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By the end of this section, you will be able to:
  • Compute the percent composition of a compound
  • Determine the empirical formula of a compound
  • Determine the molecular formula of a compound

In the previous section, we discussed the relationship between the bulk mass of a substance and the number of atoms or molecules it contains (moles). Given the chemical formula of the substance, we were able to determine the amount of the substance (moles) from its mass, and vice versa. But what if the chemical formula of a substance is unknown? In this section, we will explore how to apply these very same principles in order to derive the chemical formulas of unknown substances from experimental mass measurements.

Percent composition

The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most succinct way of representing this elemental makeup. When a compound’s formula is unknown, measuring the mass of each of its constituent elements is often the first step in the process of determining the formula experimentally. The results of these measurements permit the calculation of the compound’s percent composition    , defined as the percentage by mass of each element in the compound. For example, consider a gaseous compound composed solely of carbon and hydrogen. The percent composition of this compound could be represented as follows:

% H = mass H mass compound × 100 %
% C = mass C mass compound × 100 %

If analysis of a 10.0-g sample of this gas showed it to contain 2.5 g H and 7.5 g C, the percent composition would be calculated to be 25% H and 75% C:

% H = 2.5 g H 10.0 g compound × 100 % = 25 %
% C = 7.5 g C 10.0 g compound × 100 % = 75 %

Calculation of percent composition

Analysis of a 12.04-g sample of a liquid compound composed of carbon, hydrogen, and nitrogen showed it to contain 7.34 g C, 1.85 g H, and 2.85 g N. What is the percent composition of this compound?

Solution

To calculate percent composition, we divide the experimentally derived mass of each element by the overall mass of the compound, and then convert to a percentage:

% C = 7.34 g C 12.04 g compound × 100 % = 61.0 % % H = 1.85 g H 12.04 g compound × 100 % = 15.4 % % N = 2.85 g N 12.04 g compound × 100 % = 23.7 %

The analysis results indicate that the compound is 61.0% C, 15.4% H, and 23.7% N by mass.

Check your learning

A 24.81-g sample of a gaseous compound containing only carbon, oxygen, and chlorine is determined to contain 3.01 g C, 4.00 g O, and 17.81 g Cl. What is this compound’s percent composition?

Answer:

12.1% C, 16.1% O, 71.8% Cl

Got questions? Get instant answers now!

Determining percent composition from formula mass

Percent composition is also useful for evaluating the relative abundance of a given element in different compounds of known formulas. As one example, consider the common nitrogen-containing fertilizers ammonia (NH 3 ), ammonium nitrate (NH 4 NO 3 ), and urea (CH 4 N 2 O). The element nitrogen is the active ingredient for agricultural purposes, so the mass percentage of nitrogen in the compound is a practical and economic concern for consumers choosing among these fertilizers. For these sorts of applications, the percent composition of a compound is easily derived from its formula mass and the atomic masses of its constituent elements. A molecule of NH 3 contains one N atom weighing 14.01 amu and three H atoms weighing a total of (3 × 1.008 amu) = 3.024 amu. The formula mass of ammonia is therefore (14.01 amu + 3.024 amu) = 17.03 amu, and its percent composition is:

Questions & Answers

Question 91 in chapter 4 (finding the molarity of Calcium hydroxide), the answer given is 1.22 M and I have repeated the calculations numerous times and I keep getting 0.0122 M. Which is correct?
Hope Reply
what are the types of heat changes in chemical reaction
Chancha Reply
potassium is element number what, what is it symbol, what is it atomic weight, what it boiling point?
Vincent Reply
It is element number 19,symbol is K
Amha
atomic weight 39.0983g/mol boiling point 1047.0K
Johnson
I need 5 different and 5 similarities between compound and mixture
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Please send me answers
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Mahmood
what is matter
Arinze Reply
Matter is anything that occupied space
Rasidi
matter is anything that has mass and occupies space
Johnson
matter is anything that has mass and volume.
rafaela
matter is any thing that can that has mass
samuel
a plot of the atomic radius against atomic number shows a periodic variation with group 1elements occupying the peaks and group 4 the troughs.plot the graph
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what is the periodic table all about
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samuel
it's about predicting the different types of matter around us and organizing them in an elegant way
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what is boyle law
Daniel Reply
the law states that at a constant absolute temperature the volume of a fixed of a gas is inversely proportional to the pressure of the gas...
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Ujunwa Reply
I am sorry, I don't understand the question. Are you referring to photospectometry, a machine that uses lightwaves to determine the chemical composition of a substance? Instead of the "coffee filter and solvent" method which is more crude, but simple enough to understand basic concepts.
Melissa
what is esterification?
Darey Reply
Esterification is the general name for a chemical reaction in which two reactants (typically an alcohol and an acid) form an ester(-COOR) as the reaction product....
Johnson
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Chancha
Please...what do you mean by you don't know chemistry..can you explain further
Johnson
I am afraid you may end up injuring yourself working with these chemical processes with limited lab experience. Please be extremely careful. Research chem analysis, precedure and equipment.
Melissa
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samuel
how we Learn full periodic table
muhammad Reply
how
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Michael
explain the atomic orbital
Masti Reply
2nacl➡️2na+cl2 2.58.5g 22.4l
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nuclear energy is the energy released during nuclear fission or fusion.especially when used to generate electricity
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Practice Key Terms 2

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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