Download Chemistry Calculations

Chemistry Calculations
Learning Intentions
1) How can we work out the formula mass
of a chemical?
2) What is a mole?
3) How can we use formula mass and
moles to work out grams of a chemical?
Part 1: Formula Mass
Each element has a
Relative Atomic Mass Number
e.g. Na = 23, O = 16
Task:
Use your data book to find the Atomic Mass for:
Ca, Cl, S, H, N, C, Ar
Formula Mass
If we add up all the RAM nummbers present
in a chemical we get the
Formula Mass number
Example:
H2O = 2 X Hydrogen + 1 X Oxygen Mass Numbers
= 2X1
+ 1 X 16
= A formula mass of 18
Task:
On your show me boards work out the
formula mass of:
1) NaOH
2) HCl
3) O2
4) HNO3
5) H2SO4
6) (NH4)3PO4
(NH4)3PO4
Solution:
Split into 2 halves
1) (NH4)3
N
= 14 X 1
H
=1X4
NH4 = 18
(NH4)3 = 18 X 3 = 54
2) PO4
P = 31 X 1
O = 16 X 4
PO4 = 95
54 + 95 = a formula mass of 149
Learning Intentions
1) How can we work out the formula mass
of a chemical?
2) What is a mole?
3) How can we use formula mass and
moles to work out grams of a chemical?
Part 2: The Mole
Units of Measurement:
We use units of measurement in life to give us an
easier way to see large numbers.
meter = 100 cm, liter = 1000 ml, kilogram = 1000 g
The mole is a unit of measurement
In chemistry we use it to tell us how many
atoms or molecules there are
The Mole
If I could weigh 6.02 X 1023 molecules of water
it would weigh 18g
Q) What is the formula mass of water?
18
Q) What is the relationship between the
formula mass and 1 mole of a chemical?
1 mole of a chemical is equal to
the Formula Mass in grams
Example
I have 40g of NaOH
The Formula Mass (FM) = 40
Therefore I have 1 mole NaOH
FM
1 mole
0.5 moles
0.25 moles
etc
= 40
= 40g
= 20g
= 10g
Learning Intentions
1) How can we work out the formula mass
of a chemical?
2) What is a mole?
3) How can we use formula mass and
moles to work out grams of a chemical?
Part 3: Mole Calculations
We can use moles and formula mass to work
out mass of chemicals needed/used
M = n X FM
n
= M/FM
FM = M/n
(n)
Example
If I have 9g of water, how many
moles do I have?
n = M/FM
n = 9g/18
n = 0.5
(n)
What is the mass of
0.25 moles N2?
M = n X FM
M = 0.25 X 28
M = 7g
Task:
1) If I have 9g water how many moles do I
have?
2) If I have 5g NaOH how many moles do I
have?
3) If I have 0.5 moles O2 how many grams
do I have?
Learning Intentions
1) How can we work out the formula mass
of a chemical?
2) What is a mole?
3) How can we use formula mass and
moles to work out grams of a chemical?