GenChem1-Writing Chemical Formula and Empirical/Molecular.pptx

Chemical formula, molecular, empirical formular

1. Writing Chemical Formula

2. -is a way of expressing information about the proportions of  atoms  that constitute a particular  chemical compound , using a single line of chemical element  symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and  plus  (+) and  minus  (−) signs.

3. The chemical formula of a covalent molecular compound tells us the number of atoms of each element present in the compound. E.g. Water H2O -The formula tells us that there are 2 Hydrogen atoms and 1 oxygen atom present in each water molecule.

4. The chemical formula of a covalent network tells us the ratio of the elements present. E.g. Silicon dioxide, SiO2 structure-The formula tells us there are 2 silicon atoms present for every 1 oxygen atom.

5. The chemical formula of an ionic compound tells us the ratio of the elements present. E.g. Sodium chloride, NaCl – The formula tells us there is one ion of sodium for every one ion of chlorine

6. 1. CHEMICAL FORMULA FOR ELEMENTS The chemical formula of an element is simply its’ symbol e.g. Magnesium Mg Iron Fe Phosphorus P Argon Ar

7. 2. USING VALENCY The valency of an atom or ion is the number of electrons it shares, loses or gains in a chemical reaction to become stable i.e. the number of bonds it forms with other atoms Table: Group number 1 2 3 4 5 6 7 0 Valency 1 2 3 4 3 2 1 0

8. We can follow a set of simple rules to write chemical formula: 1. Write symbols of elements present in compound 2. Put valency above each symbol 3. Cross valency over (swap and drop) 4. Cancel down ratio if necessary 5. Write correct chemical formula

9. Example Sodium chloride Na Cl Na 1 Cl 1 Na 1 Cl 1 (Not needed) NaCl

10. 3. ROMAN NUMERALS Some elements, particularly the transition metals in the centre block of the periodic table can have more than one valency. Roman Numeral Valency I 1 II 2 III 3 IV 4 V 5 VI 6

11. Example 1. Copper(II) chloride Cu Cl Cu 2 Cl 1 Cu1 Cl2 CuCl2

12. Example 2. Iron(III)fluoride Fe F Fe 3 F 1 Fe1 F3 FeF3

13. 4. PREFIXES In the names of some compounds the ratio of atoms present can be indicated by prefixes. Prefix Meaning Mono 1 Di 2 Tri 3 Tetra 4 Penta 5 Hexa 6

14. Examples: Carbon mono xide CO Nitrogen di oxide NO2 Phosphorus tri chloride PCl3 Phosphorus penta chloride PCl5 Dinitrogen tetro xide N2O4

15. 5.Polyatomic Ions Ions formed by more than one type of element Atoms of different elements held together by covalent bonds Atoms always stay together and collectively have a single charge Do not always have “-ide” ending ex: NH 4 1- = ammonium ion CO 3 2- = carbonate

16. 6. IONIC FORMULA The ionic formula shows the charges of the ions present in an ionic substance .

17. The order of common nonmetals in binary compound formulas is C, P, N, H, S, I, Br, Cl , O, F.

18. Empirical and Molecular Formula

19. Empirical Formula Empirical formulas are the simplest, whole number ratio of elements in a compound. It’s the reduced chemical formula Example: Benzene – C 6 H 6 (molecular formula) CH (empirical formula) Water – H 2 O (molecular formula) H 2 O (empirical formula) The empirical formula CAN be the same as a molecular formula

20. Solve for Empirical Formula: A compound is found to be 34.53% zinc, 14.79% nitrogen, and 50.68% oxygen. Calculate the empirical formula for this compound. 34.53% = 0.3453 x 100 = 34.53 Divide 34.53 to the atomic weight of zinc (in the periodic table). = 34.53/65.38 = 0.53mol 3. Divide by the smallest answer.

21. Solve: A compound is composed of 52.14% Carbon, 13.13% Hydrogen, and 34.73% Oxygen by mass. A)What is the empirical formula? B) If the molar mass of the compound is 138.204 g/mol, what is the molecular formula?

22. Molecular Formula Molecular formulas tell the actual number of atoms of each element in a compound Example: Glucose – C 6 H 12 O 6 (one molecule has 6 carbons 12 hydrogens and 6 oxygens)

23. Solve for Molecular Formula: C6H12O6 1. Calculate molar mass for the empirical formula 2. Find the molar mass given in the problem . 3. Find the “number” n 4. Multiply each subscript in the empirical formula by n (“number”) 5. Write the formula with the metal first or in the order in which it was given if there is no metal.  

24. Solve: A compound is composed of 52.14% Carbon, 13.13% Hydrogen, and 34.73% Oxygen by mass. A)What is the empirical formula? B) If the molar mass of the compound is 138.204 g/mol, what is the molecular formula?

25. Quiz: Determine the empirical and molecular formula for chrysotile asbestos. Chrysotile has the following percent composition: 28.03% Mg, 21.60% Si, 1.16% H, and 49.21% O. The molar mass for chrysotile is 520.8 g/mol.