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% Composition and Empirical and Molecular Formulas

% Composition and Empirical and Molecular Formulas. Mr. Shields Regents Chemistry U04 L04. Molar Composition. When we see a chemical formula we immediately know how Many and what kind of atoms are in the compound. But number of atoms can also represent # of moles of atoms

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% Composition and Empirical and Molecular Formulas

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  1. % Composition and Empirical and Molecular Formulas Mr. Shields Regents Chemistry U04 L04

  2. Molar Composition When we see a chemical formula we immediately know how Many and what kind of atoms are in the compound. But number of atoms can also represent # of moles of atoms In the compound. For example… In one mole of NaHCO3 I have 1 mol of Na, 1 mol of H, 1 mol of C and 3 moles of oxygen atoms And in 0.25 mol of NaHCO3 I can calculate that I have 0.25 mol of Na, 0.25 mol of H, 0.25 mol of C and 0.75 mol of Oxygen atoms

  3. Back to Hydrates HEAT Now try this problem… How many moles of water are there in 0.67 mol of CoCl2.6H2O Think of it this way… If I had 1 mol of CoCl2.6H2Ohow many mol of water molecules would I have if all the water was driven off? H20 H20 CoCl2.6H2O CoCl2 H20 + H20 H20 H20 Now, If I only had 0.67 mole of CoCl2.6H2O how many mol of water would I have? 0.67 x 6 = 4.02 mol

  4. Back to Hydrates – moles of Water Problem: How many moles of water are in 3 moles of CaO∙8H20 ? 3 x 8 = 24 moles Problem: Two moles of water were driven off when a sample of CaO∙8H20 was heated leaving the anhydrous salt behind. How many moles of hydrate were originally present? IF I had 1 mole of hydrate I would have 8 moles of water but I only got 2 moles of water so I only had 2/8 = 0.25 moles

  5. Moles of atoms in hydrates When a compound has NO water of hydration it is said to be Anhydrous and is called an Anhydride. Here’s another problem involving Hydrates… A 25 gram sample of a hydrate is heated until all the water is driven off. The mass of the anhydrous product remaining was 14 grams. What is the % water in the hydrate? Ans: 25 g of hydrate – 14 grams of remaining anhydrous Product = 11 grams of water 11g H20 / 25g Hydrate = 44% water

  6. Hydrates Problem: if a 15g sample of a hydrate is heated until all the water of hydration is driven off. The mass of the anhydrous product remaining is 9.5 grams. What is the percent of water in the hydrate? Answer: 15g - 9.5g = mass of the water = 5.5g % water = (5.5g/15g) x 100 = 36.67%

  7. Empirical formula DEFINITION: A chemical formula that states the simplest whole number ratios of atoms in a compound is called the EMPIRICAL FORMULA of the compound Ex. C8H16  CH2 Empirical formula’s ARE NOT NECESSARILY the same as themolecular formula, but they Can be

  8. Empirical formula Ex. C6H6 (Benzene) is NOT an empirical formula it’s the true Molecular Formula. Why? BUT Consider Methane (CH4) It’s molecular formula is also it’s empirical formula. Why? What are the empirical formula’s of the following? Is the Empirical formula the same as the molecular formula C2H4Cl2 C3H3Br3 C2H6O

  9. glucose Glucose is a carbohydrate important to sustaining life. It’s a compound with the molecular formula C6H12O6. What’s the Empirical formula? Is it the same as its Molecular formula?

  10. Empirical formulas • The molecular formula for Hydrogen Peroxide • is H2O2. What’s the empirical formula? • The simplest whole number ratio of the elements • is HO. This is it’s empirical formula What’s the empirical formula for Water? H2O

  11. Molecular Formula Obviously a chemist needs to know whether an empirical Formula is or is not also the molecular formula. For example is water H20 or H4O2 or H6O3 To determine this the Chemist needs one more piece of information. They need the compound’s molar mass. NOTE: The chemist has several ways to determine Molar Mass (Which we won’t go into) without knowing the Actual molecular formula

  12. Molecular Formula Let’s look at Benzene as an example Benzene has an empirical formula of CH therefore the Molar mass is 13.02 If the chemist determines that the true molar mass is 78.12 can the empirical formula also be the molecular formula? NO, the molar masses do not match! ?!

  13. Molecular Formula So what is the true molecular formula? If the true molar mass of Benzene is 78.12 and the Molar mass of the empirical formula is 13.02 how much greater is the molecular mass than the empirical Mass? 78.12/13.02 = 6 The molecular formula is 6x larger than the empirical Formula. So the molecular formula must be 6x the Empirical formula or C6H6

  14. Molecular Formulas Let’s try another problem. A compound has an empirical formula of CH2O. It’s Molar mass is determined to be 180g/mol. What is the molecular formula? Step 1: Empirical molar mass = 30g Step 2: Divide molar mass of the empirical formula into the actual molar mass. 180/30 = 6 Step 3: Multiply all atoms in the empirical formula by 6 This is the molecular formula C6H12O6

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