453 amu
Formula mass of NaCl = 22.98977 amu + 35.453 amu = 58.44277 amu
Practical Insights:
- Formula mass is often used to calculate the molar mass of a compound, which is the mass of one mole of the compound.
- It can also be used to determine the empirical formula of a compound, which is the simplest whole-number ratio of atoms in the compound.
