Concentration(not the “thinking very hard” kind of concentration)

Concentration MattersSometimes, how much stuff you have is less important than how concentrated it is.

Consider two bottles of Kool Aid, each with 1 scoop of powder added:

vs

Concentration MattersSometimes, how much stuff you have is less important than how concentrated it is.

Consider two bottles of Kool Aid, each with 1 scoop of powder added:

vsI’m going to drink the good one

Concentration UnitsAll units of concentration are a ratio (what else?):

amount of stuff dissolved vs how much it’s dissolved in(solute) (solvent)

The mixture of the two is a solution

There are many units out there, but most of them are bad:

Concentration UnitsAll units of concentration are a ratio (what else?):

amount of stuff dissolved vs how much it’s dissolved in(solute) (solvent)

There are many units out there, but most of them are bad:

Percent: The vinegar was listed as 5% acid. The problem is, 5% by what? The unit is unclear

5% by weight ≠ 5% by volume ≠ 5% by moles

Concentration UnitsAll units of concentration are a ratio (what else?):

amount of stuff dissolved vs how much it’s dissolved in(solute) (solvent)

There are many units out there, but most of them are bad:

Percent: The vinegar was listed as 5% acid. The problem is, 5% by what? The unit is unclear

5% by weight ≠ 5% by volume ≠ 5% by moles

Mass per volume: usually given in grams per 100 mL. This is better, because at least you know the units, but there’s still a problem:

5 g/100 mL NaCl (aq) is more concentrated than 5 g/100 mL CaCl2 (aq)

Calcium chloride is heavy, so five grams is actually less stuff (fewer formula units)

Molarity

Molarity

An Annoying DetailLet’s say I’m making that 3M salt solution. I go and get myself 6 moles of salt, and 2 liters of water:

+

An Annoying DetailWhen I mix them, it won’t be 2L anymore—the salt takes up space.

Proper ProcedurePut your 6 moles of salt in, then fill it up until the total gets to 2L.

Why Molarity?

Summary