A solution is a homogeneous mixture where one substance is uniformly dispersed throughout another, forming a single phase. The substance that dissolves is the solute, and the substance that does the dissolving is the solvent. While water is the most common solvent, other liquids can be used depending on the chemistry required. The concentration must be precisely known for any experiment or application to be accurate.
Determining the Requirements
The first step in preparing any solution is a mathematical one, focusing entirely on determining the exact amount of solute needed to achieve the desired concentration. The concentration specifies the ratio of solute to the total volume of the final solution. Common measures of concentration include percentage, often expressed as mass per volume (w/v), and molarity, the number of moles of solute per liter of solution (mol/L).
To calculate the required mass for a molar solution, you must first know the solute’s molar mass (grams per mole). This value is multiplied by the desired concentration in moles per liter and the total volume of the final solution in liters. The calculation yields the precise mass of solute in grams that must be weighed out. For instance, a 1 Molar solution of a compound with a molar mass of 50 g/mol requires 50 grams of that compound dissolved to make one liter of final solution.
Percentage concentration calculations are often simpler, such as a 10% (w/v) solution, which means 10 grams of solute is needed for every 100 milliliters of total solution. The calculation must be performed meticulously, as any error at this stage results in an incorrect concentration. The critical distinction is that the calculated mass must be dissolved in enough solvent to reach the final volume, not simply added to that volume of solvent.
Accurate Measurement of Solute
Once the required mass of the solute has been calculated, the next phase involves the practical, high-precision measurement of that substance. For solid solutes, this requires an analytical balance, a highly sensitive instrument capable of measuring mass to several decimal places, often \(0.0001\) grams. The solid is typically placed on a clean, dry weighing boat or weighing paper, and the balance is “tared” to zero out the container’s weight before the solute is added.
Adding the calculated amount of solid must be done carefully, usually with a small scoop or spatula, to avoid overshooting the target mass. Excess chemical should not be returned to the original container due to contamination risks. If the solute is a liquid, its volume is measured using precise glassware like a volumetric pipette or a graduated cylinder.
After the solute is measured, it must be quantitatively transferred into the vessel where the solution will be made, typically a volumetric flask or a beaker. It is important to rinse the weighing boat or measuring vessel with a small amount of the solvent to ensure all of the measured solute is transferred, preventing a loss of material that would reduce the final concentration. This meticulous transfer process ensures that the concentration calculated earlier remains accurate.
The Dissolution Process
With the solute accurately measured and transferred, the next step is dissolution, where the solute is dispersed into the solvent. The initial amount of solvent added must be significantly less than the final target volume, often about half to two-thirds. This volume is added to the flask or beaker to promote mixing, and the vessel is then swirled or gently agitated to help the solid dissolve completely.
A stir bar and a magnetic stirrer may be used to speed up the process by continuously mixing the solution. Full dissolution is necessary before proceeding, as undissolved particles will cause the final concentration to be lower than intended. If a solute is difficult to dissolve, gentle heating may be applied, but the solution must be cooled to room temperature before the final volume adjustment.
When dealing with strong acids or bases, the dissolution process requires caution. The mixing of some solutes and solvents, such as concentrated sulfuric acid and water, can be highly exothermic, releasing a large amount of heat. The concentrated substance must always be added slowly to the larger volume of water, never the reverse, to safely manage the heat and prevent boiling or splattering.
Adjusting to Final Volume
The final stage involves bringing the mixture precisely to its target volume and ensuring complete homogeneity. This is typically done using a volumetric flask, glassware designed to contain one exact volume, indicated by a single etched line on its neck. After the solute is fully dissolved, more solvent is added until the liquid level is near the calibration mark, often about one centimeter below it.
For the final few drops, a dropper or Pasteur pipette is used to add the solvent with maximum control. The goal is to align the bottom of the liquid’s curved surface (the meniscus) exactly with the calibration line when viewed at eye level. Filling past this line necessitates discarding the solution and starting over, as removing liquid would also remove an unknown amount of the dissolved solute.
Once the volume is correctly set, the flask is sealed with a stopper, and the solution is mixed thoroughly to ensure uniform concentration. The flask is often inverted repeatedly, while holding the stopper firmly in place. This final mixing ensures the solution is truly homogeneous, completing the process of creating a precise chemical solution.