1. Chapter 3 Composition of Substances and Solutions
Part A

2. Molecular mass of HCCl3 = 119.37 u
Atoms
Atomic Mass: Average mass of 1 atom
Atomic Mass of Cl: Average mass of 1 atom Cl = u
Molecules
Molecular Mass: Sum of atomic masses of all atoms in molecular formula. Molecular mass of HCCl3 ?
Molecular mass of HCCl3 = u

3. Formula Mass Ionic compounds
Formula Mass: Sum of atomic masses of all atoms in ionic compound’s formula.
formula mass of NaCl = u

4. Mole: Unit for Amount 12g of 12C contains 6.022 × 1023 atoms 12C
The mole is the amount of substance containing as many units as there are atoms in exactly 12 grams of 12C atoms
12g of 12C contains × 1023 atoms 12C
1 mole of 12C contains × 1023 atoms 12C
1 mole contains × 1023 units
(Always state explicitly the identity of the unit)
1 gram = × 1023 u
Avogadro’s number (NA) = number of units per mole
NA = × 1023 units.mole-1

5. Molar Mass
Atoms
Molar Mass of element: Mass per mole of atoms of the element (1 mole of atoms has × 1023 atoms )
Atomic mass of Cl: Average mass of 1 atom Cl = u
Mass of 1 mole of atoms Cl = g
Molar mass of Cl =35.45 g /mol
Molecules
Molar Mass: Sum of molar masses of all atoms in the formula.
Molecular mass of HCCl3 = u
1 mole of HCCl3 has a mass of g Molar mass of HCCl3 is g/mol

6. Calculate Molar Mass What is the molar mass of C2H5COOH? 148 g/mol6

7. Molar Mass Ionic compounds In General
Molar Mass: Sum of molar masses of all atoms in ionic compound’s formula.
Formula mass of NaCl = u
Molar mass NaCl = g/mol
In General
Molar mass (MM) Mass of per mole of substance (atom
of element, molecule, or ionic formula) in consideration.
Unit for molar mass is g/mol
MM of Cl= g/mol, MM of NaCl= g/mol

8. Calculate Molar Mass Formula is Mg3(PO4)2
What is the molar mass of magnesium phosphate?
262.9 g/mol
150.9 g/mol
333.6 g/mol
119.3 g/mol
166.9 g/mol
Formula is Mg3(PO4)2 8

9. Molar mass of an element is the mass of the element per mole of its atoms.
Atomic mass of O = u molar mass of O = g/mol
Molar mass of a molecular substance is the mass of the compound per mole of its molecules.
Molecular mass of O2=2 x u=32.00 u
Molar mass of O2 = g/mol
Molar mass of an ionic compound is the mass of the compound per mole of its formula units.
Formula mass of NaCl = u
Molar mass of NaCl = g/mol

10. Convert from grams to moles
How many moles of potassium are in 4.70 g of potassium?
4.70 g K ( 1 mol K/39.10 g K) = mol

11. Convert from moles to grams
How many grams of argon are in 4.70 mol of Ar?
4.70 mol Ar (39.95 g Ar/1 mol Ar) = 188 g Ar

12. Convert from moles to grams
Example: If we need mole Ca3(PO4)2 for an experiment, how many grams do we need to weigh out?
Calculate molar mass of Ca3(PO4)2
3mol Ca×molar mass Ca = 3 mol× g/mol = g
2mol P× molar mass P = 2 mol × g/mol = g
8 mol O× molar mass O = 8 mol× g/mol = g
1 mole Ca3(PO4)2 = g Ca3(PO4)2
Molar mass Ca3(PO4)2 = g/mol Ca3(PO4)2

13. Convert from moles to grams
Set up conversion factor so that what you want is on the top and what you start with is on the bottom
= g Ca3(PO4)2

14. Using Moles in Calculations
Start with either
Grams (Macroscopic)
Elementary units (Microscopic)
Use molar mass to convert from grams to mole
Use Avogadro’s number to convert from moles to elementary units

15. Formula unit and Mole Conversions
Calculate the number of formula units of Na2CO3 in 1.29 moles of Na2CO3.
How many moles of Na2CO3 are there in 1.15 × 105 formula units of Na2CO3 ?
= 7.77 × 1023 units of Na2CO3
= 1.91×10–19 mol Na2CO3

16. Deriving Number of Atoms from Mass for an Element
How many silver atoms are in a 85.0 g silver bracelet?
What do we want to determine?
85.0 g silver = ? atoms silver
g Ag  mol Ag  atoms Ag
= 4.75 × 1023 Ag atoms

17. Conversion from Number of atoms to Mass
How much, in grams, do 8.85 × 1024 atoms of zinc weigh?
= 961 g Zn 17

18. Conversion from formula unit to mass
Calculate the mass in grams of FeCl3 in 1.53 × formula units. (molar mass = g/mol)
= 41.2 g FeCl3