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Elemental Composition of Pure Substances

Imagine you were trying to describe types of doughnuts to your friend. You could choose to describe the doughnuts based on their physical state: donut holes, glazed donuts, and cream-filled donuts. You could also describe them according to what they are made of yeast donuts, cake donuts, or sourdough donuts. This is a loose, fun example of how we can…

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# Elemental Composition of Pure Substances

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Imagine you were trying to describe types of doughnuts to your friend. You could choose to describe the doughnuts based on their physical state: donut holes, glazed donuts, and cream-filled donuts. You could also describe them according to what they are made of yeast donuts, cake donuts, or sourdough donuts. This is a loose, fun example of how we can classify matter! We can classify matter according to the physical state (solid, liquid, gas), or we can classify it according to its composition as a pure substance or mixture!

• We are going to do a quick overview of the classification of matter according to the composition of pure substances and mixtures
• Then, we will zoom in on the definition of pure substances
• After, we will look at some pure substance examples
• We will also explore the percentage composition and empirical formulas of pure substances
• Finally, we will briefly investigate the composition of mixtures

## Classification of Matter According To Composition

When you think about classifying matter, you likely think of the three states of matter: solid, liquid, and gas. But, thinking back to the donut metaphor, we can also classify matter according to what it is made of (aka its chemical composition)! So, what makes up matter? Well, on a molecular level, all matter is made up of atoms that can form elements, compounds, and molecules! Even with all these different components, matter can be split into two broad categories based on its composition: pure substances and mixtures. We will spend most of our time on pure substances, but briefly cover mixtures! So, let’s dive into pure substances!

We will go into more detail on all the components of matter, but for a more thorough review, check out Atomic Structure!

## Definition of Pure Substances

A simple way to think about pure substances is as something that is made up of only one type of matter that always has the same properties, such as melting point, boiling point, density, etc.

Pure substances are matter that has a fixed chemical composition and properties that do not change.

Can you guess what are pure substances in your day-to-day life? I bet you may have salt (NaCl) and tin cans (Sn) in your pantry! These are perfect examples of pure substances because salt is made up of purely NaCl molecules, and tin is made up of only tin atoms.

But wait, you may have noticed a difference between salt and tin and asked how exactly salt is only made up of one type of matter when it’s two different atoms bonded together?

Pure substances can be further divided into two classes: elements and compounds.

## Pure Substance Examples

We are going to spend some time looking at elements and compounds separately and some common examples of each!

### Elements

If you think elements sound familiar, you are correctly thinking of the 118 organized in the periodic table. Tin is an example of an element!

Elements are made up of only one type of atom and cannot be broken down into any other substance.

Some common examples of elements are:

• Hydrogen (H), Oxygen (O), and Nitrogen (N) - usually occur as diatomic molecules
• Gold (Ag), Iron (Fe), and Copper (Cu) - are metals that usually exist by themselves
• Sodium (Na), Potassium (K), and Lithium (Li) - naturally occur only in compounds

Fig.1-Elements contain one type of atom

Some elements tend to exist in the form of diatomic molecules, which means two atoms of the same element are bonded together, such as hydrogen gas (H2), nitrogen gas (N2), oxygen gas (O2), fluorine gas (F2), chlorine gas (Cl2), bromine (Br2), and iodine (I2). These are still elements because the atoms are the same! Check out the Periodic table for more information on these molecules.

### Compounds

The second class of pure substances are compounds!

Compounds are substances containing identical molecules that are made up of two or more atoms of different elements bonded together in a fixed structure. They can be chemically broken down into individual elements or simpler compounds.

Thinking back to our salt and tin example, table salt is a compound. Every sample of table salt will always be composed of two bonded atoms (sodium and chlorine), but through a chemical change, it can be broken down into sodium and chlorine atoms!

Compounds are always made up of molecules of two or more atoms bonded together. But, as we saw with diatomic molecules, not all molecules are compounds!

Can you think of any common examples of compounds?

Here are some:

• Water -H2O
• Carbon dioxide -CO2
• Baking soda -NaHCO3
• Alcohol - C2H6O

Fig.2-H2O contains two different atoms from two different elements

To check your understanding of the difference between elements and compounds, identify if the following are elements or compounds: NaF, Re, I2 NH3, Si

This should have been relatively easy! If not, review the above material and try again! The correct answers are Compound, Element, Element, Compound, Element!

Before we define the other category of matter known as mixtures, it is essential to understand how to find out the exact percentage of elements that make up a compound!

## Percentage Composition and Empirical Formula

Often, if we analyze a compound, we will want to determine the ratio of all the different elements present in the compound relative to the compounds as a whole. You may think, doesn't the ratio change depending on how much of the compound we have? Well, because of a concept known as the law of definite proportions, compounds will always be composed of a fixed ratio of elements by mass regardless of the size of the sample! We can determine this mass ratio by calculating the percentage composition of the compound.

Check out Law of Definite Proportions for a deep dive into this concept!

Percentage composition tells us by mass the percentage of an individual element present in the compound.

