20 Chemistry
Chapter 11 Notes
The Mole Concept
What is a mole?
Just like a dozen means 12 of something, a mole is a certain number of things.
Specifically:
1 mol is 6.022 x 1023 somethings
This number is sometimes called Avagadro’s number. Rather than saying we have
6.022x1023 water molecules we would say we have 1 mol of water.
Converting between moles and the number of particles:
# particles 
# particles
x # mols
mol
Notice that all we’ve done is multiply the # mols by a conversion factor
6.022 x10 23 particles
1mol
Try p. 311 # 1 – 3
Converting between the number of particles and moles:
# mols 
1mol
x # particles
# particles
Notice again that all we’ve done is multiply the # particles by a conversion factor
1mol
6.022 x10 23
Try p. 312 # 4 a – d
Converting Moles to Mass
Molar Mass – the mass of 1 mol of any substance. For elements, the molar mass is the
same as the atomic mass (but it is in grams).
i.e.
the molar mass of Hydrogen is 1.008 g (1 mol of hydrogen gas has a mass
of 1.008 g
the molar mass of Oxygen is 15.999 g
the molar mass of water (H2O) is (2 x 1.008) + 15.999g = 18.015 g
To convert a the number of moles of a substance into a mass:
𝒎𝒂𝒔𝒔 = 𝑴𝒐𝒍𝒂𝒓 𝑴𝒂𝒔𝒔 𝒙 # 𝒎𝒐𝒍𝒔
OR
𝒎𝒂𝒔𝒔 =
𝒎𝒂𝒔𝒔
𝒙 # 𝒎𝒐𝒍𝒔
𝒎𝒐𝒍
See sample p. 315 then try p. 316 # 11 a-d
Converting Mass to Moles
To convert a the mass of a substance to a number of moles:
𝒎𝒐𝒍𝒔 =
𝟏
𝒙 𝒎𝒂𝒔𝒔
𝒎𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔
OR
𝒎𝒐𝒍𝒔 =
𝟏 𝒎𝒐𝒍
𝒙 𝒎𝒂𝒔𝒔
𝒎𝒂𝒔𝒔
See sample p. 316 then try p. 316 # 12 a-d
Converting from mass to the number of particles (and Vice Versa)
x
1mol
6.022 x10 23 particles
# of
particles
x
1mol
MolarMass
# of
moles
6.022 x10 23 particles
x
1mol
mass
x
MolarMass
1mol
Example : How many “formula units” are there in 7.8 grams of Na2SO4? A: 3.31x1022
Practice:
p. 318 # 13 & 14
p. 321 # 20 – 24 & p. 322 # 25,26 & p. 323 # 27 – 29
p. 326 # 31 – 35
Percent (by Mass) Composition
Percent composition tells us the percentage by mass of each element in a compound.
Consider as an example, the percentage composition of Chlorine in Calcium Chloride
(CaCl2). First we assume that we have 1 mol of calcium chloride. Why can we do this?
𝑃. 𝐶. =
#𝑚𝑜𝑙𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑥 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡
𝑥 100%
𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑
Therefore
Try p. 331 # 42 – 45
𝑃. 𝐶. =
2 𝑥 35.45
110.98
𝑥 100% = 63.9%