Molarity
A great deal of chemistry is done with solutions. A solute, solid, liquid or gas, is dissolved in a liquid solvent such as water, methylene chloride, alcohol, ether, etc. The solute is usually the substance of interest, the solvent being a medium and usually not being involved in a reaction. In these cases we want to know the number of moles of solute in a given volume of the solution.
To prepare a solution of a certain concentration of a substance we first calculate the molecular weight of the solute. Let’s say we want a water solution of potassium chromate. The formula for potassium chromate is K2CrO4. The atomic weights of potassium, chromium and oxygen are 39.10, 52.00 and 16.00. We add up 2 x 39.1, 52.0 and 4 x 16.00 that gives 194.2g/mole. The concentration of the solution is in moles per liter so we then decide the volume of solution we want. Let’s say we want to make up one liter of solution. We will use a 1000ml volumetric flask. Now let’s say we want a 0.1 molar solution (0.1M). We want one tenth mole dissolved in one liter. We weigh out 19.42g of solid K2CrO4 and place it in the dry flask. We then add a little water, enough to dissolve the solid, and swirl it around until all the solid is dissolved. We then add water to the mark. (The reason you wait until all the solid is dissolved before filling to the mark is that the volume occupied by the solid differs from the volume it contributes to the solution once it is dissolved. Thus if you filled with water to the mark while there was still undissolved solid in the flask you might see the water level go down slightly as the solid dissolved.) You now have a 0.1M aqueous solution of K2CrO4. If you want 0.01moles of potassium chromate you would divide 0.01mol by 0.1mol/liter to get 0.1liter or 100ml. 100ml of the solution would contain 0.01moles.
A solution can contain more than one solute. Each is weighed out and added to the flask before adding the solvent. Each has its own concentration in moles per liter. In preparing solutions of solids it is important determine how much water is in the solid. Some compounds such as KCl do not tend to absorb water from the atmosphere. Others, such as CaCl2 will absorb a great amount of water and must be dried in an oven before weighing. A substance that absorbs water from the atmosphere is called hygroscopic. Some compounds include water of crystallization where the water is part of the crystal structure. An example is borax, Na2B4O7* 10H2O. It is "dry" but contains a lot of water. Once dried a solid can be stored in a desiccator. A desiccator is a container that contains a layer of a dried hygroscopic material such as CaCl2 which scavenges water from the air in the container.