2. General Chemistry Faculty of Allied Medical Sciences (MGGC-101)

3. Supervision: Prof.Dr.Shehata El-Sewedy Dr.Fatma Ahmed Chemical stoichiometry

4. Outcomes By the end of this lecture, the students will be able to 1-Understand the Atomic mass 2-learn to Molecular mass 3-To differentiate between emprical and molecular formula 4-Recognize mole of atoms 5-To know determination of empirical formula 6-learn to determination of molecular formula from emprical forumla

5. Atomic mass : Atomic mass of an atom is its mass in atomic mass unit (amu) Mass of 12 C atom is exactly 12 amu Average mass of F atom is 18.998 amu ( ~19amu) Molecular mass: It is mass of the molecule in atomic mass unit, it the sum of atomic masses of all the atoms in that molecule.

6. Example: Calculate the molecular mass of CHCl 3 ? Molecular mass= 1X atomic mass of C+ 1X atomic mass of H + 3X atomic mass of Cl = 1X 12.01 + 1X1.008 + 3X35.5 = 119.37 amu Example: Calculate the molecular mass of KClO 3 ? molecular mass = 1X 39.098 + 1X 35.5 + 3X 15.999 = 122.548 amu

7. Mole of atoms: Unit used for describing large number of atoms, ions and molecules. Mole of an object contains avogadro's No of this object. e.g. 1 mol C = 6.023  10 23 atom 1 mol H 2 O = 6.023  10 23 molecule. 1 mol NO 3 = 6.023  10 23 ions. *One mole of any substance contains Avogadro’s number (6.02 x 10 23 ) of atoms or molecules.

8. Example : Calculate the mass of 12 C atom? 1 mol of 12 C contains 6.022X 10 23 atom 6.022X 10 23 atom weights 12 gm 1 atom weights X gm X gm = 12X 1/ 6.022 X 10 23 gm = 1.993 X 10 -23 gm No. of particles (atoms, molecules & ions) = no. of moles X Avogadro`s no. Note: No. of moles = mass of substance/ its atomic mass Weight/molecular Weight

9. Exercise: Calculate the No of C-atoms in 0.35 mol of C 6 H 12 O 6 ? 1 molecule C 6 H 12 O 6  6 C-atoms 1 mol C 6 H 12 O 6  6.023  10 23 molecules 0.35 mol  X molecules No. of molecules of C 6 H 12 O 6 = 0.35 x 6.023  10 23  No. of C atoms = 0.35 x 6.023  10 23  6 = 1.26  10 24 atom Exercise: What is the mass in grams of 1 mol glucose C 6 H 12 O 6 Mwt = (6  12) + (12  1) + (6  16) = 180 amu 1 mol= mass /180 Mass = 1 mol X 180 = 180 gm  1 mol of C 6 H 12 O 6 weight 180 grams.

10. Exercise: How many Cl atoms are contained in 18.29 gm Cl? 1mol of Cl  35.5 gm of Cl X mol of Cl  18.29 gm of Cl X = 18.29/35.5= 0.5159 mol of Cl atom No. of atoms= no. of moles X Avogadro`s no. = 0.5159 X 6.023  10 23 = 3.107  10 23 Cl atoms. Exercise: Calculate the no. of grams found in 4.6 X10 -4 mol of vitamin C (C 6 H 8 O 6 ) (molar mass= 176.1 gm)? No. of moles = mass / mol. Mass mol. Mass = gm/ mol 4.6 X10 -4 mol = mass/ 176.1 gm.mol -1 Mass = 4.6 X10 -4 mol X 176.1gm.mol -1 = 0.081 gm

11. Exercise: How many moles of H –atoms are contained in 500 mg of vitamin C (C 6 H 8 O 6 )? 500 mg = 0.5 gm No. of moles of vitamin C in 500 mg = 0.5 gm/ 176.1 gm. mol -1 = 2.84 X 10 -3 mol 1mol of vitamin C  8 moles of H 2.84 X10 -3 mol  ?? moles of H ?? mol of H = 2.84 X10 -3 X 8 = 2.27 X 10 -2 mol

12. Determining Chemical Formulas In order to determine a chemical formula, the mass percent of each element in the compound must be determined. Mass percentage (percent composition) The percent composition of a compound is the mass percentage of each element in the compound. We define the mass percentage of element “A” as the parts of “A” per hundred parts of the total, by mass. That is, You can look at the mass percentage of A as the number of grams of A in 100 g of the whole.

