Chemistry I

Solutions and Molarity

• Solute--a substance being dissolved

• Solvent--substance doing the dissolving
  • most often the solvent is the component present in greatest quantity

• Concentration--quantity of solute in a given quantity of solvent or solution

• Molarity--the number of moles of solute per liter of solution

                    Molarity    =    moles of solute
                                    liters of solution

• molar -- "M" -- the unit/symbol of molarity

                              M  =  mol
                                     L

• if units are given in anything other than moles and liters, you must change the units to moles and liters

Practice Problem:

• If you have 12.5 grams of sodium chloride in 1.23 liters of solution, what is the final concentration of the solution?

Answer:

1. Change grams to moles.

12.5 g NaCl   x   1 mol NaCl    =    0.21 mol NaCl
                  58.44 g NaCl

2. Calculate the molarity.

    if    M  =  mol
                 L

       then,        M   =   0.21 mol NaCl
                              1.23 L

                      M   =   0.17 M

Practice Problem:

• There are 58.5 milligrams of sodium chloride in 2.50 milliliters of solutions. Calculate the final concentration of the solution.

1. The units are not in moles and liters. You must change them.

         2.50 mL  =  0.0025 L
         58.5 mg  =  0.058 g

2. Change grams to moles.

     0.058 g  NaCl   x   1 mol NaCl   =   0.00099 mol NaCl
                        58.44 g NaCl

    if    M  =  mol
                 L

       then,        M   =   0.00099 mol NaCl
                                0.0025 L

                       M   =   0.396 M

Dilutions
M₁V₁ = M₂V₂

where,
M₁ = original molarity of the concentrated solution;
V₁ = unknown volume needed to dilute; M₂ = new molarity wanted of the new solution;
V₂ = new volume wanted of the new solution.

Titration

• a technique used to determine either the concentration of a solution of unknown molarity or the number of moles of a substance in a given sample
  • usually reactions between acids and bases
• begin with an unknown concentration of an acid or base and the other is a known concentration
• an indicator solution is used (usually phenolphthalein because it is the easiest to identify -- pink vs. clear)
• neutralization occurs when the number of moles of each solute are the same

Example

39.07 mL of a 2.0 M solution of sodium hydroxide is used to neutralize 25.0 mL of hydrochloric acid. What is the concentration of the acid?

Step 1:  Write a balanced equation:

              HCl   +   NaOH  --->  NaCl   +   H2O

Step 2:  Find the number of moles of the base:

     M = mol  so,    mol = (M)(L)  =  (2.0 M)(0.0397 L) = 0.078 mol NaOH
           L                         
 (because the ratio of NaOH to HCl is 1:1, then 0.078 mol is also 
  the number of moles of HCl)

Step 3:  Calculate the concentration of HCl using the number of moles 
              from Step 2 and the volume of the acid used.

         M  = mol     =   0.078 mol HCl   =    3.12 M HCl
               L              0.025 L

---

Test Review -