Solutions are homogenous (evenly-distributed) mixtures of two or more chemicals. Solutions can exist as solids, liquids, or gases.
All solutions contain a solvent and one or more solutes. The solvent, often water, is the chemical that’s most abundant. The solute is the chemical(s) that’s less abundant.
How to Dissolve Solids and Scale Down Experiments
For experiments, you will often need to dissolve solutes in solid form to make solutions of a particular strength (strength is measured by ion disassociation). Plan one hour for every 2-4 solutions you need to prepare. You will need a balance to weigh out the solute and a graduated cylinder to measure the solvent (if it’s water).
First, determine the concentration (weight percent or Molarity, see below) and amount (milliliters) of solution you need from your lab procedure. Second, calculate the amount of solute needed in grams, using one of the formulas given below. Next, weigh out the solute and add it to a mixing beaker. Finally, measure the volume of water needed in milliliters with a graduated cylinder and add it to the beaker. Stir the solution until all of the chemical dissolves.
Break up hard lumps of chemical with a mortar and pestle or by careful crushing in a plastic bag with a hammer. Chemicals dissolve more quickly by gently heating the solution and stirring.
You might consider scaling down when experiments call for large amounts of chemical. Scaling down reduces safety hazards, chemical cost, and waste disposal.
Most experiments can be scaled down by dividing the solute and solvent by a factor of your choosing. For example, an experiment calling for 50 g of a solvent and 250 ml of water can be scaled down by a factor of 10 so that it uses only 5 g of solvent and 25 ml of water. You can make scaling down easier by using smaller beakers, test tubes, and other measuring equipment.
When making chemical solutions, always use the appropriate safety equipment.
How to Make Molar Solutions
Molar (M) solutions are based on the number of moles of chemical in one liter of solution. A mole consists of 6.02×1023 molecules or atoms. Molecular weight (MW) is the weight of one mole of a chemical. Determine MW using a periodic table by adding the atomic mass of each atom in the chemical formula.
Example: For the MW of CaCl2, add the atomic mass of Ca (40.01) to that of two Cl (2 x 35.45) to get 110.91 g/mole. Therefore, a 1M solution of CaCl2 consists of 110.91 g of CaCl2 dissolved in enough water to make one liter of solution.
Once the molecular weight of the solute is known, the weight of chemical to dissolve in a solution for a molar solution less than 1M is calculated by the formula:
For example, to make 100 ml of 0.1 M CaCl2 solution, use the previous formula to find out how much CaCl2 you need:
Now you can make your solution: dissolve 1.11 g of CaCl2 in sufficient water to make 100 ml of solution. The amount of water needed will be slightly less than 100 ml.
A balance and a volumetric flask are used to make molar solutions. A procedure for making a molar solution with a 100 ml volumetric flask is as follows:
If you don’t have a volumetric flask you can use a 100ml graduated cylinder instead. Just add the solute to the graduated cylinder and then add distilled water until you reach the 100ml mark in the side of the cylinder.
How to Make Weight Percent (Wt%) Solutions
In weight percent solutions, the weight of the solute is divided by the weight of the solution (solute + water) and multiplied by 100. Since the density of water is 1 g/ml, the formula to calculate the amount of solute that must be mixed for a weight percent solution is:
As an example, to make 100 ml of 10% NaCl (table salt) solution, use the previous formula to find out how much NaCl you need:
Now you can make your solution: dissolve 11.1 g NaCl in 100 ml of water.
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