Composition of Solution

The Composition of Solutions

Molarity

1. Moles of solute per liter of solution

What's the molarity of a solution that contains 5.5 moles of sodium chloride in 10.5 liters of solution?
HIGHLIGHT ANSWER--> M = moles / liters, or (5.5 moles) / (10.5 liters) = 0.52 M

HARDER

If I have 3.50 grams of sodium chloride in 1250 mL of a solution, what's the molarity?
HIGHLIGHT ANSWER-->

To find molarity, we need to convert grams to moles and milliliters to liters.  To convert grams to moles, we first need to divide the number of grams by the molar mass of sodium chloride.  (3.5/58.5) = 0.060 moles.  To convert milliliters to liters, multiply by 0.001.  (1250 x 0.001) = 1.25 liters.  In the final step, divide the number of moles by the number of liters to get the molarity.  Since 0.060 / 1.25 = 0.048, the molarity of the solution is 0.048 M.

Concentration of Ions in Solution

Ionic compounds dissociate in solution, multiplying the molarity by the number of ions present

What is the Chloride Concentrations[Cl-] in the following solutions?
2.0M NaCl [Cl-]= 1 x 2.0M =2.0M         NaCl => Na+ + Cl-  
1.5M AlCl3 [Cl-]= 3 x 1.5M= 4.5M         AlCl3 = > Al3+ + 3 Cl-
2.0M CaCl2 [Cl-]=  2 x 2.0M= 4.0M        CaCl2 = > Ca2+ + 2 Cl-

Moles from Concentration

Liters of solution x molarity = moles of solute

How many moles of CaCl2 would be used in the making of 5.00 x 102 cm3 of a 5.0M solution?
HighLight Answer--> First    5.00 x 102 cm3  = 0.500 L

                                   Given: Molarity = 5.0 M (moles/L)
                                                Volume = 0.500 L

                            # of moles of CaCl2 = 5.0 moles/L x 0.500 moles = 2.5 moles of CaCl2
How Many Grams of CaCl2 would be used?
HighLight Answer-->   Given:  # of moles of solute = 2.5 moles (from our answer other example)
          

Molar mass of solute (CaCl2) = 111 g/mole (from the periodic table)

Mass of CaCl2 = 2.5 moles x 111 g/mole

                                        Answer: Mass of CaCl2 = 280 g (when rounded correctly)

 

Solutions of Known Concentration

Standard solution - a solution whose concentration is accurately known

Preparation of Standard solutions  

How much? (Volume Liters)

 How strong (Molarity)?

What does it weigh (Molar Mass)?

  

grams required to prepare the standard

liters X

 mol X grams =
L mol

 

How many gram of NaOH would you need to make 2.5L of a 2.0M solution?
HighLight Answer--> 2.5L x 2.0M x 40. g/mol= 200grams NaOH

 Dilution

Using a Volumetric Flask dilute of a volume (aliquot)  of solution with water. This does not change the number of moles present. 

Solving dilution problems

M1V1 = M2V2

What volume aliquot of 12.0M HCl should be used to make a solution that is 1.50M with a volume of 500.mL?
HighLight Answer-->                          Since MV  =  MV

12.0M x V = 1.5M x 0.500L      (Note 500mL =0.500L)

                                                               V=  62.5mL (of 12.0M HCl)

This is added to a volumetric flask and water is added to bring the solution up to 500.mL 

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References-misterguch.brinkster.net/ molaritytutorial.html

 

 

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