SOLUTON CONCENTRATION

• Many chemical reactions involve reactants that are dissolved in water. These reactants are referred to as solutions and are a very important part of chemistry.
• A solution is a homogeneous mixture of two substances: solute and solvent.
• Solute is the substance being dissolved (present in lesser amount), while solvent is the substance doing the dissolving (present in larger amount).
• An aqueous solution is one in which water is the solvent.

Solution Concentrations:

• The amount of solute in a solution is variable. Solutions that have little solute are called dilute, while solutions that have large amount of solute are called concentrated.
  

  

• Concentration can be define in general terms as the amount of solute dissolved in a certain amount of solution:

MOLARITY

• The most common unit of concentration used in the laboratory is molarity (M). Molarity is defined as:

Examples:

1. What is the molarity of a solution containing 1.4 mol of acetic acid in 250 mL of solution?

2. What is the molarity of a solution prepared by dissolving 60.0 g of NaOH in 0.250 L of solution?

3. What is the molarity of a solution that contains 75 g of in 350 mL of solution?

USING MOLARITY

• Molarity can be used as a conversion factor between moles of solute and volume of solution. Examples below show how amount of solute can be calculated from volume and molarity of solution.

Examples:

1. How many moles of nitric acid are in 325 mL of solution?

2. How many grams of KCl would you need to prepare 0.250 L of 2.00 M KCl solution?

3. How many grams of are in 325 mL of 4.50 M solution of ?

USING MOLARITY

• Examples below shown how volume of solution can be calculated from amount of solute and molarity of solution.

Examples:

4. What volume (L) of 1.5 M HCl solution contains 6.0 moles of HCl ?

5. What volume (mL) of 2.0 M NaOH solution contains 20.0 g of NaOH ?

6. How many mL of a 0.300 M glucose intravenous solution is needed to deliver 10.0 g of glucose to the patient?

ION CONCENTRATION

• The concentration of ionic substances in solution reflects the concentration of the solute before it dissolves.
• The concentration of the ions dissolved in solution can be calculated by noting the number of each ion present in the formula unit.
• For example, a solution has the following ion concentrations:

Examples:

1. Determine the molar concentrations of and in a solution.
2. Which solution below has the greatest concentration of ?
   a)
   b)
   c)

|dilution

• Solutions are often prepared from more concentrated ones by adding water. This process is called dilution.
• When more water is added to a solution, the volume increases, causing a decrease in concentration. However, the amount of solute does not change.
  

  

  
  

  

  
  

  

Volume and Concentration are inversely proportional

• The amount of solute depends on the concentration and the volume of the solution. Therefore,

Examples:

1. What is the molarity of the final solution when 75 mL of 6.0 M KCl solution is diluted to 150 mL ?

2. What volume (mL) of 0.20 M HCl solution can be prepared by diluting 50.0 mL of 1.0 M HCl ?

SOLUTION STOICHIOMETRY

• In aqueous reactions, amounts of reactants and products are often specified in terms of volume and concentrations. A chemical equation, however, is based on mole relationships between reactants and products.
• A knowledge of conversion of volume and concentration data into mole quantities and subsequent calculations based on the stoichiometric relationships in the chemical equation are referred to as solution stoichiometry.
• The general concept for doing these calculations is shown below:
  

  

Examples:

1. What volume (in L ) of a 0.150 M KCl solution will completely react with 0.150 L of a solution as shown below:

2. If it takes 55.6 mL of a 0.10 M NaOH solution to neutralize 125 mL of an HCl solution, what is the concentration of the HCl ?

ACID-BASE TITRATIONS

• Principles of solution stoichiometry can be used to solve problems in a common laboratory procedure called titration.
• In a titration, a substance of known concentration is reacted with another substance of unknown concentration. For example, consider the acid-base reaction between HCl and NaOH shown below:

• When a solution of NaOH with a known concentration is added slowly to a known volume of HCl with an unknown concentration, the and ions combine to form water. (Note that the and ions forming NaCl are omitted from this discussion, since they are spectator ions).
• After addition of enough to neutralize all of the present (equivalence point), the solution becomes neutral and the titration is complete. The equivalence point is usually detected by addition of an indicator, a dye that changes color based on the acidity of the solution.
• Stoichiometric calculations based on concentration of know substance ( NaOH ) and volume of both solutions can yield the concentration of the unknown substance (HCl).
  

  

ACID-BASE TITRATIONS

Examples:

1. The titration of 10.00 mL of an HCl of unknown concentration requires 12.54 mL of a 0.100 M NaOH solution to reach the equivalence point. What is the concentration of the unknown HCl solution?

2. The titration of a sample of an solution of unknown concentration requires 22.87 mL of a 0.158 M KOH solution to reach equivalence point. What is the concentration of the unknown solution?