Percentage composition chemistry class 11: formula, definition, examples, NCERT

So far we have dealt with multiple entities present in a given sample. But often there is information about the percentage of a particular element in a compound necessary. Suppose you are given an unknown or new compound, the first question you would ask is what is its formula or what are its constituent parts, and in what ratio are they present in the given compound?

Even for known compounds, such information provides a check that the given sample contains the same percentage of elements that are present in a pure sample. In other words, the purity of a given sample can be verified by analyzing this data. In this article, we will discuss the percentage composition chemistry class 11, so let’s get started…

Nutritional composition in the jar

The packaged foods you eat usually have nutritional information on the label. The label on a jar of peanut butter (above) shows that the serving size is assumed to be 32g. The label also indicates the masses of different types of compounds present in each serving. One serving contains 7g protein, 15g fat, and 3g sugar. By calculating the amount of protein, fat, or sugar in a serving of peanut butter and converting it into percentages, we can determine the peanut butter’s composition in percent by mass.

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What is percentage composition?

Percentage composition definition: The percentage composition of a compound is a ratio of the amount of each element to the total amount of individual elements in a compound, which is then multiplied by 100. 

The composition percentage of a given compound is nothing but the ratio of the amount of each element present in the compound to the total amount of each element present in the compound multiplied by 100. Here we measure the amount in grams of the element, present in the solution. The percentage composition of a compound is an expression of its composition with respect to all elements present. The importance of this calculation of composition is found in chemical analysis.

Percentage composition formula

The percentage composition of a given element can be expressed by the following formula:

$$\% C_E = \frac{g^E}{g^T} \times 100$$

Here $\% C_E$ is the percentage composition of element E. This is the value that we want to calculate. The numerator on the right-hand side of the formula gives the total amount of element E present in the compound. On the other hand, the denominator is an expression for the total of all the elements present in the compound. We multiply this ratio by 100 to get the percentage composition of the compound.

Mass Percentage Composition

Mass percentage composition is used to denote the concentration of an element in a compound or a component in a mixture. We use this term to indicate the total mass percentage of each element present in a compound. It’s important to note that we can calculate mass percent composition by dividing the mass of one component by the total mass of the mixture. Then multiply this ratio by 100. We also call it mass percent (w/w) %.

Mass Percentage of an element formula

The mass percent of an element is given as follows:

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

Determining the Mass Percent from a Chemical Formula: solved examples

Let’s understand this section by taking an example of water $(H_2O)$. Since water contains hydrogen and oxygen, the percentage composition of both these elements can be calculated as follows:

\begin{aligned} &\text { Mass } \% \text { of an element = }\\ &\frac{\text { mass of that element in the compound } \times 100}{\text { molar mass of the compound }}\\ &\text { Molar mass of water }=18.02 \mathrm{~g}\\ &\text { Mass } \% \text { of hydrogen }=\frac{2 \times 1.008}{18.02} \times 100=11.18\\ &\text { Mass } \% \text { of oxygen }=\frac{16.00}{18.02} \times 100=88.79 \end{aligned}

Let us take one more example. What is the percentage of carbon, hydrogen, and oxygen
in ethanol.

What is the percentage of carbon, hydrogen, and oxygen in ethanol? Solution: Molecular formula of ethanol is: $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$ Molar mass of ethanol is: $(2 \times 12.01+6 \times 1.008+16.00) g=46.068 g$ Mass percent of carbon $$ =\frac{24.02 g}{46.068 g} \times 100=52.14 \% $$ Mass percent of hydrogen $$ =\frac{6.048 g}{46.068 \mathrm{~g}} \times 100=13.13 \% $$ Mass percent of oxygen $$ =\frac{16.00 g}{46.068 g} \times 100=34.73 \% $$

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Frequently Asked Questions – FAQs

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