1. Chemistry Calculations

2. Learning Intentions How can we find the formula mass of a substance?
How can we use formula mass and moles to figure out grams of a substance?

3. Relative Atomic Mass Each element has a
Relative atomic Mass Number (RAM)
e.g. Na = 23.0, O = 16.0
We always round to the tenths place
Task:
Use your periodic table to find the RAM for:
(always round to tenths place)
Ca, Cl, S, H, N, C, Ar

4. Formula Mass Formula Mass If we add up all the RAM numbers present
in a chemical compound we get the
Formula Mass
Example:
H2O =
2 X Hydrogen + 1 X Oxygen
RAM Numbers
= 2 X X16.0
= A formula mass of 18.0g (tenths place)

5. the Formula Mass in grams
The Mole
If I could weigh 6.02 X 1023 molecules of water
it would weigh 18.0g
The formula mass of water=18.0g
1 mole of a substance is equal to
the Formula Mass in grams
Therefore the formula mass, molar mass or molecular mass of water = 18.0g

6. Task:
Work out the formula mass of:
NaOH
HCl O2
HNO3
H2SO4
(NH4)3PO4

7. Example I have 40.0g of NaOH The Formula Mass (FM) = 40.0g
Therefore I have 1 mole NaOH
FM (1 mole) = 40.0g
2.00 mole = 80.0g
0.500 moles = 20.0g
0.250 moles = 10.0g
etc

8. Mole Calculations M = n X FM n = M/FM FM = M/n
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
(M)
(n)
(FM)

9. Example If I have 9.0g of water, how many moles do I have?
n = M/FM
n = 9.0g/18.0
n = 0.50
What is the mass of
0.25 moles N2?
(n)
M = n X FM
M = 0.25 X 28.0
M = 7.0g

10. Task: 1) If I have 90.0g water how many moles do I have?
2) If I have 5.00g NaOH how many moles do I have?
3) If I have 0.50 moles O2 how many grams do I have?