Percentage Composition

Percentage Composition

By Claudine Francisco

Quarter 2 Lesson 8

Percentage Composition

The mass of each element in a compound compared to the entire mass of the compound multiplied by 100 percent is called the percentage composition of the compound.

Quarter 2 Lesson 8

% Composition = mass of a substance

formula mass

​

For example, let us calculate the percentage composition of magnesium carbonate. MgCO_3.

Quarter 2 Lesson 8

Multiple Choice

First, you have to determine the mass of Magnesium, Carbon and Oxygen.

What is the mass of Magnesium?

• 24.31 g

• 25.31 g

Multiple Choice

What is the mass of Carbon?

• 12.01 g

• 10.12 g

Multiple Choice

What is the mass of Oxygen?

• 15. 00 g

• 16.00 g

Multiple Choice

Since there are 3 molecules of Oxygen in Magnesium Carbonate, you have to multiply the mass of Oxygen to 3. What is the answer?

• 45.00 g

• 48.00 g

Multiple Choice

You can now get the SUM of the mass of the three elements...

Mg + C + O₃ = 24.31 + 12.01 + 48 = ?

• 84.32 g

• 89.15 g

Multiple Choice

DIVIDE the mass of the element to the total mass of the compound then multiply to 100 to get the percent composition.

$$Mg\ =\ \left(\frac{24.31}{84.32}\right)\times100\ =\ ?$$  

• 14.24 %

• 28.83 %

Multiple Choice

DIVIDE the mass of the element to the total mass of the compound then multiply to 100 to get the percent composition.

$$C\ =\ \left(\frac{12.01}{84.32}\right)\times100\ =\ ?$$  

• 14.24 %

• 56.93 %

Multiple Choice

DIVIDE the mass of the element to the total mass of the compound then multiply to 100 to get the percent composition.

$$O\ =\ \left(\frac{48}{84.32}\right)\times100\ =\ ?$$  

• 28.83 %

• 56.93 %

The Percentage Composition of MgCO₃:

Mg = 28.83%

C = 14.24%

O = 56.93%

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If you will add them altogether, the result is 100%. ​

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Let's Try!​

Multiple Choice

What is the Percentage Composition of CaO?

Ca = ?

O = ?

Remember that their sum should be equal to 100%.

• Ca = 71.47%

  O = 18.53%

• Ca = 71.47%

  O = 28.53%

• Ca = 50.47%

  O = 28.53%

Multiple Choice

What is the Percentage Composition of H₂O₂?

H₂ = ?

O₂ = ?

Remember that their sum should be equal to 100%.

• H₂ = 6.88%

  O₂ = 93.12%

• H₂ = 5.88%

  O₂ = 94.12%

• H₂ = 4.88%

  O₂ = 95.12%

Multiple Choice

What is the Percentage Composition of Fe₂O₃?

Fe₂ = ?

O₂ = ?

Remember that their sum should be equal to 100%.

• Fe₂ = 69.94%

  O₃ =30.06%

• Fe₂ = 69.84%

  O₃ =30.16%

• Fe₂ = 30.06%

  O₃ = 69.94%

There are two types of formulas:

Actual whole number ratio of the elements in a compound.

Molecular Formula

Lowest whole number ratio of the elements in a compound

Empirical Formula

For us to understand this, let us have an example.

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	​EMPIRICAL FOMULA
​H	​	​	​	​	​
O	​	​	​	​	​

What is the mass of Hydrogen and Oxygen?

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	​EMPIRICAL FOMULA
H	5.88	​	​	​	​
O	94.12	​	​	​	​

​The mass is in the given.

Next, determine the atomic mass of Hydrogen and Oxygen.

Check your periodic table.

Multiple Choice

What is the atomic mass of Hydrogen and Oxygen?

• H = 1

  O = 16

• H = 16

  O = 1

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	​EMPIRICAL FOMULA
H	5.88	1	​	​	​
O	94.12	1​6	​	​	​

Now, divide the mass to the atomic mass of each element.

Multiple Choice

$$H\ =\ \frac{5.88}{1}=?$$  

• 5.88

• 15.88

Multiple Choice

$$O\ =\ \frac{94.12}{16}=?$$  

• 5.88

• 15.88

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	​EMPIRICAL FOMULA
H	5.88	1	5.88	​	​
O	94.12	16	5.88	​	​

Divide the number of moles of each element by the one with the SMALLEST VALUE. Since both elements have the same number of moles, we will divide them to each other.

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	​EMPIRICAL FOMULA
​H	5.88	1	5.88	1​	​
O	94.12	1​6	5.88	1​	​

The mole ratio of both elements is 1.

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	​EMPIRICAL FOMULA
H	5.88	1	5.88	​1	​
​O	94.12	1​6	5.88	1	​

The mole ratio indicates the number of moles in the compound. Meaning, H and O has 1. Together, they can be written as: HO

Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​ELEMENT	MASS (g)	ATOMIC MASS (g/mol)	MOLE​	MOLE RATIO	EMPIRICAL FOMULA
​H	5.88	1	5.88	​1	​
​O	94.12	16	5.88	​2	​

HO

What is the mass of this empirical formula?

Multiple Choice

Compute the TOTAL MASS of HO.

• HO = 15g

• HO = 17g

• HO = 19g

H = 1g

O = 16g

​

1+16 = 17

​

HO = 17​g

​

This is the Empirical Formula mass.​

Now divide this to the molecular mass. Let us recall the problem.

​

​Calculate the empirical and molecular formula of a compound consisting of 5.88%H and 94.12% O. Its molecular mass is found to be 34. Identify the compound.

​

What is the molecular mass?​

H = 1g

O = 16g

​

1+16 = 17

​

HO = 17​g

​

17g is the Empirical Formula mass.​