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Topic 5: Formulae, Equations and Amounts of Substance
5.3 Amounts of Substance
5.3.1 The Mole Concept
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What is the fundamental unit in chemistry that measures the amount of a substance?
Mole
One mole contains
6.02
Avogadro's constant is approximately
6.02 x 10²³
particles per mole.
What formula is used to calculate the number of moles given the number of entities and Avogadro's constant?
n
=
n =
n
=
N
N
A
\frac{N}{N_{A}}
N
A
N
In the formula
n
=
n =
n
=
N
N
A
\frac{N}{N_{A}}
N
A
N
,
N
A
N_{A}
N
A
represents Avogadro's
What is the numerical value of Avogadro's number?
6.02
×
1
0
23
6.02 \times 10^{23}
6.02
×
1
0
23
Avogadro's number links the macroscopic amount of a substance to the number of microscopic
entities
Avogadro's number is a constant that relates the number of
particles
in a mole to the amount of the substance.
What formula is used to calculate the number of particles in a given number of moles using Avogadro's number?
N
=
N =
N
=
n
×
N
A
n \times N_{A}
n
×
N
A
Avogadro's number is equal to
6.022
×
1
0
23
6.022 \times 10^{23}
6.022
×
1
0
23
Molar mass is the mass of one mole of a substance expressed in grams per mole (
g/mol
).
What formula is used to calculate the number of moles given the mass and molar mass of a substance?
n
=
n =
n
=
m
M
\frac{m}{M}
M
m
The molar mass of a compound can be calculated by adding the atomic masses of all its
elements
Molar mass is typically expressed in
grams per mole
(g/mol).
Steps to calculate the molar mass of a compound
1️⃣ List each element in the compound.
2️⃣ Find the atomic mass of each element from the periodic table.
3️⃣ Multiply the atomic mass by the number of atoms of each element.
4️⃣ Sum the resulting values to get the total molar mass.
What is the SI unit for measuring the amount of a substance?
Mole
One mole contains
6.022
×
1
0
23
6.022 \times 10^{23}
6.022
×
1
0
23
entities, which is Avogadro's constant.
Avogadro's number links the macroscopic amount of a substance to the number of microscopic
entities
What formula is used to find the number of moles
(
n
)
(n)
(
n
)
given the number of entities
(
N
)
(N)
(
N
)
?
n
=
n =
n
=
N
N
A
\frac{N}{N_{A}}
N
A
N
In the formula
n
=
n =
n
=
N
N
A
\frac{N}{N_{A}}
N
A
N
,
N
N
N
represents the number of entities
To find the number of moles in
1.8066
×
1
0
24
1.8066 \times 10^{24}
1.8066
×
1
0
24
molecules of glucose, you would divide by Avogadro's number.
What is the numerical value of Avogadro's number
(
N
A
)
(N_{A})
(
N
A
)
?
6.022
×
1
0
23
6.022 \times 10^{23}
6.022
×
1
0
23
Avogadro's number provides a link between the macroscopic amount of a substance and the number of microscopic
entities
What is the definition of molar mass
(
M
)
(M)
(
M
)
?
Mass of one mole
The formula to calculate moles from mass is
n
=
n =
n
=
m
M
\frac{m}{M}
M
m
.
What units is molar mass typically expressed in?
g / mol
\text{g / mol}
g / mol
The molar mass of NaOH is calculated as
22.99
+
22.99 +
22.99
+
16.00
+
16.00 +
16.00
+
1.01
=
1.01 =
1.01
=
40.00
g / mol
40.00 \text{ g / mol}
40.00
g / mol
, resulting in 0.5 moles in 20 grams of NaOH.
Steps to calculate the molar mass of a compound
1️⃣ List each element in the compound
2️⃣ Find the atomic mass of each element from the periodic table
3️⃣ Multiply the atomic mass by the number of atoms
4️⃣ Sum the resulting values
What is the molar mass of H₂O (water)?
18.016 \text{ g / mol}</latex>
To convert mass to moles, use the formula
n
=
n =
n
=
m
M
\frac{m}{M}
M
m
, where
M
M
M
is the molar mass.
To convert moles to mass, use the formula m = n \times
M
</latex>.
How many moles are in 50 grams of NaCl (sodium chloride)?
0.856
moles
0.856 \text{ moles}
0.856
moles
The molar mass of H₂O is
18.016
g / mol
18.016 \text{ g / mol}
18.016
g / mol
, so 2 moles of H₂O have a mass of 36.032 grams.
Balancing chemical equations requires determining the
molar ratios
of reactants and products.
Steps to balance a chemical equation using molar ratios
1️⃣ Write down the unbalanced equation
2️⃣ Determine the molar ratios by balancing atoms
3️⃣ Identify the moles of each substance
In the balanced equation
2
H
2
+
2\text{H}_{2} +
2
H
2
+
O
2
→
2
H
2
O
\text{O}_{2} \rightarrow 2\text{H}_{2}\text{O}
O
2
→
2
H
2
O
, how many moles of oxygen react with 2 moles of hydrogen?
1
mole
1 \text{ mole}
1
mole
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