Final answer:
To calculate the percent by mass, divide the mass of the element in 1 mole of the compound by the molar mass of the compound and multiply by 100%. This percentage reflects the element's contribution to the compound's overall mass, and for a pure compound, it will always be the same.
Step-by-step explanation:
To calculate the percent by mass of each element in a compound, you would use the mass of the element in 1 mole of the compound and the molar mass of the compound itself. You simply divide the mass of the element by the molar mass of the compound and then multiply by 100%. This will give you the percent by mass of that element in the compound. For example:
% by mass = (mass of element in 1 mol × 100%) / molar mass of compound
The percent composition of a compound will always add up to 100%. This indicates the relative mass contribution of each element to the compound's total mass. If a compound is pure, its percent composition will be constant. An example to illustrate this is as follows:
- For water (H2O), with molar mass approximated to 18 g/mol:
- The mass of hydrogen in 1 mol of water is about 2 g (since there are two moles of hydrogen atoms).
- The mass of oxygen in 1 mol of water is about 16 g.
- To find the percent by mass of hydrogen: (2 g × 100%) / 18 g = 11.11%
- To find the percent by mass of oxygen: (16 g × 100%) / 18 g = 88.89%
Note that when you add these percentages together, they equal 100%.