13. Example: Calculate the percentage composition of C 12 H 22 O 11 ? Answer: You can calculate %O by the same way or be subtracting the %H and C from 100%: % O = 100 – (42.1 +6.4) = 51.5 %

14. Example: Formaldehyde, CH 2 O, is a toxic gas with a pungent odor. Large quantities are consumed in the manufacture of plastics, and a water solution of the compound is used to preserve biological specimens. Calculate the mass percentage of the elements of formaldehyde. Answer: % O = 100 - (40.0+6.7%) = 53.3

15. Types of Chemical Formula 1- Emprical Formula Represent the relative number of each atom in a molecule. e.g. H 2 O 2 has empirical formula HO i.e. 1 H : 1O e.g. Glucose C 6 H 12 O 6  CH 2 O 1 C : 2 H : 1O 2- Molecular Formula Represent the actual number of each atom in a molecule. e.g. C 6 H 6 has 6 C-atoms and 6 H-atoms

16. Determination of Empirical Formula Ex: A compound composed of C, H, O with the weights 7.2, 1.2 and 9.69 grams respectively. What is the empirical formula ? Answer: C H O  Mass (grams) : 7.2 1.2 9.69 Moles (grams/at wt): 7.2/12 1.2/1 9.69/16 0.6 1.2 0.6  Divide by (the least no.): 1 2 1 CH 2 O

17. Ex: Compound composed of C H O with mass % 38.71, 6.44 and 51.58 respectively. What is the empirical formula? Answer: C H O  Assume 100 grams: 38.71 6.44 51.58  Moles: 3.22 6.44 3.22  % by 3.22: 1 2 1 CH 2 O

18. Determination of molecular formula from emprical forumla M.F = n  E.F Where n =

19. Ex: Compound composed of C, H, O has mass % of 26.7, 2.2 and 71.1 respectively and has molecular weight 90 g/mol. What is its molecular formula? Answer: C H O  Assume 100 grams: 26.7 2.2 71.1 g  Moles: 2.22 2.2 4.4  % by 2.2: 1.01 1 2 CHO 2 n = = = 2 M.F = 2  CHO 2 = C 2 H 2 O 4

20. Example: Compound composed of Na, Cr, O with mass % 17.5, 39.7 and 42.8 respectively. What is the empirical formula? Answer: Na Cr O  Assume 100 grams: 17.5 39.7 42.8  Moles: 0.761 0.763 2.68  divided by the least number, 0.761: 1 1 3.52  according to this, we get Na 1 Cr 1 O 3.5, in this case the subscripts are not all integers. They can be made into integers my multiplying each one by 2; then we get the empirical formula Na 2 Cr 2 O 7.

21. Quiz time

22. 1-Calculate the molecular mass of Al 2 (SO 4 ) 3. 2-How many atoms or molecules present in a)1.0 mol of Na b)0.25 mol of hydrogen c)2.781 mol of KNO 3 3-How many grams in a)0.25 mol H 2 O b)2 mol H 2 4-How many moles present in 9.8 gm of H 2 SO 4 5-Compound composed of Na, Cr, O with mass % 17.5, 39.7 and 42.8 respectively. What is the empirical formula?

23. 7-Formaldehyde, CH 2 O, is a toxic gas with a pungent odor. Large quantities are consumed in the manufacture of plastics, and a water solution of the compound is used to preserve biological specimens. Calculate the mass percentage of the elements of formaldehyde. 6-Compound composed of C, H, O has mass % of 26.7, 2.2 and 71.1 respectively and has molecular weight 90 g/mol. What is its molecular formula?

24. Student Question

25. * Compound composed of C, H, O has mass % of 26.7, 2.2 and 71.1 respectively and has molecular weight 90 g/mol. What is its molecular formula? * Compound composed of C H O with mass % 38.71, 6.44 and 51.58 respectively. What is the empirical formula? * Calculate the percentage composition of C 12 H 22 O 11 ? * Calculate the No of C-atoms in 0.35 mol of C 6 H 12 O 6 ? * How many Cl atoms are contained in 18.29 gm Cl?

26. Assignments Group A and Group B Samar Mostafa- Omnia Mosaad- Esraa Kamel- Reem khaled- Shrouk Essam Emprical Formula Ahmad Rada Kamel – Ahmad Mohamad Abd El Salam- Ahmad El Said- Ahmad Alaa Molecular Formula Sara Lotfy- Shimaa Saied – Esraa Sameh-Ahmad Shadra-Ibrahim El- Habet- Ibrahim El Said Elemental analysis Aya Ibrahim – Shreen Hassan – Mohamad Ahmad Torky- Mohamad Montaser- Doaa Sabri Mass spectroscopy

27. R ECOMMENDED TEXTBOOKS : 1-Raymond Chang. Chemistry. 10th ed. 2009 2-Zumdehl. International edition. 2009