The concentration of a solution is the starting point for chemical calculations. It is necessary to use mass, volume, and amount in moles appropriately and to convert among them.
Basic Knowledge
Main concentration expressions: ① mass percent concentration = (mass of solute/mass of solution)×100 [%], ② molar concentration = amount of solute in moles / solution volume [mol/L], ③ molality = amount of solute in moles / mass of solvent [mol/kg], ④ mole fraction = amount of solute in moles / total amount in moles. If the density ρ of the solution is known, mass percent and molar concentration can be converted into each other: c = 1000ρw/(100M).
Mass % concentration (w = msolute/msolution×100)
Molar concentration c (n/V [mol/L])
Molality m (n/msolvent(kg) [mol/kg])
Conversion equation (c = 1000ρw/(100M))
Dilution equation (c1V1 = c2V2)
ppm (concentration of trace components at the 10-6 level)
Further Study (Principles and Applications)
Molar concentration is volume-based and depends on temperature. Molality and mole fraction do not depend on temperature, so they are used in calculations involving colligative properties. Environmental analysis uses ppm (10-6) and ppb (10-9); chlorine in tap water is approximately 0.4 ppm. Dilution calculations are also essential in pharmacy and clinical testing.
- Molar concentration = volume-based; molality = solvent-mass-based
- Molality is used for colligative properties
- Dilution equation c1V1=c2V2
- ppm ≒ mg/L (for water)
- Density is essential for converting between mass % and molar concentration
- Solution volume is not the simple sum of solute and solvent volumes
- Prepare standard solutions using a volumetric flask
Important Notes (Common Confusions and Frequent Mistakes)
① Distinguish molar concentration (mol/L) from molality (mol/kg) by their units. ② Mass percent is divided by the mass of the solution, not the mass of the solvent. ③ During dilution, the amount of solute in moles remains constant. ④ Volume is not the simple sum of solute and solvent volumes.
Practice
- When 20 g of NaOH (M=40) is dissolved in 500 g of water, find the molality.
- Find the molar concentration of HCl (density 1.18 g/cm3, mass %=35%, M=36.5).
- How many times must 2.0 mol/L sulfuric acid be diluted to obtain 0.10 mol/L?