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DETERMINING CHEMICAL FORMULAS

DETERMINING CHEMICAL FORMULAS. What if you discovered the cure for cancer?. Any new chemical that is discovered that is a potential health care treatment must have its molecular formula submitted to the Ministry of Health.

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DETERMINING CHEMICAL FORMULAS

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  1. DETERMINING CHEMICAL FORMULAS

  2. What if you discovered the cure for cancer? • Any new chemical that is discovered that is a potential health care treatment must have its molecular formula submitted to the Ministry of Health. • How do you determine the molecular formula of some new compound that neither your nor anyone may have never seen before? • It’s actually not that hard…all you need is a device called a mass spectrometer and some chemistry fundamentals – some of which are hundreds of years old!

  3. A mass spectrometer is a lab instrument that measures the molar mass of a compound. The machine can also give you a reading of each element in the compound based on mass and percentage. Look at this reading from the Cassini Spacecraft as it flew by one of Saturn’s moons – Enceladus. The Mass Spectrometer

  4. Law of Constant Composition • A compound contains elements in certain fixed proportions (ratios) and in no other combinations. • This law is important in formula determination because it basically says that each compound has only one formula…if you find something that matches a known formula then that is what you have found – no arguments. • Water will always be H2O…Find that and you have found water.

  5. Empirical vs. Molecular • Before you can determine the molecular formula of a compound you must first determine the empirical formula. • An empirical formula is the lowest whole number ratio of elements in a compound. • Think of empirical as a formula in lowest terms. • The molecular formula of a compound is the actual number of atoms in one molecule of a compound. • Think of molecular formulas as seeing the compound out on the street. • Example: Glucose C6H12O6 (molecular formula) CH2O (empirical formula)

  6. Determining Empirical Formulas • In order to determine the empirical formula for a compound you need the percent composition for it… • The mass spectrometer will give you this…or the question will tell you! • Let’s try an example of a problem in which we must determine the percent composition.

  7. C = 40.00g ÷ 12.01g/mol = 3.33mol C = 3.33mol ÷ 3.33mol = 1 H = 6.71g ÷ 1.01g/mol = 6.64mol H = 6.64mol ÷ 3.33mol = 2 O = 53.29g ÷ 16.00g/mol = 3.33mol O = 3.33mol ÷ 3.33mol = 1 Therefore the empirical formula for lactic acid is CH2O. Steps to Solve Assume 100g (%  g) g  mol (÷ by g/mol) Divide by lowest mol Sample Problem 1: The substance that gives sour milk that lovely taste is lactic acid. It consists of 40.00% Carbon and 6.71% Hydrogen with the remainder being oxygen. Determine its empirical formula…Or Else!!!

  8. Empirical Is Not Enough! • If you look at this slideshow carefully – you will have noticed both glucose (C6H12O6) and lactic acid (C3H6O3) have the same empirical formula – but they are very different molecules it terms of properties and function within the human body! • Many molecules can have the same empirical formulas so it is absolutely necessary to be able to determine the molecular formula for any compound. • In order to determine the molecular formula we need two pieces of information: • Empirical Formula (or a way to get it…like percent composition…a la the mass spectrometer) • Molar mass of the compound (The almighty mass spectrometer will give us this too!)

  9. Behold!...The Mass Spec!

  10. Sample Problem 2: Determine the molecular formula of a compound containing 85.7% carbon and 14.3% hydrogen by mass. The molar mass of the compound is 84g/mol. Solve…Or Else!!! • We do not have the empirical formula but we do have a way to get it. (by using the Percent Composition)! • C = 85.7g ÷ 12.01g/mol = 7.136mol ÷ 7.136mol = 1 • H = 14.3g ÷ 1.01g/mol = 14.16mol ÷ 7.136mol = 2 • Therefore the empirical formula is CH2. • Molar Mass = 84g/mol = 6 Empirical Formula Mass 14.03g/mol • This means that the molecular formula is 6 times larger than the empirical formula so we need to multiply the empirical formula by 6 to get the molecular formula. 6 x CH2 = C6H12 • The Molecular formula is C6H12.

  11. Sample Problem 3: Determine the molecular formula of a compound with the empirical formula P2O3 and a molar mass of 220.0g/mol. • We are given the empirical formula outright so you go directly to the second step of the solving process. • Molar Mass = 220.0g/mol = 2 Empirical Formula Mass 109.94g/mol • This means that the molecular formula is 2 times larger than the empirical formula so we need to multiply the empirical formula by 2 to get the molecular formula. 2 xP2O3 = P4O6 • The molecular formula is P4O6.

  12. That’s All I Got!... “I know it’s a lot to think about… So I will leave you to it.”

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