how to calculate the mass percentage of an element in a compound
Answers
Answer:
The mass percent composition of a molecule shows the amount that each element in the molecule contributes to the total molecular weight. The contribution of each element is expressed as a percentage of the whole. This step-by-step tutorial will show how to determine the mass percentage of a molecule's composition.
An example of potassium ferricyanide
Calculate the mass percent composition of each element in a molecule of potassium ferricyanide, K3Fe(CN)6.
Solution
Step 1: Find the atomic mass of each element present in the molecule
The first step in finding the mass percent is to find the atomic weight of each element in the molecule. K3Fe(CN)6 is composed of potassium (K), iron (Fe), carbon (C), and nitrogen (N). Using the periodic table:
Atomic mass of K: 39.10 g/mol
Atomic mass of Fe: 55.85 g/mol
Atomic weight C: 12.01 g/mo
Atomic mass of N: 14.01 g/mol
Step 2: Find the weight combination of each element.
The second step is to determine the total weight combination of each element. Each KFe(CN)6 molecule contains 3 K, 1 Fe, 6 C, and 6 N atoms. Multiply these numbers by the atomic mass to get the mass contribution of each element.
Mass contribution K = 3 x 39.10 = 117.30 g/mol
Mass contribution of Fe = 1 x 55.85 = 55.85 g/mol
Mass contribution C = 6 x 12.01 = 72.06 g/mol
Mass contribution N = 6 x 14.01 = 84.06 g/mol
Step 3: Find the total molecular weight of the molecule.
The molecular weight is the sum of the mass contributions of each element. Simply add up each bulk post to find the grand total.
Molecular weight of K3Fe(CN)6 = 117.30 g/mol + 55.85 g/mol + 72.06 g/mol + 84.06 g/mol
Molecular weight of K3Fe(CN)6 = 329.27 g/mol
Step 4: Find the mass percent composition of each element.
To find the mass percent composition of an element, divide the mass contribution of the element by the total molecular weight. This number must be multiplied by 100% to express it as a percentage.
Fork:
Mass percentage of composition K = mass contribution K/molecular weight K3Fe(CN)6 x 100%
Mass percentage of composition K = 117.30 g/mol/329.27 g/mol x 100%
Mass percentage of composition K = 0.3562 x 100%
Mass percentage of composition K = 35.62%
For Fe:
Mass percentage of Fe composition = mass contribution of Fe/molecular weight of K3Fe(CN)6 x 100%
Mass percentage of composition Fe = 55.85 g/mol/329.27 g/mol x 100%
Mass percentage of composition Fe = 0.1696 x 100%
Mass percentage of composition Fe = 16.96%
Why:
Mass percentage of composition C = mass contribution C/molecular weight K3Fe(CN)6 x 100%
Mass percent of composition C = 72.06 g/mol/329.27 g/mol x 100%
Mass percentage of composition C = 0.2188 x 100%
Mass percentage of composition C = 21.88%
For N:
Mass percentage of composition N = mass contribution N/molecular weight K3Fe(CN)6 x 100%
Mass percentage of composition N = 84.06 g/mol/329.27 g/mol x 100%
Mass percentage of composition N = 0.2553 x 100%
Mass percentage of composition N = 25.53%.
Answer
K3Fe(CN)6 is 35.62% potassium, 16.96% iron, 21.88% carbon, and 25.53% nitrogen.
It's always a good idea to check your work. If you add up all the ingredients in percentage by weight, you should get 100%. 35.62% + 16.96% + 21.88% + 25.53% = 99.99% Where is the other 0.01%? This example illustrates the effects of significant figures and rounding errors. This example used two valid numbers after the decimal point. This allows an error of the order of ±0.01. The answer to this example is within these tolerances.
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