Molecular mass and formula mass Foundation Year Molecular mass and formula mass 2 Dr. Marwa Eid

Molecular Mass Objectives -After this Lecture the students will be able to convert between: -Moles and grams -Moles and particles

Molecular Mass “MM” Molecular Mass is conversion factor between grams and moles. Number of grams in 1 mole (n= mass/MM MM=mass/n) MM= # Protons + # Neutrons in a nucleus. = atomic mass

Molecular Mass 1 mole of Mg atoms = 24.3 g 1 mole of Cu atoms = 63.5 g 1 mole of C atoms = 12.0 g 1 mole of Mg atoms = 24.3 g 1 mole of Cu atoms = 63.5 g

Molecular Mass

Molecular Mass Learning Check Give the molar mass for each (to the tenths decimal place). A. 1 mole K atoms = ________ B. 1 mol Sn atoms = ________

Molecular Mass Solution Give the molar mass for each (to the tenths decimal place). A. 1 mole K atoms = 39.1 g B. 1 mole Sn atoms = 118.7 g

Moles and Grams I mole of Na = 22.99 g 3.5 mole (Na)= ??? g What is the mass of 3.5 moles of sodium (Na)? I mole of Na = 22.99 g 3.5 mole (Na)= ??? g Mass = (3.5* 22.99)/1 Mass = 80.5 g

Moles and Grams I mole of Fe = 55.85 g ??? mole (Na)= 25 g How many moles of iron (Fe) does 25 g of Fe represents? I mole of Fe = 55.85 g ??? mole (Na)= 25 g # moles = (25*1 )/ 55.85 # moles (n) = 0.448

One Mole a Compound 32.1 g 55.9 g 58.5 g 294.2 g 342.2 g One-Mole Quantities

Molecular Mass of a Compound “The molecular mass of a compound is the sum of the molecular mass of the elements in the formula” 1 mole of methane 1 mole of carbon and 4 moles of hydrogen The molar mass of (“one mole of”) a compound is obtained by summing the masses of the appropriate number of moles of component atoms.

Molecular Mass of a Compound

Molecular Mass of a Compound Example Calculate the molar mass of CaCl2. Solution Element Number of Moles (n) Molecular Mass (MM) Total Mass = MM*n Ca 1 40.1 g/mole 40.1 *1= 40.1 g Cl 2 35.5 g/mole 35.5*2= 71.0 g CaCl2 40.1+71= 111.1 g

Molecular Mass of a Compound Calculate the molar mass of K3PO4. Element Number of Moles (n) Molecular Mass (MM) Total Mass = MM * n K 3 39.1 g/mole 117.3*3= 117.3 g P 1 31 g/mole 31*1 = 31.0 g O 4 16 g/mole 16 * 4 = 64.0 g K3PO4 212.3 g

Molecular Mass of a Compound Calculate the molar mass of N2O4. Element Number of Moles (n) Molecular Mass (MM) Total Mass = MM * n N 2 14 g/mole 14*2= 28 g O 4 16 g/mole 16*4 = 64 g N2O4 92 g

Molecular Mass of a Compound Calculate the molar mass of H2SO4. Element Number of Moles (n) Molecular Mass (MM) Total Mass = MM * n H 2 1.008 g/mole 1.008*2 = 2.016 g S 1 32.07 g/mole 32.07*1 = 32.07 g O 4 16 g/mole 16*4 = 64.0 g K3PO4 98.09 g

Molecular Mass Learning Check What is the molar mass of each of the following? A. K2O B. Al(OH)3

Molecular Mass Solution A. K2O 94.2 g/mole 2 mole K (39.1 g/mole) + 1 mole O (16.0 g/mole) 78.2 g + 16.0 g = 94.2 g B. Al(OH)3 78.0 g/mole 1 mole Al (27.0 g/mole) + 3 mol O (16.0 g/mole) + 3 mol H (1.01 g/mole) 27.0 g + 48.0 g + 3.03 g = 78.0 g

Moles and Grams Solution Aluminum is often used for the structure of light-weight bicycle frames. How many grams of Al are in 3.00 moles of Al? 3.00 moles Al ? g Al Solution

1 mole= 6.02 X 1023 particles Avogadro’s Number Avogadro’s number is a conversion factor between moles and particles

Moles and Particles 1 dozen cookies = 12 cookies 1 mole of cookies = 6.02 X 1023 cookies 1 dozen cars = 12 cars 1 mole of cars = 6.02 X 1023 cars 1 dozen Al atoms = 12 Al atoms 1 mole of Al atoms = 6.02 X 1023 atoms Note that the NUMBER is always the same, but the MASS is very different! Mole is abbreviated mol (gee, that’s a lot quicker to write, huh?)

A Mole of Particles Contains 6.02 x 1023 Atom = 6.02 x 1023 C atoms = 6.02 x 1023 H2O molecules = 6.02 x 1023 NaCl “molecules” (technically, ionics are compounds not molecules so they are called formula units) 6.02 x 1023 Na+ ions and 6.02 x 1023 Cl– ions 1 mole C 1 mole H2O 1 mole NaCl

Avogadro’s Number as Conversion Factor 6.02 x 1023 particles 1 mole or 1 mole Note that a particle could be an atom OR a molecule!

Avogadro’s Number as Conversion Factor How many moles of CO2 are in 25*1024 molecules of CO2 ??? 1 mole CO2 = 6.02 * 1023 molecule ? mole CO2 = 25*1024 molecule # mole CO2 = (25*1024 * 1)/ 6.02 * 1023 # mole CO2 = 4.15 mole

Learning Check 1. Number of atoms in 0.500 mole of Al a) 500 Al atoms b) 6.02 x 1023 Al atoms c) 3.01 x 1023 Al atoms 2.Number of moles of S in 1.8 x 1024 S atoms a) 1.0 mole S atoms b) 3.0 mole S atoms c) 1.1 x 1048 mole S atoms

Learning Check How many molecules are in 2.00 moles of H2O? 1.20x1024 molecules H2O How many moles are in 8.30 X 1023 molecules of H2O? 1.38 moles H2O How many molecules are in 0.500 moles of H2? 3.01x1023 molecules H2 How many atoms are in 1.00 moles of He? 6.02x1023 atoms He How many atoms are in 1.50 moles of Hg? 9.03x1023 atoms Hg How many molecules are in 2.50 moles of O2? 1.51x1024 molecules O2 How many atoms are in 0.250 moles of Rb? 1.51x1023 atoms Rb How many moles are in 2.91 X 1022 atoms of He? 0.0483 moles Rb How many moles are in 1.23 X 1050 molecules of H2? 2.04x1026 moles H2 How many moles are in 5.25 X 1025 atoms of Au? 87.2 moles Au How many moles are in 7.14 X 1035 molecules of O2? 1.19x1012 moles O2 How many moles are in 8.11 X 1020 molecules of CH4? 0.00135 moles CH4