Molarity

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

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: The difference between a solute and solvent

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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

Since molarity is just moles of solute divide by liters of solution, we can manipulate the equation to suit the question asked. 

Example: 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?

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

Example: 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. 

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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. 

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

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

Problem: 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.

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Molarity and Chemical Reactions

Stoichiometry is used when given the known quantity of one compound and asked the find the unknown quantity of another compound or element. Now we throw molarity into the mix. 

This new Stoichiometric Chart is used anytime we are given the known quantity of a compound in units such as mL, L or M and asked to find the unknown quantity of another compound. 

Example: Molarity and Stoichiometry Calculation

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By using our new Stoichiometric Chart we can now answer stoichiometric questions dealing with molarity. 

Using our new Stoichiometric Chart it is possible to now calculate the unknown volume of a compound through molarity. 

Problem: How many milliliters of 0.325 M HCl are needed to react with 16.2 g of magnesium metal? 2 HCl (aq) + Mg (s) ----> MgCl2 + H2 (g)

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Knowing the volue and molarity of a compound allows us to determine the molarity of an unknown compound. 

Problem: What is the molarity of a hydrobromic acid solution if it takes 34.12 mL of HBr to completely neutralize 82.56 mL of 0.156 M Ca(OH)2? 2 HBr (aq) + Ca(OH)2 (aq) ----> CaBr2 (aq) + 2 H2O (l)

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Problem: Iron (III) can be oxidized by an acidic K2Cr2O7 solution according to the net ionic equation: Cr2O72- + 6 Fe2+ + 14 H+ -----> 2 Cr3+ + 6 Fe3+ + 7 H2O If it takes 30.0 mL of 0.100 M K2Cr2O7 to titrate a 25 mL Fe2+ solution, what is the molar concentration of Fe2+?

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