Empirical and Molecular Formula Practice

1. A compound contains 18.8% sodium, 29.0% chlorine, and 52.2% oxygen, by mass. If the molar mass of the compound is 122.44 g/mol, determine the empirical and molecular formulas.

2. A 4.99 gram sample of a compound contains 1.52 grams of nitrogen atoms and 3.47 grams of oxygen atoms. The molar mass of the compound is between 90.0 g and 95.0 g. Determine the empirical and molecular formulas. Also, calculate the actual molar mass of this compound.

 

3. Ascorbic acid is another name for Vitamin C. It is composed of 40.92% carbon, 4.58% hydrogen, and 54.50% oxygen, by mass. Determine the empirical formula for ascorbic acid.

4. A 100 gram sample of compound of sodium contains 36.5 grams of sodium, 25.4 grams of sulfur, and 38.1 grams of oxygen. Does the empirical formula for the compound lead you to believe it is sodium sulfite or sodium sulfate?

5. Fat makes up a major portion of all soaps. A fat used in many soaps is 76.5% carbon, 12.2% hydrogen, and 11.3% oxygen. What is its empirical formula? What is the molecular formula of a fat with a molecular mass of 705 g?

6. A sample of a substance is determined to be composed of 0.89 grams of potassium, 1.18 grams of chromium, and 1.27 grams of oxygen. Calculate the empirical formula of this substance.

7. Strychnine, a deadly poison, has a molecular mass of 334 amu and a percentage composition of 75.42% carbon, 6.63% hydrogen, 8.38% nitrogen, and the balance oxygen. What is the molecular formula of strychnine?

(Hint: DO NOT round up the number found for carbon in the empirical formula.)

8. A 10 gram sample of a compound contains 7.22 grams of magnesium and 2.78 grams of nitrogen. What is its empirical formula?

9.An organic compound with a molecular mass of 140u is 68.54% carbon, 8.63% hydrogen, and 22.83% oxygen. What is the molecular formula of the compound?

10. To find the experimental empirical formula of a compound, a student heats a coil of magnesium ribbon 35.00 cm long in a crucible. Water is added to the ash left in the crucible and the crucible is reheated until dry.

Data from the experiment:

• Mass of empty crucible and cover - 20.74g
• Mass of one meter of magnesium ribbon - 0.72g
• Mass of crucible, cover, and final product - 21.17g

What is the empirical formula for the oxide of magnesium that was the final product?

Answers:

1. Empirical and molecular formula - NaClO₄

2. Empirical formula - NO₂. Molecular formula - N₂O₄. Molar mass = 92.04 g/mol

3. Empirical formula - C₃H₄O₃

4.  Empirical formula- Na₂SO₃, sodium sulfite

5. Empirical formula- C₉H₁₇O Molecular formula- C₄₅H₈₅O₅

6. Empirical formula- K₂Cr₂O₇

7. Molecular formula- C₂₁H₂₂N₂O₂

8. Empirical formula- Mg₂N₃

9. Molecular formula- C₈H₁₂O₂

10. Empirical formula- MgO