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Some Basic Concepts of Chemistry - Percentage Composition

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Percentage composition is defined as the percentage by mass of each element present in a compound.

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It is calculated by dividing the total mass of a specific element in one mole of the compound by the molar mass of the compound, then multiplying by 100100.

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The sum of the mass percentages of all constituent elements in a compound must equal 100%100\%.

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This concept is crucial for determining the empirical formula and molecular formula of a substance when the elemental analysis data is provided.

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Molar mass of a compound is the sum of the atomic masses of all the atoms present in a molecule, expressed in g/molg/mol or uu.

📐Formulae

\text{Mass % of an element} = \frac{\text{Mass of that element in the compound}}{\text{Molar mass of the compound}} \times 100

Molar Mass=∑i=1n(Number of atoms of element i×Atomic mass of element i)\text{Molar Mass} = \sum_{i=1}^{n} (\text{Number of atoms of element } i \times \text{Atomic mass of element } i)

💡Examples

Problem 1:

Calculate the mass percent of different elements in Ethanol (C2H5OHC_2H_5OH).

Solution:

Molar mass of C2H5OH=(2×12.01)+(6×1.008)+(16.00)=46.068 g/molC_2H_5OH = (2 \times 12.01) + (6 \times 1.008) + (16.00) = 46.068 \text{ g/mol}. \n Mass % of Carbon (CC) = 24.0246.068×100=52.14%\frac{24.02}{46.068} \times 100 = 52.14\%. \n Mass % of Hydrogen (HH) = 6.04846.068×100=13.13%\frac{6.048}{46.068} \times 100 = 13.13\%. \n Mass % of Oxygen (OO) = 16.0046.068×100=34.73%\frac{16.00}{46.068} \times 100 = 34.73\%.

Explanation:

We first find the molar mass of the molecule by summing the atomic masses of 22 Carbon, 66 Hydrogen, and 11 Oxygen atoms. Then, we apply the percentage composition formula for each element individually.

Problem 2:

Calculate the mass percentage of Water of Crystallization in Blue Vitriol (CuSO4⋅5H2OCuSO_4 \cdot 5H_2O). [Atomic masses: Cu=63.5,S=32,O=16,H=1Cu = 63.5, S = 32, O = 16, H = 1]

Solution:

Molar mass of CuSO4⋅5H2O=63.5+32+(4×16)+5×(2×1+16)=249.5 g/molCuSO_4 \cdot 5H_2O = 63.5 + 32 + (4 \times 16) + 5 \times (2 \times 1 + 16) = 249.5 \text{ g/mol}. \n Mass of 55 moles of H2O=5×18=90 gH_2O = 5 \times 18 = 90 \text{ g}. \n % of H2O=90249.5×100=36.07%H_2O = \frac{90}{249.5} \times 100 = 36.07\%.

Explanation:

To find the percentage of a specific group like water of crystallization, treat the group (H2OH_2O) as a single unit. Divide the total mass of the water molecules by the total molar mass of the hydrated salt.

Percentage Composition - Revision Notes & Key Formulas | CBSE Class 11 Chemistry