Fig.3-Conceptualization of percentage composition of an element in a compound.

The formula for percent composition is:

$$\%\,Composition=\frac{mass\,of\,element\,present\,in\,compound}{total\,molecular\,mass\,of\,compound}*100\%$$

Let’s play this out in a quick and simple example.

Find the percentage composition of sodium and chlorine in NaCl?

1) The first step is to determine the molecular mass of NaCl by using the periodic table to find the sum of Na and Cl atomic mass. Note, if there was more than one Na or Cl atom you would multiply the mass by that number.

The mass of Na = $$22.99\frac{g}{mol}$$

The mass of Cl = $$35.45\frac{g}{mol}$$

So the sum: $$22.99\frac{g}{mol}+ 35.45\frac{g}{mol}= 58.44 \frac{g}{mol}$$

2) The second step is to divide the individual atomic masses by the sum total molecular mass of the compound.

Na: $$(\frac{22.99\frac{g}{mol}}{58.44\frac{g}{mol}})*100\% = 39.3\%$$

Cl: $$(\frac{35.45\frac{g}{mol}}{58.44\frac{g}{mol}}) *100\%= 60.7\%$$

So, NaCl is made up of 39.3% Na and 60.7% Cl.

This is a very simple demonstration of how to find percent composition. Please check out Molecular and Empirical Formulas, where we will cover more advanced examples!

### Empirical Formula

The simplest whole-number ratio of the elements in a compound is referred to as the empirical formula.

So, let’s say a compound had a molecular formula of C6H10O4. The empirical formula would be C3H5O2. It’s similar to reducing fractions down to their simplest form!

We can often use percentage compositions to determine the empirical formula of a compound.

## Composition of Mixtures

As we come to an end, we are going to look at the second type of substance: mixtures!

To understand mixtures, let’s say we took NaCl and dissolved it into water. What does the combination of these two pure substances create? The mixture of saltwater.

Mixtures are comprised of two or more substances physically mixed together that don’t lose their individual chemical properties and can be separated back into individual substances through physical change.

Fig.4-Gold and Iron are elements that when mixed together become a mixture and yet maintain their individual chemical properties.

The main differences between compounds and mixtures are:

1. In mixtures, pure substances are physically “mixed” together, whereas, in compounds, atomic elements are chemically bonded together.
2. This means that salt and water have the same chemical properties before and after creating saltwater. However, when the compound NaCl, is separated into its atomic components, pure sodium becomes a shiny metallic solid, and chlorine is a greenish gas that, when bonded together, forms a new compound of white crystals of salt with new properties.

Some examples of mixtures include things like soil, tea/juices, gasoline, and gold mixed with iron!

This is just a brief overview of the composition of mixtures. Check out the composition of mixtures to find out about the two types of mixtures and how to identify them!

## Elemental Composition of Pure Substances - Key takeaways

• Matter can be classified according to its composition into two categories: pure substances and mixtures
• Pure substance refers to any matter with fixed chemical composition and its properties always remain constant.
• It can be divided into elements and compounds
• Elements are composed of one type of atom (think the periodic table of elements)
• Compounds are composed of two or more atoms of different elements bonded together (H2O, CO2)
• We can use percent composition and empirical formulas to figure out the ratio of elements in the compound
• Mixtures are made up of two or more pure substances physically mixed together.

## References

1. Brown, Theodore L, H E. LeMay, Bruce E. Bursten, Catherine J. Murphy, Patrick M. Woodward, and Matthew Stoltzfus. Chemistry: The Central Science. , 2015. Print.
2. Fraser, Simon. Classifying Matter According to Its Composition, LibreTexts: Chemistry, 2020.

To calculate the mass of a pure substance, you can look up the atomic mass of the element(s) on the periodic table, multiply the mass by 1 g/mol, and then add them up if it's a compound.

The two types of pure substances are elements and compounds. Elements contain one type of atom while compounds are made up of two or more atoms of different elements. They both have a fixed chemical composition and unchanging properties.

Here are ten examples of pure substances. Five of the examples will be elements and five will be compounds.  Gold, Iron, Tin, Hydrogen, and Carbon are all elements. Water, Alcohol, Ammonia, Baking Soda, and Carbon Dioxide are all compounds.

The amount of elements a pure substance contains depends on the type of pure substance. An element only contains one type of element. A compound can contain two or more elements.

## Elemental Composition of Pure Substances Quiz - Teste dein Wissen

Question

What are the two ways we can classify matter?

it's physical state and chemical composition

Show question

Question

What are the two broad categories for the composition of matter?

pure substances and elements

Show question

Question

True or False. Pure substances are made up of one type of atom or compound and have a fixed chemical composition with properties that do not change.

True

Show question

Question

Element and Compounds are ____, while anything made up of two or more ______ are ______

mixtures, compounds, mixtures

Show question

Question

Identify the composition of the following. Decide if it is a pure substance or a mixture If it is a pure substance, specify what kind.

Methane (NH3) = pure substance compound

Iodine (I2) = pure substance, element

Show question

Question

True or False. All of the gold atoms found in a gold bar would be an example of a compound.

