Chapter 4: Concentration of Solutions

Molarity

the number of moles of solute per liter of solution

equivalence point

the point at which the two solutions used in a titration are present in chemically (stoichiometrically) equivalent amounts

Dilution

the process of adding solvent to lower the concentration of solute in a solution

When solvent is added to a solution, what happens to the number of moles?

they do not change moles solute before dilution = moles solute after dilution

Concentration

A measurement of how much solute exists within a certain volume of solvent greater amount of solute = higher concentration

A sample of 70.5 mg potassium phosphate is added to 15.0 mL of 0.050 M silver nitrate, resulting in the formation of a precipitate. a) write the molecular equation b) What is the limiting reaction? c) calculate the theoretical yield, in grams, of the precipitate that forms.

K3PO4 (ag) + 3AgNO3 (aq) = Ag3PO4 (s) + 3KNO3 (aq) 3.32 x 10-4 K3PO4 7.5 X 10 -4 aGno3 AgNO3 is the limiting factor .00075AgNO3 moles (1 mol Ag3PO4/3 mol AgNO3) = 2.5 E-3 2.5 e-4 moles (418.6046 g Ag3PO4/1 mole) = .10465115g

Molarity equation

M = moles of solute/liters of solution

equation that shows relationship between moles in concentrated solution and dilute solution

M concentrated x V concentrated = M diluted x V diluted

Cannot use ______ ________ when converting between grams and moles in a solution. What must be used instead.

Molar mass Because it is a solution and not a pure substance. moles of solute = M x V if you know moles of one reactant you must determine moles of the other reactant if you are solving for that

Tiration

The process, operation, or method of determining the concentration of a substance in solution by adding to it a standard reagent (standard and indicator) of known concentration in carefully measured amounts until a reaction of definite and known proportion is completed, as shown by a color change or by electrical measurement, and then calculating the unknown concentration.

When calculating molarity for separate ions make sure to

calculate in terms of the amount of ions 0.025 M of Ca(NO3)2 Ca = 0.025 M (NO3)2 = 0.050 M

concentration of an electrolyte

concentration of ions the dissolve in a solution depends on the chemical formula of a compound 1.0 M solution of NaCl is 1.0 M Na+ and 1.0 M Cl- Concentration of electrolyte solution can be specified as either the compound used to the make the solution or the ions in the solution

What is the molar concentration of each ion present in a 0.025 M aqueous solution of calcium nitrate?

mol NO3-/L = (0.025 mol Ca(NO3)2/L)(2 mol NO3-/1 mol Ca(NO3)3) 0.050 M NO3 Ca2+ = .025 M

moles =

molarity x volume

Using moles in dilute solution to calculate liters of concentrated solution to provide the same amount of moles

moles in dilute = (L solution) (molarity mol/L) Liters of concentrated solution to get the same amount of moles in diluted = (moles in diluted)(molarity L/mol)

What types of chemical reactions have be conducted with a titration?

neutralization, precipitation or oxidation-reduction reactions

indicator

A compound that changes color in the presence of an acid or a base

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

0.100 M = moles/.025L = .0025 moles HNO3 Ca(OH)2 + 2HNO3 = 2H2O + Ca(NO3)2 Grams of Ca(OH)2 = .0025 moles of HNO3(1 mole Ca(OH)2/2 moles 2HNO3)(74 g Ca(OH)2/ 1 mol Ca(OH)2 = .0925g Ca(OH)2

How many grams of Na2SO4 are there in 15 mL of .50 M Na2SO4? How many mililiters of .50M Na2SO4 solution are needed to provide 0.038 mol of this salt?

1.1 g 76 mL

How many mL of 3.0 M H2SO4 are needed to make 450 mL of 0.10 M H2SO4?

15 mL

The quantity of Cl- in a municipal water supply is determined by titrating the sample with Ag+. The precipitation reaction taking place during the titration is: Ag+ (aq) + Cl- (aq) = AgCl (s) 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 10.0g , what percentage of Cl- does it contain?

2.02 x 10-3 mol Ag+ 0.0717% Cl-

One commercial method used to peel potatoes is to them in a NaOH solution for a short time and then remove the potatoes and spray off the peels. The NaOH concentration is normally 3 to 6 M, and the solution must be analyzed periodically. In on such analysis, 45.7 mL of .500 M H2SO4 is required to neutralize 20.0 mL of NaOH solution. What is the concentration of the NaOH solution?

2.28 M NaOH

Grams of Na2SO4 are required to make .350 L of .500 M Na2SO4?

24.6 G Na2SO4

Standard solution (titrant)

a solution of known concentration measure with burette so exact amount can be determined