Empirical Formula Worksheet Answer Key

Mastering Empirical Formula: A Comprehensive Guide with Worksheet and Answer Key

Determining the empirical formula of a compound is a fundamental concept in chemistry. Understanding this process is crucial for students aspiring to master stoichiometry and chemical analysis. This article provides a comprehensive guide to calculating empirical formulas, complete with a practice worksheet and detailed answer key, designed to solidify your understanding and build your confidence. We will break down the process step-by-step, clarifying any potential confusion and equipping you with the tools to tackle even the most challenging problems.

Introduction: What is an Empirical Formula?

The empirical formula represents the simplest whole-number ratio of atoms of each element present in a compound. It doesn't necessarily reflect the actual number of atoms in a molecule (that would be the molecular formula), but rather the smallest ratio that maintains the correct proportions. For example, glucose has a molecular formula of C₆H₁₂O₆, but its empirical formula is CH₂O, representing the 1:2:1 ratio of carbon, hydrogen, and oxygen atoms. Understanding the distinction between empirical and molecular formulas is key to success in this area.

Step-by-Step Guide to Calculating Empirical Formula

Calculating the empirical formula involves several key steps, each requiring careful attention to detail. Let's break down the process:

1. Determine the Mass of Each Element:

This step usually involves being given the percentage composition of each element in the compound, or the mass of each element in a sample. If given percentages, assume you have a 100g sample, making the percentage directly equivalent to grams.

2. Convert Mass to Moles:

Using the molar mass of each element (found on the periodic table), convert the mass of each element from step 1 into moles. Remember, moles are a measure of the amount of substance, and the conversion factor is:

Moles = mass (grams) / molar mass (g/mol)

3. Determine the Mole Ratio:

Divide the number of moles of each element by the smallest number of moles calculated in step 2. This step gives you the simplest whole-number ratio of the elements.

4. Express the Ratio as a Formula:

Write the empirical formula using the whole-number ratios obtained in step 3 as subscripts for each element.

Illustrative Example: Finding the Empirical Formula of a Compound

Let's work through an example to illustrate the process. A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Let's determine its empirical formula:

1. Mass of Each Element (assuming a 100g sample):

• Carbon: 40.0g
• Hydrogen: 6.7g
• Oxygen: 53.3g

2. Moles of Each Element:

• Carbon: 40.0g / 12.01 g/mol = 3.33 moles
• Hydrogen: 6.7g / 1.01 g/mol = 6.63 moles
• Oxygen: 53.3g / 16.00 g/mol = 3.33 moles

3. Mole Ratio:

Divide each mole value by the smallest number of moles (3.33 moles):

• Carbon: 3.33 moles / 3.33 moles = 1
• Hydrogen: 6.63 moles / 3.33 moles ≈ 2
• Oxygen: 3.33 moles / 3.33 moles = 1

4. Empirical Formula:

The empirical formula is CH₂O.

Dealing with Non-Whole Number Ratios

Sometimes, the mole ratios calculated in step 3 might not be whole numbers. If this happens, you need to multiply each ratio by a small integer (like 2, 3, or 4) until all ratios become whole numbers. This ensures the formula reflects the simplest whole-number ratio.

Empirical Formula Worksheet

Now, let's test your understanding with a worksheet. Remember to follow the steps outlined above.

Problem 1: A compound contains 74.97% carbon, 5.61% hydrogen, and 19.42% oxygen. Determine its empirical formula.

Problem 2: A sample of a compound weighing 0.250g contains 0.112g of nitrogen and 0.138g of oxygen. Find the empirical formula of the compound.

Problem 3: A 1.25g sample of a compound is analyzed and found to contain 0.625g of iron and 0.625g of sulfur. Determine the empirical formula of this compound.

Problem 4: A compound is analyzed to contain 26.58% potassium, 35.35% chromium, and 38.07% oxygen. Determine the empirical formula.

Problem 5: A compound is composed of 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen. What is its empirical formula?

Empirical Formula Worksheet Answer Key

Here are the solutions to the problems in the worksheet. Check your work and review any areas where you may have encountered difficulties.

Problem 1:

1. Mass (assuming 100g): C = 74.97g, H = 5.61g, O = 19.42g
2. Moles: C = 6.24 mol, H = 5.55 mol, O = 1.21 mol
3. Mole Ratio: C = 5.16, H = 4.59, O = 1. Multiply by 2 to get whole numbers: C = 10, H = 9, O = 2
4. Empirical Formula: C₁₀H₉O₂

Problem 2:

1. Moles: N = 0.112g / 14.01 g/mol = 0.008 mol, O = 0.138g / 16.00 g/mol = 0.0086 mol
2. Mole Ratio: N = 1, O = 1.07 (approximately 1).
3. Empirical Formula: NO

Problem 3:

1. Moles: Fe = 0.625g / 55.85 g/mol = 0.0112 mol, S = 0.625g / 32.07 g/mol = 0.0195 mol
2. Mole Ratio: Fe = 1, S = 1.74. Multiply by 2 to get whole numbers: Fe = 2, S = 3.48 (approximately 3 and half). Multiplying by 2 again may be necessary based on the level of precision and rounding. In this case, the ratio should be approximately Fe2S3.
3. Empirical Formula: Fe₂S₃ (consider the margin of error)

Problem 4:

1. Mass (assuming 100g): K = 26.58g, Cr = 35.35g, O = 38.07g
2. Moles: K = 0.68 mol, Cr = 0.68 mol, O = 2.38 mol
3. Mole Ratio: K = 1, Cr = 1, O = 3.5. Multiply by 2: K = 2, Cr = 2, O = 7
4. Empirical Formula: K₂Cr₂O₇

Problem 5:

This problem is a repeat of the illustrative example. The empirical formula is CH₂O.

Frequently Asked Questions (FAQ)

Q1: What is the difference between an empirical formula and a molecular formula?

A: The empirical formula shows the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual number of atoms of each element in a molecule. For example, the empirical formula for glucose is CH₂O, while the molecular formula is C₆H₁₂O₆.

Q2: What if I get a very small or very large number when calculating moles?

A: This could indicate an error in your calculations. Double-check your work, particularly the mass values and molar masses used. Also be mindful of rounding errors.

Q3: How can I check if my empirical formula is correct?

A: You can calculate the molar mass of the empirical formula and compare it to the given molar mass of the compound (if provided). If the molar mass of the empirical formula matches the molecular mass, then the empirical formula is also the molecular formula. If not, the molecular formula is a whole-number multiple of the empirical formula.

Conclusion: Mastering Empirical Formula Calculations

Calculating empirical formulas is a fundamental skill in chemistry, requiring careful attention to detail and a strong understanding of stoichiometry. By following the steps outlined in this guide, practicing with the provided worksheet, and reviewing the answers, you will build confidence in your ability to solve even the most complex empirical formula problems. Remember to always double-check your work and understand the underlying concepts to truly master this essential chemical calculation. Good luck!