Percent Composition
Percent composition measures the relative quantities of each element in a compound. It represents the percentage, typically by mass, of each element compared to the total mass of the compound.
For example, in water (H2O), percent composition helps us understand how much of the total mass comes from hydrogen (H) and how much from oxygen (O). It is a useful concept in chemistry for identifying substances, comparing compounds, and analyzing their properties. [1-4]
Formula
To calculate the percent composition of an element in a compound, we use a simple formula: [1-4]
Percent composition = (Mass of element/ Mass of compound) x 100
Here,
– Mass of element: Combined mass of all atoms of a specific element in the compound.
– Mass of compound: Sum of the masses of all elements in the compound (or the molar mass of the compound).
Thus, percent composition expresses the fraction of a compound’s mass contributed by a specific element as a percentage.
Example: If the mass of an element in a compound is 10 grams and the total mass of the compound is 50 grams, then:
Percent Composition = (10 g/50 g) x 100 = 20%
It means that the element makes up 20% of the compound’s mass.
Applications [1-4]
- General: Used to check the purity of chemical substances, such as metals or pharmaceutical drugs
- Food Science: Used to calculate the nutrient content of food, such as the percentage of carbohydrates, fats, and proteins.
- Environmental Science: Helps analyze air pollutants or soil samples to monitor environmental health.
- Material Science: Used to study the composition of alloys, ceramics, and other materials for industrial use.
- Forensic Chemistry: Helps identify the composition of unknown substances in crime scene investigations.
- Medicine: Determines the composition of medical compounds to ensure proper dosages.
- Agriculture: Analyzes fertilizers to ensure they contain the right balance of nutrients.
Disadvantages [1-4]
- No Structural Information: Percent composition does not provide details about the arrangement of atoms or the structure of the compound.
- Limited for Complex Mixtures: It may not give meaningful insights for mixtures with numerous components or where the exact proportions vary.
- Accurate Data is Required: Accurately determining percent composition depends on precisely measuring the masses of each element, which may be challenging in some cases.
- Ambiguity in Molecular Formulas: Different compounds can have the same percent composition but different structures, which can lead to potential misinterpretation.
- Dependency on Measurement Conditions: Percent composition can vary due to experimental errors or differences in the conditions (e.g., temperature and humidity) under which the data were obtained.
Example Problems with Solutions
Problem 1: Determine the percent composition of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl). (Atomic masses of Na and Cl are 23 g and 35.5 g)
Solution
1. Calculate the masses of each element:
Sodium: 1 x 23 g = 23 g
Chlorine: 1 x 35.5 g = 35.5 g
Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol
2. Calculate the percent composition:
Sodium: (23/58.5) x 100 = 39.32%
Chlorine: (35.5/58.5) x 100 = 60.68%
Problem 2: Calculate the percent composition of hydrogen (H) and oxygen (O) in water (H2O). (Atomic masses of H and O are 1 g and 16 g)
Solution
1. Calculate the masses:
Hydrogen: 2 x 1 g = 2 g
Oxygen: 1 x 16 = 16 g
Molar mass of H2O = 2 + 16 = 18 g/mol
2. Calculate the percent composition:
Hydrogen: (2/18) x 100 = 11.11%
Oxygen: (16/18) x 100 = 88.89%
Problem 3: Find the percent composition of carbon (C) and oxygen (O) in CO2. (Atomic masses of C and O are 12 g and 16 g)
Solution
1. Calculate the masses:
Carbon (C): 1 x 12 g = 12 g
Oxygen (O): 2 x 16 g = 32 g
Molar mass of CO2 = 12 + 32 = 44 g/mol
2. Calculate the percent composition:
Carbon: (12/44) x 100 = 27.27%
Oxygen: (32/44) x 100 = 72.73%
Problem 4: Determine the percent composition of nitrogen (N) and hydrogen (H) in ammonia (NH3). (Atomic masses of N and H are 14 g and 1 g)
Solution
1. Calculate the masses:
Nitrogen: 1 x 14 g = 14 g
Hydrogen: 3 x 1 g = 3 g
Molar mass of NH3 = 14 + 3 = 17 g/mol
2. Calculate the percent composition:
Nitrogen: (14/17) x 100 = 82.35%
Hydrogen: (3/17) x 100 = 17.65%
Problem 5: Calculate the percent composition of carbon, hydrogen, and oxygen in glucose (C6H12O6). (Atomic masses of C, H, and O are 12 g, 1 g, and 16 g)
Solution
1. Calculate the masses:
Carbon: 6 x 12 g = 72 g
Hydrogen: 12 x 1 = 12 g
Oxygen: 6 x 16 = 96 g
Molar mass of C6H12O6 = 72 + 12 + 96 = 180 g/mol
2. Calculate the percent composition:
Carbon: (72/180) x 100 = 40%
Hydrogen: (12/180) x 100 = 6.67%
Oxygen: (96/180) x 100 = 53.33%