Structure 1.4 – Counting particles by mass : the mole

S1.4.5 – Molar concentration

📌 Concentration

  • Molar concentration is determined by the moles of solute and volume of solution
  • In solutions, the number of moles is represented as a concentration

NUMBER OF MOLES IN A SOLUTION = [CONCENTRATION (in mol/dm3) / 1000] X VOLUME (in cm3)

📌 Dilutions

  • In chemistry, the term standard solutions refers to solutions prepared at a known concentration
  • The exact mass of solute is measured and then carefully added to the solvent in a volumetric flask
  • Standard solutions can be diluted to get different concentrations for specific experiments/reactions
  • Since the number of moles must remain standard throughout the dilution the following formula is used

C1V1 = C2V2

where C1 is the initial concentration, C2is the desired concentration and V2is the desired volume
Using this, we can determine how much (volume) of the initial concentration solution is required

QUESTION : Determine the final concentration of a 75 cm3 solution of HCl of concentration
0.40 mol dm?, which is diluted to a volume of 300 cm3.

ANSWER : (75 x 0.4) = (300 x y)
y = 0.1 mol dm3

📌 Serial dilutions

  • Serial dilution is where the concentration is reduced by a fixed amount at each step
  • A series of solutions of known concentrations can be used under UV-vis spectroscopy which measures absorbance
  • Doing so allows us to plot a calibration curve to then determine the concentration of a solution with knowing the absorbance