General Chemistry

Faculty of Allied Medical Sciences

(MGGC-101)

Supervision:Prof.Dr.Shehata El-Sewedy

Dr.Fatma Ahmed

Chemical stoichiometry

Outcomes By the end of this lecture, the

students will be able to1-Understand the Atomic mass

2-learn to Molecular mass3-To differentiate between emprical and

molecular formula 4-Recognize mole of atoms

5-To know determination of empirical formula6-learn to determination of molecular formula from emprical forumla

Atomic mass:Atomic mass of an atom is its mass in atomic mass unit (amu)Mass of 12C atom is exactly 12 amuAverage 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.

Example:Calculate the molecular mass of CHCl3?

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 amuExample:Calculate the molecular mass of KClO3?

molecular mass = 1X 39.098 + 1X 35.5 + 3X 15.999 = 122.548 amu

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 1023 atom 1 mol H2O = 6.023 1023

molecule. 1 mol NO3 = 6.023 1023 ions.

*One mole of any substance contains Avogadro’s number (6.02 x 1023) of atoms or molecules.

Example : Calculate the mass of 12C atom? 1 mol of 12C contains 6.022X 1023 atom 6.022X 1023 atom weights 12 gm 1 atom weights X gm

X gm = 12X 1/ 6.022 X 1023 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

Exercise: Calculate the No of C-atoms in 0.35 mol of C6H12O6?

1 molecule C6H12O6 6 C-atoms 1 mol C6H12O6 6.023 1023 molecules 0.35 mol X molecules

No. of molecules of C6H12O6 = 0.35 x 6.023 1023 No. of C atoms = 0.35 x 6.023 1023 6 = 1.26 1024 atom

Exercise: What is the mass in grams of 1 mol glucose C6H12O6

Mwt = (6 12) + (12 1) + (6 16) = 180 amu 1 mol= mass /180 Mass = 1 mol X 180 = 180 gm 1 mol of C6H12O6 weight 180 grams.  

 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 1023

= 3.107 1023 Cl atoms.Exercise:Calculate the no. of grams found in 4.6 X10-4 mol of vitamin C (C6H8O6) (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

Exercise:How many moles of H –atoms are contained in 500 mg of vitamin C (C6H8O6)?

500 mg = 0.5 gmNo. 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

Determining Chemical FormulasIn 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 , %100 whole theof massin whole A"" of mass A"" % mass

You can look at the mass percentage of A as the number of grams of A in 100 g of the whole.

Example: Calculate the percentage composition of C12H22O11?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 %

Example:Formaldehyde, CH2O, 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

Types of Chemical Formula 1- Emprical Formula Represent the relative number of each atom in a molecule. e.g. H2O2 has empirical formula HO i.e. 1 H : 1Oe.g. Glucose C6H12O6 CH2O

1 C : 2 H : 1O

2- Molecular Formula Represent the actual number of each atom in a molecule. e.g. C6H6 has 6 C-atoms and 6 H-atoms

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

CH2O

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 CH2O

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

wtFEwtFM

..

Where n =

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

CHO2

n = = = 2wtFEwtFM

..

4590

M.F = 2 CHO2 = C2H2O4

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 Na1Cr1O3.5, 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 Na2Cr2O7. 

Quiz time

1-Calculate the molecular mass of Al2(SO4)3.2-How many atoms or molecules present in a)1.0 mol of Nab)0.25 mol of hydrogenc)2.781 mol of KNO3 3-How many grams ina)0.25 mol H2Ob)2 mol H2

4-How many moles present in 9.8 gm of H2SO4 5-Compound composed of Na, Cr, O with mass % 17.5, 39.7 and 42.8 respectively. What is the empirical formula?

7-Formaldehyde, CH2O, 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?

Student Question

* 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 C12H22O11?

* Calculate the No of C-atoms in 0.35 mol of C6H12O6?

* How many Cl atoms are contained in 18.29 gm Cl?

AssignmentsGroup 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

RECOMMENDED TEXTBOOKS:

1-Raymond Chang. Chemistry. 10th ed. 20092-Zumdehl. International edition. 2009