Ch.4 - Chemical Quantities & Aqueous ReactionsWorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Ever wonder how a mass amount such as moles can be converted into the volume amount of liters? Well, molarity serves as the bridge between moles and liters. 

Understanding Molarity

Concept #1: Definition of Molarity


We're going to say the molarity serves as a connection that allows us to interconvert between moles and liters. Basically, if I have molarity, I can go from moles to liters or liters to moles.
We're going to say, for example, we have 5.8, capital M means molarity, so that really means 5.8 molar, 5.8 molar NaCl solution really means, we have a 5.8 moles of NaCl per 1 liter of solution. So, just remember, whatever your molarity is, it's that number in moles over 1 liter. Now, the formula simply for molarity is molarity equals moles of solute over liters of solution.
Now, the terms of solute and solution, we really haven't talked about until now. What we should realize is that we talked about mixture before. We said that mixtures could be heterogeneous or homogenous and we're going to say that a typical mixture consists of basically two parts.
We have a smaller amount of one substance, which is called the solute and it's dissolved in a larger amount of another substance called the solvent. So, the smaller solute gets dissolved by the larger solvent. We're going to say together, solute plus solvent give us the solution. 

Anytime that you hear the term molarity used, just think that we are talking about how many moles of a solute are in a liter of solution

Solute and Solvent

Forgot what's the difference between a solute and a solution? Here's a reminder: 

Concept #2: The difference between a solute and solvent


Here we have an image. Here we're going to say that this represents pure water. Pure water is known as the universal solvent. That just means that it's able to dissolve tons of different types of compounds. Now, if you take some table salt or maybe even some sugar and you just sprinkle some in there, it's going to dissolve in there. So we're going to have little specs of sugar or salt mixed in there. Those little specs of sugar or salt will represent your solute. They're the solute because they're so much smaller than the total volume of the liquid. So, the little bits of sugar or salt that you pour in there are your solute, the water is the solvent and it dissolves it, together they formed a solution. This is our pure solvent. We say we throw in some salt or sugar. The solvent dissolves it and together they form our solution. So, these are the terms that we need to be aware of. And when we get to calculation on Molarity, it's going become essential that you guys remember, whatever your Molarity is, it's that number in moles over 1 liter.

In a homogeneous mixture, the smaller amount is the solute, and the larger amount is the solvent. When dissolving a solute into a solvent they make a solution. 

Molarity Calculations

Example #1: 2.64 grams of an unknown compound was dissolved in water to yield 150 mL of solution. The concentration of the solution was 0.075 M. What was the molecular weight of the substance?

concentrated solution can become a diluted solution with the addition of water. 

Example #2: A solution is prepared by dissolving 0.1408 mol calcium nitrate, Ca(NO3)2, in enough water to make 100.0 mL of stock solution. If 20.0 mL of this solution is then mixed with an additional 90 mL of deionized water, calculate the concentration of the calcium nitrate solution. 

We know how to calculate the molarity of a compound, but what do we do when we need the molarity of ions within the compound? Let's see. 

Practice: What is the molarity of calcium ions of a 650 mL solution containing 42.7 g of calcium phosphate?

Now let's try connecting molarity with an equation from the past, density. 

Practice: A solution with a final volume of 750.0 mL was prepared by dissolving 30.00 mL of benzene (C6H6, density = 0.8787 g/mL ) in dichloromethane. Calculate the molarity of benzene in the solution.