III.4 B ASIC LABORATORY PROCEDURES
III.4.8 Dissolution
Dissolution is a process by which a solute forms a solution in a solvent. The maximum quantity of the dissolved compound depends on the nature of the solvent and the solute, the temperature and the pressure, in the case of gases.
When a solid is to be dissolved, it should be pulverized in a mortar to increase the contact surface of it. Dissolution can also be accelerated by heating and stirring. Stirring of liquids can be made by tools made of glass or metal. If homogenization needs relatively long time, the stirrer can be electrically operated. A frequently used device is the magnetic stirrer. A magnetic stirrer is a laboratory device that employs a rotating magnetic field to cause a stir bar immersed in a liquid to spin very quickly, thus stirring it. The stir bar is a small iron bar covered by glass or plastic. This procedure ensures effective homogenization of the solution.
Concentration of solutions provides information on the quantitative relation of solute and solvent. Definition of the most frequently used concentrations is discussed in Chapter V.) Depending on the quantity of solute and solvent, the solution can be unsaturated, saturated or supersaturated.
Solubility is the property of a substance to dissolve in a solvent to form a homogeneous solution. It is numerically characterized by the amount of solute (g) dissolved in 100 g of solvent. Solubility is the concentration of the saturated solution. A saturated solution is one in which the dissolved solute is in equilibrium with the non-dissolved solute; adding more solute does not increase the concentration of the solution.
Solubility of a substance fundamentally depends on the physical and chemical properties of the substance and solvent as well as on the temperature, pressure (in the case of dissolution of gases in liquids) and the pH of the solution.
Solutions with definite concentrations can be prepared in two ways:
Preparation of a solution from a solid substance. When a solid substance is 1.
available, the required amount of substance is weighed out on a balance and transferred into a volumetric flask. When the compound is dissolved, the flask is filled with the solvent.
Preparation of a solution from a “stock solution”. This way a diluted solution is 2.
prepared from a more concentrated solution of known concentration (“stock solution”).
62 The project is supported by the European Union and co-financed by the European Social Fund Calculation examples for making of solutions
III.4.8.1
What volume (cm3) of 96.0 m/m% concentrated sulphuric acid (ρ = 1.86 g/cm3) is 1.
needed to prepare 0.40 dm3 of 10.0 m/m% (ρ = 1.09 g/cm3) sulphuric acid solution?
Mass of the 0.40 dm3(ρ = 1.09 g/cm3) sulphuric acid solution:
m = V . ρ = 400.0 cm3 1.09 g/cm3 = 436.0 g Mass of sulphuric acid in the solution:
100.0 g solution 10.0 g sulphuric acid
436.0 g x g sulphuric acid
x = 43.6 g Mass of containing 43.6 g of sulphuric acid:
in 100 g solution there is 96.0 g solute
in y g 43.6 g
y = 45.42 g
Volume of the 45.42 g concentrated sulphuric acid:
𝑉 = 𝑚
𝜌 = 45.42
1.86 = 24.42 𝑐𝑚3
Thus, 24.42 cm3 of 96 m/m% concentrated sulphuric acid is needed.
54.0 g of 14.0 m/m% and 96.0 g of 6.0 m/m% sodium chloride solutions are 2.
mixed. Calculate the m/m% concentration of the obtained solution.
These types of problems can be calculated by the mixing equation, considering that mass is an additive property:
= (m1 + m2) (m/m%)3 (m/m%)2
(m/m%)1
m1 + m2
= (54.0 g + 96.0 g) (m/m%)3
54.0 g 14.0 % + 96.0 g 6.0 %
8,88 % = (m/m%)3
Thus, concentration of the diluted sodium chloride solution is 8.9 m/m%.
Identification number:
TÁMOP-4.1.2.A/1-11/1-2011-0016 63
Experimental task: Preparation of solutions of different III.4.8.2
concentrations
III.4.8.2.1. Preparation of solution from a solid substance
Prepare a solution of given volume and concentration from the substance provided by the instructor. There are five major steps in preparation of the solution:
• Calculate the required amount of substance. Pulverize the compound in a mortar, then measure the required quantity into a suitable dish (beaker, weighing dish (for hygroscopic substances), etc.). Depending on the required accuracy, laboratory or analytical balance should be used.
• With continuous stirring dissolve the substance in a quantity of water that is much less than the volume of the dish. In the case of slowly dissolving compounds it is practical to gently warm up the solution.
• The total amount of the solution (cooled back, if it is needed) is transferred into a volumetric flask by means of a long stemmed analytical funnel. Rinse the beaker with small portions of the solvent several times and collect the solvent portions in the volumetric flask by means of a long stemmed analytical funnel used before.
• Fill the volumetric flask with the solvent up to the neck of the flask. Meanwhile, homogenize the content by careful shaking. After homogenization, carefully fill the flask to the mark. Move your eye to the level of the mark on the neck of the flask and line it up so that the circle around the neck looks like a line, not an ellipse. Then add distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark on the neck of the flask. Take care that no drops of liquid are in the neck of the flask above the mark.
• After closing the measuring flask, homogenize the solution.
III.4.8.2.2. Preparation of solution from a ’stock-solution’
Prepare a solution of a given volume and concentration from the solution (of known density and m/m%) provided by the instructor. There are five major steps in preparation of the solution:
• Calculate how much stock solution you need. Measurement of volume of liquids is simpler then measuring their mass. With knowledge of density, calculate what a volume is needed from the stock-solution.
• Pour a small portion of distilled water into the volumetric flask through a long stemmed analytical funnel.
• The previously calculated volume of the stock solution has to be measured with appropriate volumetric glassware (generally with a pipette or - in case of serial sampling - with a burette) and transfer the total amount of the solution into a volumetric flask by means of a long stemmed analytical funnel. Rinse the funnel with small portions of the solvent several times and collect the solvent portions in the volumetric flask.
• Fill the volumetric flask with the solvent up to the neck of the flask. Meanwhile, homogenize the content by careful shaking. After homogenization, very carefully fill the flask to the mark. Move your eye to the level of the mark on the neck of the flask and line it up so that the circle around the neck looks like a line, not an ellipse. Then add distilled water a drop at a time until the bottom of the meniscus lines up exactly
64 The project is supported by the European Union and co-financed by the European Social Fund with the mark on the neck of the flask. Take care that no drops of liquid are in the neck of the flask above the mark.
• After closing the measuring flask, homogenize the solution.