Working with Solutions

Question 1

Review: Concentration & Solubility

Answer 1

Concentration
The quantity of solute per unit quantity of solvent or solution
Examples:
amount concentration (mol/L)
“100 g H2O” concentration (g/100 g H2O)

Solubility
The maximum possible concentration of a solute at specific temperature and pressure

Question 2

Other ways to measure concentration - percent composition

Answer 2

Percentage concentration
Percentage volume/volume (% V/V)
(for liquid-liquid solutions)
Percentage weight/volume (% W/V [g/mL] )
(for solid-liquid solutions)
Percentage weight/weight (% W/W)
(solid-liquid solutions or solid-solid solutions)

Question 3

Special case of %W/W concentration: “parts per” concentration

Answer 3

Scientists working in the medical or environmental fields often measure concentrations that are extremely low
Expressing these concentrations as amount concentrations or percentage concentrations gives awkward numbers to work with!
For this reason, very dilute concentrations are often expressed in parts per million (ppm, 1:106), parts per billion (ppb, 1:109), and even parts per trillion (ppt, 1:1012)

Question 4

Parts per concentration formula

Answer 4

Cppm = msolute/msolution x 10^6 (million)
make sure to change the symbol for ppx!!!

Question 5

Preparing solutions

Answer 5

Many chemical reactions involve aqueous solutions!

In order to get reliable data, it is important that the reactant solutions be carefully prepared with known concentrations

We always start with a standard solution: a solution for which the precise concentration is known

Question 6

Volumetric Flask

Answer 6

Standard solutions are prepared using a volumetric flask
First, a known quantity of solute is dissolved in about one quarter of the volume of solvent
Then, additional solvent is carefully added until the bottom of the solution’s meniscus lines up with the calibration mark on the flask

Question 7

Dilution

Answer 7

Usually, our standard solution is more concentrated than we need for our reaction
We can reduce the concentration of the solution by adding more solvent – this process is called dilution
Achieving a desired concentration requires transferring precise volumes of liquid, so graduated pipettes are often used

Question 8

calculations involving dilution

Answer 8

KEY CONCEPT: the amount (moles) of solute remains constant!
n1 is the moles of solute in concentrated (original) solution
n2 is the moles of solute in diluted (new) solution
n1=n2
since n=CV, C1V1=C2V2
Be careful! V2 is the TOTAL final volume, not the volume of new solvent added!