Chemistry Chapter Ten Mrs Luckett Empirical Formula Chemistry Chapter Ten Mrs Luckett

Empirical Formula When we cook rice, we commonly use one cup of rice and one cup of water. However, we can double, triple, etc the recipe if we need to. + = We do this in chemistry too! The formula for some compounds shows the basic ratio of elements.

Empirical Formula What is an empirical formula? Gives us the smallest whole-number ratio of subscripts for each element found in a compound You have an empirical formula when you can’t divide the subscripts by the same number to get a whole number For example: CH4 Different compounds can have the same empirical formula!

Are these examples empirical? H2O NaCl C2H4

Empirical and Molecular formula foldable Flaps: Change percent to mass in grams Convert mass to moles Divide each lowest moles Multiply and Plug in subscripts

Sample Problem #1 A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step One: Even conversion (change % to grams by assuming 100 gram sample) 25.9% Nitrogen = 25.9 grams N 74.1 % Oxygen = 74.1 grams O

Sample Problem #1 Step Two: convert grams to moles 14.007 g N A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step Two: convert grams to moles 25.9 g N x 1 mole = 1.85 mol N 14.007 g N 74.1 grams O x 1 mole = 4.63 mol O 15.999 g O

Sample Problem #1 Step Three: divide by the lowest number of moles A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step Three: divide by the lowest number of moles 1.85 mol N/ 1.85 = 1 4.63 mol O / 1.85 = 2.5 Are these both whole numbers? No! Now we need to do one more step

Sample Problem #1 A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? Step Four: multiply by lowest whole number possible (if necessary) Can I multiply by one to get whole numbers? NO Can I multiply by two to get whole numbers? YES N: 1 x 2 = 2 O 2.5 x 2 = 5 Now plug it into the formula: N2O5

Now you try!  Calculate the empirical formula of each compound. 94.1% O, 5.9% H 67.6% Hg, 10.8% S, 21.6% O