To calculate the percent atomic by weight of an element in a compound, you need to know the atomic masses of the elements present in the compound. The percent atomic by weight represents the proportion of the total weight of the compound contributed by a particular element.
Here's the step-by-step process to calculate the percent atomic by weight:
Determine the atomic mass of each element in the compound. You can find the atomic mass of an element on the periodic table, usually listed below the element's symbol. The atomic mass is typically expressed in atomic mass units (amu).
Multiply the atomic mass of each element by the number of atoms of that element in one molecule of the compound. This can be obtained from the chemical formula of the compound.
Add up the atomic masses obtained in step 2 to find the molar mass of the compound. The molar mass is expressed in grams per mole (g/mol).
Calculate the percent atomic by weight for a specific element by dividing the product of step 2 (atomic mass of the element) by the molar mass of the compound obtained in step 3. Then multiply the result by 100 to get the percentage.
Here's the formula to calculate the percent atomic by weight (P) of an element:
P = (Atomic mass of the element × Number of atoms of the element) / Molar mass of the compound × 100
Let's illustrate this with an example:
Example: Calculate the percent atomic by weight of oxygen (O) in water (H2O).
The atomic mass of oxygen (O) is approximately 16.00 amu.
In one molecule of water (H2O), there is one atom of oxygen. Therefore, the atomic mass of oxygen in water is 16.00 amu.
The molar mass of water (H2O) is calculated as follows: Molar mass = (2 × atomic mass of hydrogen) + atomic mass of oxygen = (2 × 1.008 amu) + 16.00 amu = 2.016 amu + 16.00 amu = 18.016 amu
Now, calculate the percent atomic by weight of oxygen in water: P = (16.00 amu / 18.016 amu) × 100 = 88.81%
Therefore, oxygen represents approximately 88.81% of the total atomic weight of water.