EMPIRICAL FORMULAS

Methanal vs. acetic acid Both substances have C:H:O in a 1:2:1 ratio, but they are completely different substances.

Methanal vs. acetic acid The EMPIRICAL FORMULA for both these substances is CH2O. The MOLECULAR FORMULA for acetic acid is C2H4O2.

EMPIRICAL VS. MOLECULAR FORMULAS EMPIRICAL FORMULAS show the simplest whole number ratio of elements in a compound. MOLECULAR FORMULAS show the element symbols and the exact no. of each type of atom in a molecular compound. **formulas for ionic compounds are ALWAYS empirical.

We can use the percent composition to determine the empirical formula of a compound.

EXAMPLE 1: Find the empirical formula of a compound with 35 EXAMPLE 1: Find the empirical formula of a compound with 35.4% sodium and the remainder nitrogen. STEP 1: Always assume you are working with 100 g of the compound.

EXAMPLE 1: Find the empirical formula of a compound with 35 EXAMPLE 1: Find the empirical formula of a compound with 35.4% sodium and the remainder nitrogen. STEP 2: Calculate the amount of each element in moles using n=m/MM

EXAMPLE 1: Find the empirical formula of a compound with 35 EXAMPLE 1: Find the empirical formula of a compound with 35.4% sodium and the remainder nitrogen. STEP 3: Determine the whole number ratio by dividing each element mole value by the lowest number.

EXAMPLE 1: Find the empirical formula of a compound with 35 EXAMPLE 1: Find the empirical formula of a compound with 35.4% sodium and the remainder nitrogen. STEP 4: Multiply each number by an integer to obtain all whole numbers.

***Assume 100g of substance*** EXAMPLE 2: Determine the empirical formula of a compound that contains 69.9% iron and 30.1% oxygen by mass. ***Assume 100g of substance*** STEP 1: Calculate the amount of each element in 100g.

EXAMPLE 2: Determine the empirical formula of a compound that contains 69.9% iron and 30.1% oxygen by mass. STEP 2: Divide the amount of each element by the smallest amount.

EXAMPLE 2: Determine the empirical formula of a compound that contains 69.9% iron and 30.1% oxygen by mass. STEP 3: Find an equivalent whole number ratio and write the empirical formula.

Tips for Solving Empirical Formula Problems Don’t round until the very end. If the value is 0.95 round to 1, if the value is 0.05 round to 0, if between 0.45 to 0.55 round to 0.5. See table 6.2 for additional tips for converting subscripts to in empirical formulas.

Determining the Empirical Formula by Experiment You can calculate a molecular formula using the periodic table, for example you know that pure copper will oxidize to become CuO. But, as a scientist you need to confirm this by experimentation. This can be done using a variety of methods such as a synthesis reaction.

HOMEWORK: pp. 292 #1 (practice) pp. 293 #1-5, 7