This document discusses empirical formulas and how to determine them from percentage composition or experimental data. It provides examples of calculating empirical formulas from the mass percentages of elements in a compound or the grams of elements in a sample. It also explains how to determine the molecular formula of a compound from the empirical formula and experimental molar mass. Key steps include converting percentages to grams of elements, calculating moles of each element, and dividing by the smallest ratio of elements to give whole number ratios in the empirical formula. The molecular formula is found by dividing the experimental molar mass by the molar mass of the empirical formula.

1. Empirical Formulas Chapter 7 section 3

2. Empirical Formulas An empirical formula shows the simplest ratio among atoms in a compound.

3. Determining an Empirical Formula from Percentage Composition Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance.

4. Step 1: Convert to grams Assume that you have a 100.0 g sample, and convert the percentages to grams. for C: 60.0%  100.0 g = 60.00 g C for H: 13.4%  100.0 g = 13.40 g H for O: 26.6%  100.0 g = 26.60 g O

5. Step 2: Convert grams to moles (divide given amount by molar mass) 60.00 g C 12.01 g C 13.40 g H 1.01 g H 26.60 g O 16.00 g O

6. Step 3: Divide each by the smallest decimal to get whole numbers . *These numbers will be the subscripts. The formula can be written as C 5 H 13.3 O 1.66 , but you divide by the smallest subscript to get whole numbers. The empirical formula is C 3 H 8 O

7. Try this one. 69.60 g Mn 54.94 g Mn 30.40 g 0 16.00 g 0 = 1.27 mol Mn = 1.90 mol O 1.27 1.27 = 1 mol Mn 1.90 1.27 = 1.5 mol O *Still not whole numbers. We have to get rid of the 1.5. If we multiply the 1.5 by 2 it becomes 3. But whatever we do to one side we have to do to the other. 1 x 2 = 2 Mn, 1.5 x 2 = 3 O Mn 2 O 3 69.6% Mn and 30.4% O

8. Determining a Molecular Formula from an Empirical Formula The empirical formula for a compound is P 2 O 5 . Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.

9. Step 1: Find the molar mass of the empirical formula. The molar mass of the empirical formula P 2 O 5 . 2  molar mass of P = 61.94 g/mol + 5  molar mass of O = 80.00 g/mol molar mass of P 2 O 5 = 141.94 g/mol

10. Step 2: Molar mass of compound divided by molar mass of empirical formula. molar mass of compound

11. Step 3: multiply subscripts from empirical formula by answer from step 2. n (empirical formula) = 2 (P 2 O 5 ) = P 4 O 10 Molecular formula P 4 O 10

12. Try this one. C 1 x 12.01 = 12.01 H 1 x 1.01 = +1.01 13.02 g/mol 78.00 g/mol 13.02 g/mol = 5.99, we can say 6 Multiply all subscripts by 6, so 6(CH), and you get… C 6 H 6 CH 78 g/mol

13. Homework Empirical vs. Molecular worksheet 1-18 Read chapter 7 section 3. Look at the examples.