Types of Chemical Reactions and Solution Stoichiometry – Part 2

Chapter 4Part 3

Calculations

All calculations must use factor label method or dimensional analysis

Ratio is NOT acceptable

Using Molarities inStoichiometric Calculations

Using Molarity to Calculate Grams of Solute

SAMPLE EXERCISE 4.13. How many grams of Na2SO4 are required to make 0.350 L of 0.500 M Na2SO4?

Plan: Use the definition of molarity to determine the number of moles of solute, and then convert moles to grams using the molar mass of the solute.

Solve: Determine moles of solute

Using Molar Mass to determine grams

Calculating Molar Concentrations of Ions

SAMPLE EXERCISE 4.12. What are the molar concentrations of each of the ions present in a 0.025 M aqueous solution of calcium nitrate?

Plan: Use the subscripts in the chemical formula of the compound to determine the relative concentrations of the ions.

Solve: The chemical formula for calcium nitrate is Ca(NO3)2. Because there are two NO3

– ions for each Ca2+ ion in the compound, each mole of Ca(NO3)2 that dissolves dissociates into 1 mol of Ca2+ and 2 mol of NO3

–. Thus, a solution that is 0.025 M in Ca(NO3)2 is 0.025 M in Ca2+ and 2 0.025 M = 0.050 M in NO3

–.

Acid – Base Titration

The analytical technique in which one can calculate the concentration of a solute in a solution.

Titration

Titration

Volumetric analysis

Titration –deliver a measured volume of a known concentration of titrant into a solution containing the analyte.

Titrant – solution of known concentration that is added

Analyte- solution of unknown concentration that can be analyzed

Measure the volume of titrant added to neutralize the analyte using a color indicator.

Titration - Use known volumes of each solution and known concentration of titrant to calculate the concentration of the analyte.

Acid Base Titrations

Equivalence point – stoichiometric equivalent quantities of acid and base

Indicator – chemical that changes color at the end point

Acid-base indicators change color at end point.

Indicator is chosen so end point corresponds to equivalence point of titration.

Using Mass Relations in a Neutralization Reaction

SAMPLE EXERCISE 4.15. How many grams of Ca(OH)2 are needed to neutralize 25.0 mL of 0.100 M HNO3?

Solution

Determining the Quantity of Solute by Titration

SAMPLE EXERCISE 4.16. The quantity of Cl– in a municipal water supply is determined by titrating the sample with Ag+. The reaction taking place during the titration is below. The end point in this type of titration is marked by a change in color of a special type of indicator. (a) How many grams of chloride ion are in a sample of the water if 20.2 mL of 0.100 M Ag+ is needed to react with all the chloride in the sample? (b) If the sample has a mass of 10.0 g, what percent Cl– does it contain?

Solution. (a) Plan: We begin by using the volume and molarity of Ag+ to calculate the number of moles of Ag+ used in the titration. We can then use the balanced equation to determine the moles of Cl– in the sample and from that the grams of Cl–.

Solution

Comment: Chloride ion is one of the most common ions in water and sewage. Ocean water contains 1.92% Cl–. Whether water containing Cl– tastes salty depends on the other ions present. If the only accompanying ions are Na+, a salty taste may be detected with as little as 0.03% Cl–.

(b) Solve:  

Conclusion

Use Molarity to calculate molarity and then use stoichiometry to determine moles or grams in a chemical reaction.

Use titration to neutralize solution of unknown concentration with color indicator. The titrant has known concentration and measured volume. Using stoichiometry, the unknown concentration can be determined.