Solution concentration and ways to express it. Percent concentration
Concentration is a quantity expressing the amount of solute in a given mass or volume of solution; saying «concentrated» or «dilute» gives no exact information, so percent, molar and normal (an older term still used in school) concentrations are used. Percent concentration (mass fraction) is the ratio of the mass of solute to the mass of solution, multiplied by 100 %: C% = m₁/m₂·100 %, where m₁ is the mass of solute and m₂ the mass of solution (solute + solvent). For example every 100 g of a 12 % glucose solution contains 12 g of glucose and 88 g of water. The other quantities follow: m₁ = C%·m₂/100 %, m₂ = m₁·100 %/C%, and the mass of water m(H₂O) = m₂ – m₁. If the amount of substance (mol) is given, first find m₁ = n·M. The mass and volume of a solution are not equal, so percent concentration uses the mass of the solution. On dilution the mass of solute is unchanged while the mass of the solution grows by the mass of water added.
Practical (with the teacher, with safety precautions): weigh 5 g of table salt and 45 g of water, prepare the solution and calculate its percent concentration. Compare the result with 10 %.