How to calculate Molarity
• Molarity is a concentration
– Think: orange juice concentrate
• 1 can OJ in 1 pitcher of water = full strength OJ (or 1M, keep reading)
• Molecular weight of another salt: KCl
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Atomic Mass of Cl = 35.5
Atomic Mass of K = 39.1
Total Molecular weight of KCl = 74.6
This is the mass of 1 mole of KCl
• Great!!!…but…can’t measure that in the lab…hmmm
– We can measure grams/mole
• So, if we add 74.6g of KCl to 1L of H2O, we end up with a
1 Molar solution or 1M
Test your knowledge…
• I add 74.6g of KCl to 500mL of water. What is the
molarity?
• I add 37.3g of KCl to 500mL of water, what is the
molarity?
• I add 37.3g of KCl to 1L of water, what is the
molarity?
What is pH?
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Stands for “power of hydrogen”
Measures free H+ in solution
pH 1-6 = ACID (lots of free H+)
pH 7 = neutral (free H+ = free OH-)
pH 8-14 = BASE (lots of free OH-)
• Fantastic, but how the heck do we measure it?
Using pH paper of course!!!
• Use a small amount of
paper for each
solution
• Dip, then lay out to dry
• Compare to color
scale
Serial Dilution – Procedure 2
Transfer 1mL of
1M from Tube 1
FIRST
Measure pH
of 1M HCl
Then transfer 1mL
Then transfer 1mL
1) MIX
2) pH
3) pipet
1) MIX
2) pH
3) pipet
1) MIX
2) pH
9mL
H2O
9mL
H2O
9mL
H2O
2mL
1mL
1mL
1mL
1M
100mM
10mM
1mM
1) pH
2) pipet
SAVE! And use for Procedures 3 & 4
Parafilm and Vortex
Hold down to mix, neat!
Stretch to cover tube to
mix contents
Bromocresol purple – pH indicator
• Purple when basic
• Yellow when acidic
• Add 4 drops to each
tube (a 1 time thing!!)
• To test your antacids: adding 100mM acid 1 drop at a
time & mixing!
• Can increase to 5-10 drops if taking a lot of acid
• Watch for a quick yellow color change after mixing
Buffer quick info
• A good buffer strongly resists change in pH
• Can work at any pH!
– Acidic buffers keep the solution acidic
• Example: a buffer found in most colas - - phosphoric acid
& sodium phosphate
– Basic buffers keep the solution basic
• Example: a buffer found in aspirins and other medications
- - calcium carbonate