2. Empirical Formula The simplest formula that represents the whole number ratio between the elements in a compound.

3. Reduce the subscripts in a formula to their simplest whole number ratio. Empirical Formula

4. The molecular formula for hydrogen peroxide is H 2 O 2, its empirical formula is HO. Empirical Formula

5. Subscripts are usually thought of as a ratio of atoms. Empirical Formula

6. Subscripts can also be thought of as a ratio of moles of atoms. Empirical Formula

7. To determine an empirical formula, one must determine the mole ratio. Empirical Formula

8. A mole is the number of atoms needed to equal the atomic mass of an element. Moles

9. The atomic mass of an element in grams is equal to one mole of its atoms. Moles

10. One mole of anything is equal to 6.02 X 10 23 Moles

11. Determine the empirical formula of a compound that is composed of 36.5% Na, 25.4% S, and 38.1% O. Sample Problem

12. You can count on the elements being given in the problem in the same order they are found in the formula. HINT:HINT:

13. Find the number of moles of each element present. Step 1

14. Since the amount of each element is given in percentage, you must convert the percentage to a mass... Step 1

15. If 100 grams of the sample are assumed, the percentages given are the same as grams. Step 1

17. Determine the whole number mole ratio. Step 2

19. Use the mole ratio for the subscripts of each element in the formula. Step 3

20. The empirical formula is Na 2 SO 3 Step 3

21. The formula mass of the sample problem is 126 u. If the molecular mass is known to be 378 u, what is the molecular formula? Finding a molecular formula from an empirical formula

22. Divide the molecular mass by the empirical formula mass to get a whole number. Solution 378 126 = 3

23. Multiply each subscript in the empirical formula by this whole number. Solution

24. You now have the molecular formula - Na 6 S 3 O 9 Solution

25. Hydrated Crystals