# Chemistry Calculations by bKvSFpe

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```									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 Mass Number
e.g. Na = 23, O = 16

Use your data book to find the mass numbers for:
Ca, Cl, S, H, N, C, Ar
Formula Mass
If we add up all the mass numbers 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
1)   NaOH
2)   HCl
3)   O2
4)   HNO3
5)   H2SO4
6)   (NH4)3PO4
(NH4)3PO4
Solution:
Split into 2 halves
1) (NH4)3                 2) PO4
N     = 14 X 1            P = 31 X 1
H     =1X4                O = 16 X 4
NH4 = 18                  PO4 = 95
(NH4)3 = 18 X 3 = 54

54 + 95 = a formula mass of 149
Part 2: The Mole
Units of Measurement:
We use units of measurement in life to give us an
easier way to see large numbers.
metre = 100 cm, litre = 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

The next slide is Higher
understand what a mole is
The Mole
Example:
If I had 1 mole of grapefruits I would have
602,214,150,000,000,000,000,000 (6.02 X 1023) grapefruits

which would be the same size as

If I had 1 mole of H2O I would have
6.02 X 1023 molecules of water.
How big is a mole?
• A mole of sand grains would cover the United States
in approximately one centimetre of sand.
• A human body contains roughly one hundred trillion
cells; there are roughly six billion people on Earth; so
the total number of human cells on the planet is very
close to one mole.
• If you had exactly one mole of sheets of paper, you
could make one million equal stacks from sea level on
the earth that would pass the sun.
• If you had a mole of pennies, you could give out
enough money to everyone in the world so that they
could spend a million dollars every hour, day and
night, for the rest of their lives.
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            = 40
1 mole        = 40g
0.5 moles     = 20g
0.25 moles    = 10g
etc
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

What is the mass of
(n)         0.25 moles N2?
M = n X FM
M = 0.25 X 28
M = 7g
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?
Chemistry Calculations 2
Last Lesson
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?
Mole Calculations

M = n X FM
n  = M/FM
FM = M/n

(n)
1) What is the formula mass of Mg(NO3)2
2) What is the formula mass of potassium
carbonate?
3) Calculate the number of moles of 5.6g of CO
4) Calculate the number of moles of 8.2g calcium
nitrate
5) Calculate the mass of 0.2 moles of
magnesium oxide
6) Calculate the mass of 0.25 mole of NaCl
Learning Intentions
1) What is concentration?
2) What are the units for concentration?
3) How do we calculate moles in solutions?
Part 1: Concentration
This bottle of acid is labelled “conc.”
What does that mean?

Today we learn how to work out how “strong” a solution is
Concentration
“ the amount of solute (the thing
that gets dissolved) dissolved in
a volume of solvent (the thing
that does the dissolving)”
Part 2: Units for Concentration
Moles per litre written as mol/L or mol l-1

Note: on bottles of solutions we may see
concentration labelled as “M” (e.g. 1M)
Part 3: Mole Calculations
To work out the exact concentration of a
solution we use this triangle:
Number of
Moles

Volume
(L)

Concentration
(mol/L)
Example
If I want to make 0.5L of KOH (potassium hydroxide)
that has a concentration of 1mol/L how many moles
KOH do I need to dissolve?

n=CXV
n = 1 mol/L X 0.5L
n = 0.5 moles KOH needed
Example 2
What is the concentration of a solution of HCl that
contains 0.1 moles of HCl dissolved in 250cm3
Convert to litres!
C = n/V
C = 0.1/0.25L
C = 0.004 mol/L
Using both triangles together
For some calculations we need to use
both triangles together
Calculate the number of grams of NaOH needed to
make 50cm3 of NaOH with a concentration of 2 mol/L

First Triangle
n=?               n=CXV
C = 2 mol/L       n = 2mol/L X 0.05L
V = 0.05L         n = 0.1 mole NaOH
Second Triangle
M =?                M = n X FM
n = 0.1 moles       M = 0.1 moles X 40
FM = 40.            M = 4g
4g NaOH to make 50cm3 solution with a concentration of 2 mol/L

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