12D - Expressing Concentrations

Question 1

Def: What are the 6 ways to represent concentration, w/ units.

Answer 1

1. Molarity (M)
   Mol-solute / vol-soln
2. Molality (m)
   Mol-solute / Mass-solvent
3. Mole Fraction
   Mol-solute / Mol-Total
4. Parts by Mass
   Mass-solute/Mass-solution * factor
5. Mole percent
   mol-solute / mole-total * 100%
6. Parts by volume
7. Mass-solute/Volume-solution * factor

Factors:
- Percent by mass = 100 %
- ppm = 10^6
- ppb = 10^9

Question 2

Theory: Which concentration expression does not relate solute and solution? What does it relate?

Answer 2

Molality.
Solute and solvent

Question 3

Theory: Why is molality better to work with than molarity?

Answer 3

Volume changes with temperature. Mass does not. Temp changes constantly. Molarity will change and molality willnot.

Question 4

Theory: How do we go from PBM to Molarity?

Answer 4

Assume 100 g.
mass = n * MM for mass of solute.
V = mass/density for volume of soln.

Question 5

Theory: How do we go from molarity to molality?

Answer 5

Assume litre of solution.
V = mass/density for mass of solution.
Subtract mass of solute from mass of solution to get mass of solvent.

Question 6

Theory: How do we go from PBM to Molality?

Answer 6

Assume 100 g of soln.
Subtract mass of solute from mass of solution to get mass of solvent.

Question 7

Theory: How do we convert from molarity to mole fraction?

Answer 7

Assume 1 L of soln.
V = mass/density for mass of solution.
Find mass of solute.
Calculate moles.