Empirical Formula Practice Problems with Solutions

📚 Topic Summary

The empirical formula represents the simplest whole number ratio of atoms in a compound. It's derived from experimental data, such as percentage composition or mass data, and involves converting these values into moles. The mole ratios are then simplified to the smallest whole numbers to obtain the empirical formula. This formula provides essential information about the elemental composition of a substance without specifying the actual number of atoms in a molecule (as given by the molecular formula).

To determine the empirical formula, you typically need to:

1. Convert percentage composition to mass (if needed).
2. Convert mass to moles using the molar mass of each element.
3. Divide each mole value by the smallest mole value to obtain the simplest mole ratio.
4. If necessary, multiply the mole ratios by a whole number to obtain whole number subscripts.

🧪 Part A: Vocabulary

Match the term with its definition:

Answers: 1-C, 2-A, 3-B, 4-E, 5-D

⚗️ Part B: Fill in the Blanks

The empirical formula gives the simplest __________ ratio of atoms in a compound. To find it, you must convert mass data into __________, then find the smallest whole number __________. For example, if you have a compound with a ratio of 1:1, the empirical formula is simply __________.

Answers: whole number, moles, ratio, AB

🔬 Part C: Critical Thinking

A compound is found to contain only carbon and hydrogen. Combustion analysis shows that 10.0 g of the compound produces 30.0 g of $CO_2$ and 16.4 g of $H_2O$. Determine the empirical formula of the compound.

Solution:

• Calculate moles of C: $30.0 \, g \, CO_2 * (1 \, mol \, CO_2 / 44.01 \, g \, CO_2) * (1 \, mol \, C / 1 \, mol \, CO_2) = 0.682 \, mol \, C$
• Calculate moles of H: $16.4 \, g \, H_2O * (1 \, mol \, H_2O / 18.02 \, g \, H_2O) * (2 \, mol \, H / 1 \, mol \, H_2O) = 1.82 \, mol \, H$
• Find the ratio: $C_{0.682}H_{1.82}$
• Divide by smallest: $C_{0.682/0.682}H_{1.82/0.682} = CH_{2.67}$
• Multiply to get whole numbers: $CH_{2.67} * 3 = C_3H_8$
• Empirical Formula: $C_3H_8$