To calculate the atomic weight (also known as atomic mass or molar mass) of a substance from its symbol and chemical formula, you need to determine the individual atomic masses of the elements present in the compound and then sum them up according to the stoichiometry of the formula.
Here's a step-by-step guide:
Identify the chemical formula of the substance. For example, let's consider water, which has the chemical formula H₂O.
Identify the elements present in the formula and their corresponding subscripts. In the case of water, we have hydrogen (H) and oxygen (O), with subscripts 2 and 1, respectively.
Find the atomic masses of each element. The atomic masses of elements can be found on the periodic table. For hydrogen (H), the atomic mass is approximately 1.01 atomic mass units (amu), and for oxygen (O), it is approximately 16.00 amu.
Multiply the atomic mass of each element by its subscript in the formula. In our example of water (H₂O), we have:
- Hydrogen (H): 1.01 amu × 2 = 2.02 amu
- Oxygen (O): 16.00 amu × 1 = 16.00 amu
Sum up the results from step 4 to calculate the atomic weight (molar mass) of the compound. For water (H₂O): Atomic weight (molar mass) = 2.02 amu + 16.00 amu = 18.02 amu
Therefore, the atomic weight of water (H₂O) is approximately 18.02 amu.
Following these steps, you can calculate the atomic weight of any substance using its symbol and chemical formula.