MOLARITY

I. Introduction

Knowing the concentration of a solution can help chemists to precisely mix and remix substances to create a solution using the Molarity of that solution. In daily life, knowing the Molarity can be helpful in mundane tasks as using laundry detergent, or as important as knowing the dosage of a drug to take.

II. Discussion

The concentration of a solute in a solution is expressed as its Molarity. It tells you how much of a solute is present. Molarity is the moles of solute in a solution divided by the liters of solution.

Molarity (M) = Moles of solute / Liters of solution

There are three steps to converting grams of a substance to moles. (If given the number of grams of solute present instead of the number of moles of solute) 1. Determine how many grams are given in the problem.2. Calculate the molar mass of the substance.3. Divide step one by step two.* To determine the mass of a molecule, simply add up all of the atomic masses for the atoms that make up the molecule.

Examples:

1. 4.35 moles of KMnO4 are dissolved in enough water to give 750mL of solution. Determine the molarity of the solution. (1L = 1000mL)

Given: 4.35 moles of KMnO4, 750mL .750LFind: Molarity Solution: M= (moles)/ (L) M= 4.35 moles KMnO4 .750 L solution

Molarity = 5.80 moles KMnO4 / L solution

2. Calcuate the molarity when 75.0 grams of MgCl2is dissolved in 500.0 mL of solution.

3. 49.8 grams of KI is dissolved in enough water to make 1.00 L of solution. What is the molarity?

OSMOLARITY

I. Introduction

Osmolarity is a concept that allows you to determine if water will move from one side of a membrane to the other side. That is: will Osmosis occur? Water will always move across a membrane into the solution with the higher osmolarity.

II. DiscussionOsmolarity is the product of a solutions molarity and the number of particles produced per formula unit if the solute dissociates. The equation for osmolarity is

Osmolarity = molarity x i

i represents the number of particles produced from the dissociation of one formula unit of solute.

* The units of osmolarity are osmols (osm).

Examples:

1. Determine the osmolarity of 2M CaCl2Given: 2M, 3 = i

*the value of i is 3 because three ions are produced from the dissociation of one CaCl2 formula unit. CaCl2 Ca2+ + 2Cl-

Find: OsmolaritySolution: Osmolarity = Molarity x i = 2M x 3

Osmolarity = 6 osmol

2. Calculate the osmolarity (osmol) of the following solutions.a) 5.2 M Na2CO3 Given: 5.2M, 3 = i ( Na1+ +CO2+ ) Find : Osmolarity Solution: Osmolarity = Molarity x i = 5.2M x 3

Osmolarity = 15.6 osmol

b) 0.15 M Al(NO3)3 Given: .15M, 4 = i (Al3+ + NO31+) Find: Osmolarity Solution: Osmolarity = Molarity x i = 0.15M x 4

Osmolarity = 0.6 osmol

III. Reference

http://www.chemteam.info/Solutions/Molarity-problems-answers.htmlhttp://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson56.htmhttp://fog.ccsf.cc.ca.us/pherrman/documents/OsmolarityS10.pdf