Empirical Formulas and Molar Mass: Part 1-Determination of an Empirical Formula 1

Objectives -Explain what empirical formulas are -Be able to determine empirical formulas using charges or using experimental data 2

Empirical Formulas Empirical formula (“formula unit”) -the lowest whole number ratio of atoms in an ionic compound There are two ways to determine the empirical formula for a compound 1.Using charges (use your ions and this is easy) C CRISS O S S 2.Mathematically (yippee!!!) 3

Method #1 – Charges EX: Write the empirical formula for the compound formed by Na & Pc. K & Ne Sr & Cld. Cu & Cl No compound Na 3 P SrCl 2 CuCl or CuCl 2 4

Method #2 – Mathematically Step 1:Use the info given in the problem and DA with the atomic mass of the element (from periodic table- round to 3 SDs) to find moles Step 2:Take all the mole values and divide them by the SMALLEST one to figure out a ratio Step 3: Use the answers as subscripts in the empirical formula 5

Law of Conservation of Mass Chemical Equation Na + Cl  NaCl Reactants  Products Mass of Reactants = Mass of Products Example: Na + Cl  NaCl 5.0g + 5.0g = ? 10.0g 6

Atomic Mass of an Element = Mole Each element has a unique atomic mass and this is a standard for each element Atomic mass = mass of 1 mole of an element The atomic masses are from the periodic table and we use grams 1 mole O = 6.02 x atoms O = g O 1 mole Mg= 6.02 x atoms Mg= g Mg 7

Empirical Formulas 7.06 g of silver combine with an excess of fluorine to produce 8.30 g of a compound Silver + Fluorine  Ag ? F ? 7.06g 1.24g 8.30g 7.06 g Ag 1.24 g F =.0654 moles Ag =.0653 moles F Found by subtracting! Ag 1 F 1 ANSWER = AgF 8

Empirical Formulas A compound contains 24.58% K, 35.81% Mn, and 40.50% O. Find the empirical formula (assume working with 100 grams of the compound and change percentages to grams) g K g Mn g O =.629 mole K =.652 moles Mn = 2.53 moles O ANSWER = KMnO 4 9

Uneven Empirical Formulas When figuring empirical formulas mathematically, sometimes the resulting numbers don’t come out so clean You can’t just assume and round how you choose 10

.05 Rule This 0.05 rule allows for experimental error that occurs causing varied number values: –If a value is within.05 of a whole number (+0.05 or ), then the value may be rounded –Ex: 1.96 can be rounded to cannot be rounded to could be rounded to cannot be rounded to 2 If one value is not within.05 of a whole number, all the values must be multiplied by an integer so all values fall within.05 of whole numbers 11

Uneven Empirical Formulas 4.35 g sample of zinc is combined with an excess of the element phosphorus g of compound are formed. Calculate the empirical formula. Zinc + Phosphorous  Zn ? P ? 4.35g 1.37g 5.72g Found by subtracting! 4.35 g Zn 1.37 g P =.0665 moles Zn =.0442 moles P Not within.05 of a whole number X 2 = 3.00 or 3 X 2 = 2.00 or 2 ANSWER = Zn 3 P 2 12

Let’s Do It!!! A compound is found to contain 72.3% Fe and 27.7% O by weight. Calculate the empirical formula. Assume in 100 g of compound there would be 72.3 g Fe and 27.7 g O 13

72.3g Fe 1 mole Fe X —————— 55.8 g Fe = 1.30 mole Fe 27.7g O 1 mole O X —————— 16.0 g O = 1.73 mole O 1.30 mole 1.73 mole 1.30 mole = 1.00 =1.33 X 3 = 3.00 = 3 X 3 = 3.99 = 4 Fe 3 O 4 14

Objectives -Explain what empirical formulas are -Be able to determine empirical formulas using charges or using experimental data 15