True

Show question

Question

Which of the following are elements? Select all that apply and explain your reasoning.

H2O

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Question

Which of the following are true of compounds?

They are a type of mixture

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Question

If we wanted to determine, how much oxygen is in a molecule of water, what do we need to find?

the percentage composition of Oxygen

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Question

Which of the following is not a mixture?

ammonium oxide

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Question

What is a mixture?

matter that is comprised of two or more substances mixed together

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Question

Calculate the percent composition of hydrogen in water?

11.19%

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Question

What is an empirical formula?

the simplest whole-number ratio of elements in the compound

Show question

Question

If the molecular formula of a compound was C2H16O4, what is the correct empirical formula?

C2H16O4

Show question

Question

What is true of the law of definite proportions?

it explains the composition of matter into pure substances and mixtures

Show question

Question

How is a compound classified according to its composition matter? what does that mean?

compounds are pure substances, so the elemental composition of compounds is bonded atoms from two or more elements that have the same properties.

Show question

Question

What is percent composition?

the number of atoms in each compound

Show question

Question

Two unknown compounds are being tested. Compound A contains 20 g of hydrogen and 120 g of oxygen. Compound B contains 4 g of hydrogen and 32 g of oxygen. According to the law of definite proportions are they the same?

They are different compounds because the ratio of the elements (oxygen and hydrogen) is different between the compounds.

Show question

Question

True or False. The law of definite proportions implies that the source of the compound does not affect the fixed mass ratio of elements but the amount of the compound does.

true

Show question

Question

Select the statements below where the law of definite proportions is correctly applied.

When pure H2O is taken from a huge waterfall, it has a bigger ratio of oxygen to hydrogen elements than from a faucet.

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Question

What does the law of multiple proportions state?

when two elements can combine to form multiple different compounds, one element with a fixed mass combines with different masses of the second element, whose mass ratio can be expressed as small, whole numbers.

Show question

Question

Which of the following compound pairs could be explained by the law of multiple proportions?

CO, H2O

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Question

What percent of CO2 is carbon?

27.27%

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Question

What is the mass ratio between the hydrogen of CH2 and CH4?

1:2

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Question

How did Joseph Proust discover the law of definite proportions?

He made artificial copper carbonate in a lab and compared it to naturally occurring Copper carbonate and found that each sample of Copper carbonate contained the same weight proportions of the three elements involved: copper, carbon, and oxygen.

Show question

Question

If the mass of compound AB is 152 g, and element A has a 35% composition, how many grams is element B?

98.8 grams of B

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Question

Identify which law of proportions explains the following statement:

Two samples of table salt have the same ratio of Na to Cl

law of definite proportions

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Question

What are the exceptions to the law of definte proportions?

Isotopes and elements can produce different compounds

Show question

Question

Which of the following could be the empirical formulas of compounds?

C2H10O4

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Question

Fill in the blank.

________ formulas express the ______ of elements in the compound. While _______ formulas express the _____ of  each element in the compound.

Empirical formulas, number; Molecular formulas, ratio

Show question

Question

What is the empirical formula of the following compounds:

• C10H14O2
• H2O
• S2Cl2
• C12H22O11

• C5H7O
• H2O
• SCl
• C12H22O11

Show question

Question

What are the two types of data we can use to calculate empirical formulas?

percent mass composition or combustion analysis data

Show question

Question

Which of the following is the correct order of steps to determine the empirical formula from mass percentage composition?

1. Convert the percent to moles

2. Convert the moles to grams

3. Divide by the smallest gram value

4. Multiply by a common factor to get the whole number

Show question

Question

A compound contains 49.94% sulfur and 50.06% oxygen. What is the empirical formula?

SO2

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Question

Determine the empirical formula of a compound that is 38.76g Cl and 61.24 g O.

Cl2O7

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Question

14.325 g of a  compound containing only carbon, hydrogen, and oxygen was combusted with excess oxygen. The combustion process resulted in 13.974 g of carbon dioxide and 8.326 g of water. Based on this data, determine the empirical formula.

CH3O2

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Question

A hydrocarbon was reacted with oxygen in a combustion reaction. 49.205 g of carbon dioxide and 14. 391 g of water were formed. What is the empirical formula of this compound?

C7H10

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Question

If the empirical formula is known, what other information is necessary to determine the molecular formula of the compound?

the molecular mass of the compound

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Question

The empirical formula of a compound is NH2 and its molecular mass is 96.18 g/mol.  What is the molecular formula of this compound?

N6H12

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Question

If a compound has an empirical formula of S3O5, what are possible molecular formulas?

S6O10

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Question

True or False. Molecular and Empirical formulas are always different.

True

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Question

A compound is composed of 40.0 % carbon,  53.3 %  oxygen,  and 6.7 %  hydrogen by mass. The atomic mass is 120.104 g/mol. Determine the molecular formula of the compound?

C4H8O4

Show question

Question

True or False. Broadly, Molecular and Empirical Formulae are used to determine the composition of pure compounds.

True

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