Empirical Formula EQ: How is the empirical formula calculated with experimental data?

Quiz Review Definition of the Mole! 1-step vs 2-step problems Tutoring Wed/Thur A or B block Finish ALL mole conversion WS in packet Quiz Friday over Empirical and Molecular Formula

Empirical Formula vs Molecular Formula The molecular formula of a compound indicates how many moles of each element are in 1 mole of the compound. The empirical formula indicates the ratio of the atoms of an element in a compound.

Example: Glucose: C 6 H 12 O 6 There are 6 moles of carbon in 1 mole of glucose because of the subscript 6 after the C in the formula. Now look at the ratio of the atoms in glucose. Glucose is C 6 H 12 O 6. The ratio of atom is C 1 H 2 O 1. Therefore the empirical formula is CH 2 O.

PRACTICE What is the empirical formula of C 6 H 12 ? CH 2 What is the empirical formula of C 6 H 6 ? CH

Empirical Formula Using Experimental Data We will now learn how to calculate an empirical formula from experimental data. Example: We have a compound that has 70% iron and 30% oxygen. **It is easiest to assume we have a 100g sample of the compound so the percent's are equivalent to the mass (g) of the compound. Let’s begin.

Step #1 – Convert % to grams Calculate the number of grams of each element in 100g of the compound by converting the %  grams. 70% iron is 70g of iron 30% oxygen is 30g of oxygen

Step #2 – Convert grams to moles Convert grams to moles with the conversion factor 1 mol = molar mass (g) 70g Fe 1 mole Fe 1.25 moles Fe g Fe 30g O 1 mole O 1.87 moles O g O

Step #3 – Divided by Fewest # moles Divide each number by the smallest number of moles calculated moles Fe 1 moles Fe moles O 1.5 moles O 1.25

Whole # vs Decimal If your results give you a whole #, use them! (.1 or.9 can round to a whole #) Our results did not give us a whole number ratio.(½= x2, ⅓= x3, ¼ = x4) Most of the time, step #3 will yield whole number ratios, but occasionally you will have to go on to step #4.

Step #4 – Multiply to eliminate decimal If necessary, multiply by a factor so that all of the numbers are whole numbers. Since the mole ratio in the problem is 1:1.5, multiply both numbers by 2. 1 mole Fe x 2 = 2 moles Fe 1.5 moles O x 2 = 3 moles O

Final Answer: Fe 2 O 3 Congratulations, you can determine an empirical formula from experimental data. Now, let’s learn how to determine the MOLECULAR FORMULA from the empirical formula. Remember, the empirical formula is the lowest whole number ratio, the MOLECULAR FORMULA is the actual formula of the compound

Determine the Molecular Formula Example Problem: Given a compound with an empirical formula of CH 2 and a molecular mass of 42, what is the Molecular Formula?

Step #1- Mass Empirical Calculate the molar mass of the empirical formula. CH 2 is composed of one carbon atom and two hydrogen atoms. Therefore the molar mass is 14.03g.

Step #2 – Divide MM/EM Divide the molecular mass given in the problem by the empirical molar mass calculated. 42g 3 14g **This is the mass ratio.

Step #3 – Multiply Multiply the subscripts of the empirical formula by the mass ratio to determine the molecular formula. CH 2 X 3 = C 3 H 6 The molecular formula is C 3 H 6

Packet Problems % Cu, 11.2% O 2.40% C, 6.7% H, 53.3% O