1. Chemistry I Honors Solutions Lesson #3 Concentration

2. Introduction and Review We have already discussed the concept of Solubility as a measurement that tells us the maximum amount of a solute that can be dissolved in a specified amount of a solvent. Along with that came three vocabulary terms: – Dilute – a solution in which the amount of solute is far less than the maximum possible amount – Concentrated – the amount of solute is almost equal to the maximum amount. – Saturated – the amount of solute dissolved is equal to the maximum amount.

3. Concentration Now we move to a calculation that gives us a numerical value for the ratio of solute to solvent. Instead of general descriptions (like dilute, concentrated) we will have an exact indication of how the solution is “built”. There are about five different ways to describe solution concentration. The most commonly used description is called Molarity. The next slide shows you how it is calculated.

4. Molarity Molarity is defined as the ratio of the number of Moles of the Solute divided by the number of Liters of the total Solution Volume. Molarity=Moles of Solute Liters of Solution

5. Quick Notes on This The abbreviation for Molarity is a capital “M” In addition, you will see a solution described as a (number) “molar” solution. Example – “…a 2 molar solution of NaCl…” Note that in that last bullet point, only the solute is indicated. – The solvent is water – it is the most common solvent. – Chemist’s will typically assume that water is the solvent, and will not bother writing it. – If the solvent is something else (alcohol, benzene, etc), then the solvent will be indicated.

6. Back to the Calculation Molarity=Moles of Solute Liters of Solution Moles can be calculated from grams – just like we did in the mole theory unit. (use the formula weight) You may be given volume in milliliters. Remember that 1 liter is equal to 1000 milliliters.

7. A Molarity Pyramid Molarity Moles of Solute Liters of Solution

8. Possible Calculations Obviously, solving for Molarity is the most common calculation here. But, using the pyramid, the question could ask you to calculate the number of moles of solute. Another possibility would be a question that asks for the volume of the solution in liters. Finally, it is possible for te problem to provide a lot of information and then ask for the formula weight of the solute. You will need to use the pyramid from the calculation of moles from formula weight for this.

9. Finally, Dilution Dilution is a process in which a concentrated solution (has a high Molarity) is “diluted” – its concentration is reduced. The way to do this is to simply add solvent – water. – Remember that Molarity is a ratio. So, adding water will increase the volume, but reduce the ratio of solute to solvent because the denominator of the calculation is now larger.

10. The Dilution Equation In this equation, “M” is the molarity and “V” is the volume. The subscript “I” represents the initial values (before the dilution) and the “f” subscript represents the final values (after the dilution). The problems will give you three of the four variables and ask you for the missing value. It will simply be multiply and divide.

11. Vocabulary You will occasionally see the initial solution called a “Stock” solution. This will refer to a highly concentrated solution that is kept in the Chemical Stockroom. We buy solutions in high concentrations and then dilute them for use in the lab because it is more economical that way. “Stock”Dilute