Percent Composition Study Guide


We all know the importance of drinking water in our day-to-day lives. Remaining hydrated 💧 throughout the day is extremely important for the normal functioning of all the body organs and systems. It is because water constitutes approximately 60% of our bodies! This water content can also be stated as: the percentage composition of water in the human body is 60%. You might be wondering what a percentage composition is. Let’s find out.

Percent Composition


The ratio of every element’s amount to the overall amount of available components contained in the compound multiplied by 100 is the percent makeup. The amount is expressed in the grams of the various components present.

Any compound’s percentage composition reflects its makeup in the perspective of all of the components present. As a result, it aids in the chemical examination of a particular substance.

% mass = (mass of element in 1 mole of the compound) / (molar mass of the compound) x 100%

Percentage Composition Formula

%CE = gE / gT × 100

The percentage composition of the element E is shown here as %CE. This is the amount that we will compute. The entire amount of element E in the compound is shown in the numerator on the right side. The denominator, on the other hand, is an expression representing the sum of the amounts of all the constituent elements in the compound. To obtain the composition’s percentage form, we multiply this ratio by 100.


Many commercialized products contain the bicarbonate of soda (sodium hydrogen carbonate), and NaHCO3 is the formulation. Let’s now calculate the mass percent of Na, H, C, and O in sodium hydrogen carbonate. To begin, use the Periodic Table to seek up the atomic weight of the elements. The masses of the atoms are discovered to be:

  • Na – 23
  • H – 1
  • C – 12
  • O – 16

Next, calculate how many grams of every element seem to be in a mole of NaHCO₃:

  • 23 g (1 mol) of Na
  • 1 g (1 mol) of H
  • 12 g (1 mol) of C

48.00 g (3 mole x 16 gram per mole) of O 1 mole of NaHCO₃ has the following mass: 23g + 1g +12g + 48 g = 84 g

And the elemental mass percentages become

  • % Mass of Na = 23 g / 84 g x 100 = 27.36 %
  • % Mass of H = 1 g / 84 g x 100 = 1.20 %
  • % Mass of C = 12 g / 84 g x 100 = 14.30 %
  • % Mass of O = 48 g / 84 g x 100 = 57.14 %

Mass Percentage Composition

% mass = (mass of element in 1 mole of the compound) / (molar mass of the compound) x 100%

Calculating mass percent composition involves comparing an element’s molar mass to the compound’s overall molar mass. In the end, a percentage is obtained, which is helpful when contrasting different compounds. You can also write it like this:

(Mass of Solute / Mass of Solution) x 100% = mass percent

Typically, the gram is used as the unit of mass. The term mass percent is sometimes referred to as weight percent or w/w%. The total mass of all the atoms in a mole of the compound is the molar mass. The total of all the mass percentages ought to equal 100 percent.

Calculating the Mass Percentage

Mass % of element = mass of element / mass compound × 100%

Question 1. An unknown sample weighing 8.76 g is discovered to contain 4.5 g of O. What is the mass percentage of oxygen?

Answer. Substitute the given units in the formula.

Mass % of element = mass of element / mass compound × 100%%O = 4.5g / 8.76g × 100% = 51%

Question 2. What is the mass percentage of each element in NH4NO3?

Answer. Firstly, let’s figure out how many elements we have.2N = 2 × 14.007 = 28.0144H = 4 × 1.01 = 4.043O = 3 × 16 = 48

After writing the elements, we have to find and write the mass of each element by taking the number of the element and multiplying it by the molar mass of the element. Add all the masses of the elements to get the total mass of the compound, which is 80.054 g.

Total mass = 80.054 g

Now we have to find out the mass percentage of each element.

Just substitute the units in the formula.Mass % of element = mass of element / mass compound × 100%%N = 28.014 / 80.054 × 100% = 35%%H = 4.04 / 80.054 × 100% = 5.0%%O = 48 / 80.054 × 100% = 60%

The total mass percentage of all the elements should sum up to 100%.


You may determine the percentage composition of a compound experimentally, and you could use all these data to calculate a compound’s empirical formula. Any compound’s percentage composition reflects its makeup in the perspective of all of the components present.


1. What does percentage composition depend on?

By dividing the mass of every element by the compound’s entire mass, the compound’s percent composition (by weight) may be derived.

2. What is percent composition? Why is it important in chemistry?

A compound’s percentage composition is essential since it helps us figure out what percentage of every element constitutes that complex. In nature, everything is chemically or physically bonded with other elements. To figure out how much of a substance is in a sample, you must first figure out the proportion of the specimen.

3. Does percent composition change in a compound?

Considering that a compound is pure, the percentage composition of that compound will always be the same.

4. In what other ways can you apply percentage composition?

Comparing the percentage composition achieved practically to the theoretical value estimated from the compound’s formulation can determine the purity of a chemical. Air quality and fuel combustion can all be constituted from the percentage makeup.

We hope you enjoyed studying this lesson and learned something cool about Percent Composition! Join our Discord community to get any questions you may have answered and to engage with other students just like you! Don’t forget to download our App to experience our fun VR classrooms – we promise, it makes studying much more fun! 😎


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