Calculating the Mass Percentages of the Elements

Oct 13, 2025 by magicalceylontours.com 107 views

Introduction

In chemistry, determining the molecular formula of a compound is a crucial step in understanding its structure and properties. The molecular formula is a representation of the number and types of atoms present in a molecule of the compound. In this article, we will discuss how to determine the molecular formula of a compound using the given mass percentages of its constituent elements.

Given Data

A 432.50 g sample of a compound contains the following masses of its constituent elements:

Carbon (C): 240.17 g
Hydrogen (H): 40.31 g
Oxygen (O): 95.99 g
Nitrogen (N): 56.03 g

The molar mass of the compound is given as 216.28 g/mol.

Calculating the Mass Percentages of the Elements

To determine the molecular formula of the compound, we first need to calculate the mass percentages of its constituent elements. The mass percentage of an element is calculated by dividing the mass of the element by the total mass of the sample and multiplying by 100.

Mass percentage of C = (240.17 g / 432.50 g) × 100 = 55.55%
Mass percentage of H = (40.31 g / 432.50 g) × 100 = 9.26%
Mass percentage of O = (95.99 g / 432.50 g) × 100 = 22.15%
Mass percentage of N = (56.03 g / 432.50 g) × 100 = 12.94%

Converting Mass Percentages to Grams

Next, we need to convert the mass percentages of the elements to grams. We can do this by multiplying the mass percentage of each element by the total mass of the sample.

Grams of C = 55.55% × 432.50 g = 240.17 g
Grams of H = 9.26% × 432.50 g = 40.31 g
Grams of O = 22.15% × 432.50 g = 95.99 g
Grams of N = 12.94% × 432.50 g = 56.03 g

Calculating the Number of Moles of Each Element

Now that we have the masses of each element in grams, we can calculate the number of moles of each element using the molar masses of the elements.

Molar mass of C = 12.01 g/mol
Molar mass of H = 1.008 g/mol
Molar mass of O = 16.00 g/mol
Molar mass of N = 14.01 g/mol
Moles of C = 240.17 g / 12.01 g/mol = 20.00 mol
Moles of H = 40.31 g / 1.008 g/mol = 40.00 mol
Moles of O = 95.99 g / 16.00 g/mol = 6.00 mol
Moles of N = 56.03 g / 14.01 g/mol = 4.00 mol

Determining the Molecular Formula

Now that we have the number of moles of each element, we can determine the molecular formula of the compound. The molecular formula is a representation of the number and types of atoms present in a molecule of the compound.

To determine the molecular formula, we need to find the simplest whole-number ratio of atoms of each element in the compound. We can do this by dividing the number of moles of each element by the smallest number of moles.

Smallest number of moles = 4.00 mol (N)
Moles of C / moles of N = 20.00 mol / 4.00 mol = 5
Moles of H / moles of N = 40.00 mol / 4.00 mol = 10
Moles of O / moles of N = 6.00 mol / 4.00 mol = 1.5

Since we cannot have a fraction of an atom, we need to multiply the number of moles of each element by 2 to get whole-number ratios.

Moles of C = 5 × 2 = 10
Moles of H = 10 × 2 = 20
Moles of O = 1.5 × 2 = 3
Moles of N = 4 × 2 = 8

Therefore, the molecular formula of the compound is C10H20O3N8.

Conclusion

In this article, we discussed how to determine the molecular formula of a compound using the given mass percentages of its constituent elements. We calculated the mass percentages of the elements, converted them to grams, calculated the number of moles of each element, and determined the molecular formula of the compound. The molecular formula of the compound is C10H20O3N8.

References

CRC Handbook of Chemistry and Physics, 97th ed.
Weast, R. C. (ed.) (1986). CRC Handbook of Chemistry and Physics. Boca Raton, FL: CRC Press.

Note

Q: What is the molecular formula of a compound?

A: The molecular formula of a compound is a representation of the number and types of atoms present in a molecule of the compound.

Q: How do I determine the molecular formula of a compound?

A: To determine the molecular formula of a compound, you need to follow these steps:

Calculate the mass percentages of the elements in the compound.
Convert the mass percentages to grams.
Calculate the number of moles of each element using the molar masses of the elements.
Determine the simplest whole-number ratio of atoms of each element in the compound.
Multiply the number of moles of each element by a factor to get whole-number ratios.

Q: What is the difference between the molecular formula and the empirical formula of a compound?

A: The molecular formula of a compound is a representation of the number and types of atoms present in a molecule of the compound, while the empirical formula is a representation of the simplest whole-number ratio of atoms of each element in the compound.

Q: How do I calculate the empirical formula of a compound?

A: To calculate the empirical formula of a compound, you need to follow these steps:

Calculate the mass percentages of the elements in the compound.
Convert the mass percentages to grams.
Calculate the number of moles of each element using the molar masses of the elements.
Determine the simplest whole-number ratio of atoms of each element in the compound.

Q: What is the significance of the molecular formula of a compound?

A: The molecular formula of a compound is significant because it provides information about the structure and properties of the compound. It can be used to predict the physical and chemical properties of the compound, such as its melting point, boiling point, and solubility.

Q: How do I use the molecular formula of a compound to predict its physical and chemical properties?

A: To use the molecular formula of a compound to predict its physical and chemical properties, you need to follow these steps:

Calculate the molecular weight of the compound using the atomic masses of the elements.
Use the molecular weight to predict the physical and chemical properties of the compound, such as its melting point, boiling point, and solubility.

Q: What are some common mistakes to avoid when determining the molecular formula of a compound?

A: Some common mistakes to avoid when determining the molecular formula of a compound include:

Not converting the mass percentages to grams.
Not calculating the number of moles of each element using the molar masses of the elements.
Not determining the simplest whole-number ratio of atoms of each element in the compound.
Not multiplying the number of moles of each element by a factor to get whole-number ratios.

Q: How do I verify the molecular formula of a compound?

A: To verify the molecular formula of a compound, you need to follow these steps:

Calculate the molecular weight of the compound using the atomic masses of the elements.
Compare the calculated molecular weight with the experimental molecular weight of the compound.
If the calculated molecular weight matches the experimental molecular weight, then the molecular formula of the compound is correct.

Q: What are some common applications of the molecular formula of a compound?

A: Some common applications of the molecular formula of a compound include:

Predicting the physical and chemical properties of the compound.
Designing new compounds with specific properties.
Understanding the structure and properties of biological molecules, such as proteins and DNA.
Developing new materials and technologies, such as plastics and semiconductors.