SCH4C Unit 2
Date: _______________________________
Chapter 2: Quantities in Chemistry
1. Determining the Mass of Atoms
Recall:
Particle
Proton
neutron
Electron
Relative Mass (a.m.u.)
Relative charge
Location within the atom
Isotopes
________________________________________________________________________________________________
________________________________________________________________________________________________
E.g. naturally occurring magnesium contains three isotopes: Mg-24, Mg-25, and Mg-26
That is why on the periodic table, Magnesium has the mass number that is not a whole number.
_____________ is the average mass of the 3 types of isotopes with respect to its abundance.
Radioisotopes: ___________________________________________________________________________________
 E.g. all isotopes of uranium are radioisotopes, there are no stable uranium isotopes
 Archaeological dating can be done by calculating the decay of carbon-14 and potassium-40
 Depending on the type of radiation (alpha, beta, or gamma), has different penetrating abilities (gamma being
most dangerous, can penetrate through paper, aluminum, but not lead)
2. Atomic Mass & Molecular Mass
Atomic mass: ___________________________________________________________________________________
________________________________________________________________________________________________
E.g. what is the atomic mass of Aluminum?
mAl =
Molecular mass: _________________________________________________________________________________
________________________________________________________________________________________________
E.g. what is the molecular mass of Water?
mH2O =
Formula unit mass: _______________________________________________________________________________
________________________________________________________________________________________________
E.g. what is the formula mass unit of sodium chloride?
mNaCl =
3. The Mole and Molar Mass
In 1811, a chemist named __________________________________ realized that matter contained an enormous
number of chemical entities (individual atoms, ions, formula units, or molecules). Using this idea, chemist
___________________________________ showed that approximately 602,000,000,000,000,000,000,000, or
________________________, is a convenient and measurable number of entities to deal with in chemistry. This
quantity is called the _____________.
Avogadro’s constant (NA)
ONE MOLE =
(The SI unit is mol)
e.g. 1 mol of oxygen atoms is ____________________ atoms of oxygen
e.g. 1 mol of sodium ions is ____________________ of sodium ions
e.g. 1 mol of water is ______________________ of water molecules
So why is 6.02 x 1023 used to represent the mole? The result of many experiments show that 6.02 x 1023 atoms of any
element have a mass, in grams, that is equal to the numerical value of the elements atomic mass (in relative atomic
mass units, u)
e.g.
1 atom of carbon has the mass ____________ u
1 mol of carbon has a mass of ___________ g
e.g.
1 atom of iron has a mass of __________ u
1 mol of iron has a mass of _________ g
Therefore, the numerical value of an element’s atomic mass (in u) is equal to the numerical value of the mass (in
grams) of one mole (6.02 x 1023) of the element’s atoms.
Molar Mass Unit: the mass, in grams, of one mole of a chemical quantity
𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 =
𝑚𝑎𝑠𝑠
𝑀𝑚 =
𝑎𝑚𝑜𝑢𝑛𝑡 (𝑖𝑛 𝑚𝑜𝑙𝑒𝑠)
𝑚
𝑛
Measured in units of gram per mole, or g/mol.
You may get the molar mass of an element directly from a periodic table:
e.g.
1 mol of Hydrogen has the mass of 1.01 g
therefore, the molar mass of hydrogen is MmH = 1.01 g/mol
e.g.
what is molar mass of calcium? __________________________
Summary of Quantity Symbols and Units
Symbol
n
m
Mm
N
NA
Quantity
Unit
mol
g
g/mol
atoms, ions, formula units, molecules
--- [no unit]
4. The molar mass of molecules and ionic compounds
The molar mass of a molecule, or the formula unit of an ionic compound, is equal to ___________________________
________________________________________________________________________________________________
________________________________________________________________________________________________
E.g. the molar mass of water, MH2O, is equal to the sum of the molar mass of two hydrogen atoms and the molar mass
of one oxygen atom:
MmH2O = 2(MH) + 1(MO)
= 2(1.01g/mol) + 1(16.00g/mol)
= 18.02 g/mol
E.g. what is molar mass of sodium chloride?
E.g. what is molar mass of Oxygen? (Hint: diatomic!)
MmNaCl =
MmO2 =
5. Calculating molar mass given an amount of moles
You can calculate the molar mass if an entity (__________________________________________________________)
if you know the mass (in grams) of a given amount (in moles).
E.g. Calculate the molar mass of magnesium, MMg, if 0.330 mol has a mass of 8.02 g.
𝑀𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 =
𝑚𝑎𝑠𝑠
𝑎𝑚𝑜𝑢𝑛𝑡 (𝑖𝑛 𝑚𝑜𝑙𝑒𝑠)
E.g. calculate the molar mass of strontium if 1.65 mol has a mass of 144.57 g.
6. Calculating Number of Entities from Mass
Since chemists cannot count atoms or molecules one at a time, they must “count” them indirectly by measuring out a
certain mass of substance and calculating the number of moles of chemical entities contained. Industries use a similar
method to estimate the number of objects in a large collection of objects. The mass of a small number of objects is
determined first, and this value is used to determine the total number of objects in the whole collection.
E.g. how many jellybeans are in a box that contains 250g if jellybeans, of 15 jellybeans have a mass of 10.0g?
Given:
Key value =
Conversion factor =
Required:
Solution:
required value = Key value x Conversion factor
E.g. How many oxygen molecules, O2, are in a cylinder that contains 48.0 g of oxygen gas, if 6.02 x 1023 oxygen
molecules have a mass of 32.00 g?
Given:
Required:
Solution:
E.g. what amount of iodine molecules, I2(s), is in 1.231 kg of iodine, if 2 mol of iodine has a mass of 253.8 g?
Given:
Required:
Solution:
E.g. how many molecules of water are in a glass that contains 54.0 g of water, if 6.02 x 1023 molecules of water
have a mass of 18.02g?
Given:
Required:
Solution: