UNIT 1
.1.4 - Counting particles by mass: The mole
S
.4.1:The mole
1
- The mole (mol) is the SI unit of amount of substance.One mole contains exactly the number of elementaryentities given by the
Avogadro constant.
- An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or a specified group of particles
- The Avogadro constant NAhas the units mol–1
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑁 𝑁
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑚𝑜𝑙𝑒𝑠 = 23 𝑛 = 𝑁𝐴
or 𝑁 ×𝑛
6.02×10 𝐴
.4.2: Masses of atoms
1
- Masses of atoms are compared on a scale relative to12C and are expressed as relative atomic mass Ar and relative formula mass Mr.
- Relative atomic mass and relative formula mass have no units.
- RAM (Ar): the weighted average of one atom of anelement relative to 1/12th of one atom of12.C.
- RMM(Mr): the weighted average mass of a molecule relativeto 1/12th of one atom of12C.
- RFM(Mr) : the weighted average mass of a formula unitrelative to 1/12th of one atom of12C.
- Molar Mass (M): the mass of one mole of a substance, expressed in grams
- The RAM’s are often not exact numbers because they include the mass of the other isotopes of the atoms within their weighted
averages
- There are no units for RAM, RMM and RFM.
𝑀𝑟 = 𝑠𝑢𝑚𝑜𝑓𝑡ℎ𝑒𝐴𝑟 '𝑠𝑖𝑛𝑣𝑜𝑙𝑣𝑒𝑑
.4.3: Molar mass
1
- Molar mass M has the units gmol–1
- The relationship n = m/M is given in the data booklet
𝑚𝑎𝑠𝑠 𝑚
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑚𝑜𝑙𝑒𝑠×
𝑚𝑜𝑙𝑎𝑟𝑚𝑎𝑠𝑠
= 𝑛×𝑀
𝑁 𝑚
- 𝑁𝐴
= 𝑛 =
𝑀
.4.4: Empirical formula
1
- The empirical formula of a compound gives the simplest ratio of atoms of each element present in that compound.
- The molecular formula gives the actual number of atoms of each element present in a molecule.
𝐴𝑟( 𝑡𝑜𝑡𝑎𝑙)
- %𝑏𝑦𝑚𝑎𝑠𝑠 = 𝑀𝑟
× 100
- % by mass to empirical formula ↓
element x y
% by mass
Mass in grams
Ar values
n=m/Ar
Divide all above values by the smallest value to get ratio
, (r)
Find the simplest whole number ratio
T he molecular formula
-
1. Find mass of empirical formula unit
a. Mass = Mr
2. Divide the given molar mass by the mass of the empirical formula unit to find the multiplication factor
a. M/empirical unit mass (Mr)
3 . M
ultiply all subscript values by that number
How can experimental data on mass changes in combustion reactions be used to derive empirical formulas?
-
- The change in mass can be used to find the masses of two elementary entities that have reacted together. These masses can be used to
find the percentage composition and therefore the empirical formula.
- A hydrate is a crystalline solid which has water molecules loosely attached to it.
- An anhydrate is the form of the ionic salt which has no water molecules attached to it.
- Dehydration is the process of removing water from a hydrate, usually through applied heat.
.4.5: Molar concentration
1
- The molar concentration is determined by theamountof soluteand thevolume of solution.
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑚𝑜𝑙𝑒𝑠 𝑛
- 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛× 𝑣𝑜𝑙𝑢𝑚𝑒
= 𝑐×
𝑣
𝑚
- 𝑐 × 𝑣 = 𝑛 = 𝑀
- hat are the considerations in the choice of glassware used in preparing a standard solution and a serial dilution?
W
- To prepare a standard solution, a piece of lab equipment called a volumetric flask should be used. These flasks range in size and are
carefully calibrated to a single volume. On the narrow stem is a calibration mark. The precise mass of solute is dissolved in a bit of
the solvent, and this is added to the flask. Then, enough solvent is added to the flask until the level reaches the calibration mark. A
pipette should also be used to get more accurate results.
- A serial dilution is a technique to create a series of decreasing concentrations of solutions. The original solution is diluted to make
a new solution, then some of that solution is used to make the next and so on.
- ow can a calibration curve be used to determine the concentration of a solution?
H
- The calibration curve is a technique where you use known concentrations of solutions to create a graph to show how concentration
affects the absorbance of light. This then allows us to find the concentration of an unknown solution. If we know the absorbance
reading, we can determine the concentration. This is because “The amount of absorbed light is proportional to the solution
concentration”.
- ote that
N
- The units that we will most commonly use are in terms ofmoldm-3.This is also known as ‘Molarity’ andcan have the symbol ‘M’ (a 1M
solution means a 1 moldm-3 solution)
c onvert from concentration in gdm to moldm andvice versa
-
-3 -3
−1
- 𝑔 ÷ 𝑔𝑚𝑜𝑙 = 𝑚𝑜𝑙
−1
- 𝑚𝑜𝑙 × 𝑔𝑚𝑜𝑙 = 𝑔
- Unit conversions
- 1 Litre = 1dm3
.1.4 - Counting particles by mass: The mole
S
.4.1:The mole
1
- The mole (mol) is the SI unit of amount of substance.One mole contains exactly the number of elementaryentities given by the
Avogadro constant.
- An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or a specified group of particles
- The Avogadro constant NAhas the units mol–1
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑁 𝑁
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑚𝑜𝑙𝑒𝑠 = 23 𝑛 = 𝑁𝐴
or 𝑁 ×𝑛
6.02×10 𝐴
.4.2: Masses of atoms
1
- Masses of atoms are compared on a scale relative to12C and are expressed as relative atomic mass Ar and relative formula mass Mr.
- Relative atomic mass and relative formula mass have no units.
- RAM (Ar): the weighted average of one atom of anelement relative to 1/12th of one atom of12.C.
- RMM(Mr): the weighted average mass of a molecule relativeto 1/12th of one atom of12C.
- RFM(Mr) : the weighted average mass of a formula unitrelative to 1/12th of one atom of12C.
- Molar Mass (M): the mass of one mole of a substance, expressed in grams
- The RAM’s are often not exact numbers because they include the mass of the other isotopes of the atoms within their weighted
averages
- There are no units for RAM, RMM and RFM.
𝑀𝑟 = 𝑠𝑢𝑚𝑜𝑓𝑡ℎ𝑒𝐴𝑟 '𝑠𝑖𝑛𝑣𝑜𝑙𝑣𝑒𝑑
.4.3: Molar mass
1
- Molar mass M has the units gmol–1
- The relationship n = m/M is given in the data booklet
𝑚𝑎𝑠𝑠 𝑚
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑚𝑜𝑙𝑒𝑠×
𝑚𝑜𝑙𝑎𝑟𝑚𝑎𝑠𝑠
= 𝑛×𝑀
𝑁 𝑚
- 𝑁𝐴
= 𝑛 =
𝑀
.4.4: Empirical formula
1
- The empirical formula of a compound gives the simplest ratio of atoms of each element present in that compound.
- The molecular formula gives the actual number of atoms of each element present in a molecule.
𝐴𝑟( 𝑡𝑜𝑡𝑎𝑙)
- %𝑏𝑦𝑚𝑎𝑠𝑠 = 𝑀𝑟
× 100
- % by mass to empirical formula ↓
element x y
% by mass
Mass in grams
Ar values
n=m/Ar
Divide all above values by the smallest value to get ratio
, (r)
Find the simplest whole number ratio
T he molecular formula
-
1. Find mass of empirical formula unit
a. Mass = Mr
2. Divide the given molar mass by the mass of the empirical formula unit to find the multiplication factor
a. M/empirical unit mass (Mr)
3 . M
ultiply all subscript values by that number
How can experimental data on mass changes in combustion reactions be used to derive empirical formulas?
-
- The change in mass can be used to find the masses of two elementary entities that have reacted together. These masses can be used to
find the percentage composition and therefore the empirical formula.
- A hydrate is a crystalline solid which has water molecules loosely attached to it.
- An anhydrate is the form of the ionic salt which has no water molecules attached to it.
- Dehydration is the process of removing water from a hydrate, usually through applied heat.
.4.5: Molar concentration
1
- The molar concentration is determined by theamountof soluteand thevolume of solution.
𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑚𝑜𝑙𝑒𝑠 𝑛
- 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛× 𝑣𝑜𝑙𝑢𝑚𝑒
= 𝑐×
𝑣
𝑚
- 𝑐 × 𝑣 = 𝑛 = 𝑀
- hat are the considerations in the choice of glassware used in preparing a standard solution and a serial dilution?
W
- To prepare a standard solution, a piece of lab equipment called a volumetric flask should be used. These flasks range in size and are
carefully calibrated to a single volume. On the narrow stem is a calibration mark. The precise mass of solute is dissolved in a bit of
the solvent, and this is added to the flask. Then, enough solvent is added to the flask until the level reaches the calibration mark. A
pipette should also be used to get more accurate results.
- A serial dilution is a technique to create a series of decreasing concentrations of solutions. The original solution is diluted to make
a new solution, then some of that solution is used to make the next and so on.
- ow can a calibration curve be used to determine the concentration of a solution?
H
- The calibration curve is a technique where you use known concentrations of solutions to create a graph to show how concentration
affects the absorbance of light. This then allows us to find the concentration of an unknown solution. If we know the absorbance
reading, we can determine the concentration. This is because “The amount of absorbed light is proportional to the solution
concentration”.
- ote that
N
- The units that we will most commonly use are in terms ofmoldm-3.This is also known as ‘Molarity’ andcan have the symbol ‘M’ (a 1M
solution means a 1 moldm-3 solution)
c onvert from concentration in gdm to moldm andvice versa
-
-3 -3
−1
- 𝑔 ÷ 𝑔𝑚𝑜𝑙 = 𝑚𝑜𝑙
−1
- 𝑚𝑜𝑙 × 𝑔𝑚𝑜𝑙 = 𝑔
- Unit conversions
- 1 Litre = 1dm3
