Elemental Analysis is a laboratory technique used to determine the mass percentage of each element present in a sample. By comparing the experimental mass percent of an element to its theoretical mass percent in a pure compound, chemists can determine if a sample is pure or a mixture.
The Mass Percent of a component in a mixture is calculated by dividing the mass of that specific component by the total mass of the mixture and multiplying by 100. This value allows for the comparison of different samples regardless of their total size.
If a sample contains a lower mass percentage of a target element than the pure compound should, it indicates the presence of impurities that do not contain that element.
Purity is the measure of the extent to which a substance contains only the desired chemical compound. It is expressed as a percentage, where 100% represents a perfectly pure substance with no foreign matter.
The formula for calculating purity based on mass is:
| Feature | Pure Substance | Mixture |
|---|---|---|
| Composition | Fixed and definite ratio | Variable and non-fixed ratio |
| Separation | Requires chemical reactions | Achieved by physical methods |
| Properties | Constant and characteristic | Reflects properties of components |
| Formula | Single chemical formula | No single chemical formula |
Identify the Target: In purity problems, always distinguish between the 'desired compound' and the 'total sample mass'. Students often mistakenly use the mass of a single element as the numerator instead of the mass of the whole compound.
Conservation of Mass: Ensure that the sum of the masses of all components (desired compound + impurities) equals the total mass of the sample. This is a quick way to verify your data before starting calculations.
Theoretical vs. Experimental: When given elemental analysis data, first calculate the theoretical mass percent of the element in the pure compound using the periodic table. If the experimental value differs, the sample is a mixture.
Units Check: Always ensure masses are in the same units (e.g., all grams or all milligrams) before performing division to avoid decimal errors.
Alloys as Compounds: Many students believe that solid mixtures like brass or steel are compounds because they look uniform. In reality, they are homogeneous mixtures (solid solutions) because the ratio of metals can be varied.
Uniformity vs. Purity: A clear liquid is not necessarily a pure substance. A solution of salt in water is perfectly uniform (homogeneous) but is still a mixture of two different compounds.
Formula Misuse: Never try to write a single chemical formula for a mixture (e.g., do not write for salt water). Instead, represent them as separate entities: or .
