# Stoichiometry Module: Solutions

### Solutions & Concentration

In your everyday life, you encounter solutions all the time. For example, you may add salt to water when cooking pasta. The salt dissolves in the water, resulting in a solution. Most of your "household" chemicals are solutions. If you look closely at a bottle of vinegar, you will find that it is a solution of acetic acid. Similarly, bleach is a solution of sodium hypochlorite.

All of these examples have both a solute and a solvent. The solute is the material that is dissolved while the solvent is whatever it is dissolved in. So in the salt water example, the salt is the solute and the water is the solvent. In vinegar, acetic acid is the solute and water is the solvent and in bleach, sodium hypochlorite is the solute and water is the solvent.

 While all the above examples are what chemists call "aqueous solutions" (where water is the solvent), there are other types of solutions you encounter daily. For example, steel is a solid solution where nickel and chromium are dissolved in iron. The air you breath is a gaseous solution of oxygen in nitrogen. The solutions discussed in this module will all be aqueous solutions.

The concentration of the solution tells you how much solute has been dissolved in the solvent. For example, if you add one teaspoon to two cups of water, the concentration could be reported as 1 t salt per 2 c water. The vinegar label will report that the solution is 5% by weight acetic acid. This means that that there are 5 grams of acetic acid per 100 g of solution.

There are many different units of concentration. The standard unit of concentration in chemistry is molarity (abbrieviated with M). Molarity is defined as moles of solute per liters of solution. It is important to note that molarity is per liters of solution, not per liters of solvent. For example, if one mole of salt were added to one liter of water, the resulting solution would not be 1 M (read as "one molar"). After the solution has been mixed, the volume would be recorded. The molarity could be calculated by dividing one mole by this total volume.

If 21.7 grams of table sugar (C12H22O11, molecular weight 342.3 g/mol) is dissolved in water and enough water is added to make 500 milliters of solution, what is the concentration of the resulting solution?

 .000127 M .0434 M .127 M 43.4 M
 Remember that the unit M is molarity, or moles per liter. You must convert the grams of sucrose to moles of sucrose and the milliliters of solution to liters of solution.
 Good! You can review the calculation below: