Name
AQUEOUS SOLUTIONS UNIT
Electrolytes & Molarity
Definitions
Solute – component in a solution that is present in lesser quantities (typically referred to as the solid that is dissolved in aqueous solutions)
Solvent – component in a solution that is present in larger quantities (water for aqueous solutions)
Aqueous solution – solution in which water is the dissolving substance
Ionizes – breaks into its constituent ions
Electrolytes – substances which dissociate into constituent ions in water to yield a solution that conducts electricity
Strong electrolytes – substances which completely dissociate in water & are good conductors of electricity
Weak electrolytes –substances which do not completely dissociate in water & weakly conduct electricity
Nonelectrolytes -substances with do not dissociate (produce ions) in water & do not conduct electricity
Electrolyte Practice problems: Assume all are strong electrolytes.
Practice: Z Ch4 23-26
24a) ivb) iic) iii d) iii, acetic acid only partially dissociates (weak)
26) check with MsO
Molarity
Molarity – an expression of the concentration of a solute in solution defined as the moles of solute per liter of solution.
Eg. 1Determine the molarity of a solution made 1.461 g NaCl in enough H2O to make 250.0L of solution.
Eg. 2A solution is made by dissolving 3.550g of Na2SO4 in water using a 100.0mL volumetric flask to adjust the
final volume. What is the molarity of this solution?
What is the concentration of Na ions in this solution?
Eg. 3How many mL of 0.250 M NaCl solution must be measured to obtain 0.100 mol of NaCl?
Eg. 4Calculate the number of mol of AgNO3 in 25 mL of 0.75 M AgNO3.
Eg. 5Typical blood serum is about 0.14 M NaCl. What volume of blood contains 1.0 mg NaCl?
Eg. 6A student needs to prepare 250 mL of 0.100 M Cd(NO3)2(MM=236.43) solution. How many g of the solid are
required?
Eg. 7Formalin is an aqueous solution of formaldehyde (HCHO). At high concentrations it is used as a preservative
for biologic specimens. How many grams of formaldehyde are contained in 2.5 L of 12.3 M formalin?
Practice: Z Ch4 27to29 30ab 32 33
28) 5.2M C2H5OH30a) 2.00Mb) 1.00M32) ask MsO
Dilutions
We do not always start out w/ a solid, but have a stock solution of known concentration used to make a needed solution. The process of dilution involves spreading out a specific number of mol over a large volume.
To perform a dilution we need to know the volume of the known concentration that contains the needed amount of mol for our solution.
Eg. 8What volume of 16 M sulphuric acid must be used to prepare 1.5 L of an 0.10 M solution?
Mdil xVdil = Mconc xVconc
Eg. 9How can you prepare 100mL of 0.0400M K2CrO7 from 0.200M K2CrO7 ?
Eg. 10If you dilute 25.0mL of 0.56M H2SO4 to a volume of 125mL, what is the molarity of the resulting solution?
Practice: Z Ch 4 37 41 42 38 39 40
42) Stock 2.883x10-2M Mn2+ , A 1.442x10-3M Mn2+ , B 5.768x10-5M Mn2+ , C 1.154x10-6M Mn2+
38) 0.167 M HNO340) 0.167 M Co2+ , 0.117 M Ni2+ , 0.567M Cl-
Precipitation Rxns & Net Ionic Equations
Rxn Picture:
properties of reactants :
How do you know a reaction occurred?
Describe products :
Precipitate – a solid that separates from solution & is insoluble in the solution; usually the result of a chemical rxn
Solubility Rules
The solubility rules identify substances that do or do not dissolve in aqueous solutions. They are often used to predict the products of a rxn.
Unless otherwise noted, assume solutions are aqueous.
Eg. 1Use the solubility rules to determine if the following substances are soluble.
a) Ca(NO3)2d) AgNO3
b) FeCl2e) BaSO4
c) Ni(OH)2f) CuCO3
If they were soluble identify the ions (& # of each) they would form in solution.
Eg. 2Predict whether the following substances are insoluble.
a) HgBr2d) (NH4)3PO4
b) Sr(NO3)2e) PbI2
c) Hg2Br2f) Pb(C2H3O2)2
Practice: Z Ch 4 43-44 & Brady 61to63
44) see MsO
Predicting Products of Rxns
The solubility rules are used to determine whether a reaction will occur by evaluating the solubility of reactants (in order for a reaction to occur, the ions must be free to react) and predicting whether a precipitate will form.
Assume rxns are aqueous unless otherwise noted.
Eg. 3For the following sets of reactants, determine whether a rxn will occur & if so, predict the products of the
reaction. Write balanced equations for rxns that occur.
a) Na2S + AgNO3
1) identify whether the reactants are soluble – if they are not soluble no rxn occurs
2) separate soluble reactants into constituent ions.
3) determine the possible combinations for products.
4) determine whether any of the products form precipitates – if not no rxn occurs
5) Write the chemical equation & balance.
b) MgCl2 + CuSO4
c) Na2O + CoCl3
d) Pb(NO3)2 + Fe2(SO4)3
e) AgNO3 + NH4Cl
f) Mg(C2H3O2)2 + Hg2(NO3)2
Practice: Z Ch 4 45 & 46 For Brady 53 & 54 just write the balanced equations. See MsO for answers
Writing net ionic equations
These give us a better idea of the chemistry going on in the rxn.
Only precipitates & gases are written in cmpd form.
Net ionic equations – equations in which only those species involved in the reaction are written.
Spectator ion – ion that is not involved in the reaction.
Eg. 5Write net ionic equations for the reactions in Eg. 3. We are replacing step 5 with this process.
a)
6) Determine which ions are spectators & leave them out of the equation.
7) Balance the equation.
8) Check to be sure the net electrical charge on each side is equal – if it is not something is wrong
b)
c)
d)
e)
f)
Eg. 6Write the net ionic equation for the reaction between lead (II) nitrate & potassium iodide.
Eg. 7Write the net ionic equation for the reaction between cadmium (III) chloride and sodium dichromate.
Practice: Z Ch 4for 47-49 just write net ionic equations, 50-52, 53 54 See MsO for answers
Solution Stoichiometry
Knowledge about solubility rules and use of molarity & stoichiometry allows chemists to determine the amount of precipitate that will form when two solutions or substances are mixed together.
Eg. 1 What mass of potassium dichromate is required to precipitate all of the solid that will form when mixed with 75.0mL of 0.200M Copper (II) sulphate?
Eg. 2 How much precipitate will form when 25 mL of 0.50M Na2C2O4 is mixed with 30mL of 0.40 M Zn(NO3)2?
Practice: Z Ch4 56-58, 60,61 BLB ch4, 52c, 56
56) 250. mL Na3PO458) 2.33g BaSO460) 2.9g AgCl, 0.10M NO3-, 0.075M Ca2+
Intro to Acids & Bases
Acids
There are several definitions of acids. I will introduce 2 of them here.
Arrhenius acids: substances which generate H+ or H3O+ in water
Bronsted-Lowry acid: substances which are capable of donating protons
Eg. HCl + H2O
Common everyday acids - Vinegar, citric, HCl 0.1 M gastric
Polyprotic acids are those that have more than one H+ to donate. Examples H2SO4
Bases
Again, there are several definitions of bases.
Arrhenius bases: substances which generate OH- in water.
Eg. NaOH + H2O
Bronsted-Lowry base: substances which act as proton acceptors
Eg. NH3 + H2O
Common everyday bases: Soap, NH3, drano, detergent, milk of magnesia, antacids
Properties
AcidBase
Tastesour/tartbitter
Feelburnslippery
Metal rxnyesno
Litmusredblue
Conductivityyes yes
Strength
The strength of an acid or base is determined by it’s dissociation in water. If the acid or base completely dissociates in water it is a strong acid or base. If the acid or base only partially dissociates in water it is a weak acid or base. The strength of a weak acid or base can vary.
Most acids & bases are weak. The strong acids & bases need to be memorized.
STRONG ACIDS HCl, HI, HBr, HNO3, H2SO4, HClO4, HClO3
STRONG BASES alkali hydroxides, Ca(OH)2, Sr(OH)2, Ba(OH)2,
pH is a scale often used to describe the concentration of an acid or base & typically ranges from 1 – 14.
pH = 7 is neutral, < 7 is acidic, & >7 is basic
Indicators
Acid- base Indicator: a substance, usually a weak acid itself, which changes color in different pH.
Examples include Phenolphthalein, methyl red, & BTB some of which you used last year.
Indicators are also used to indicate the end point of a reaction (when all of the initial reactant has reacted) through a color change & do not always change based on pH.
Neutralization rxns
When an acid & base react it is called a neutralization rxn because the product is water. The other product is salt.
Eg. 1Write the equation for the following acid-base reactions
a) between hydroiodic acid & calcium hydroxide.
b) between perchloric acid & chromium (III) hydroxide
c) acetic acid & potassium hydroxide
Net ionic equations for acid-base reactions
The basics for writing net ionic equations are the same, EXCEPT when writing equations that have weak acids or bases. A weak acid or base does not completely dissociate in water so it is NOT written in ionic form, but as an entire molecule.
Eg. 2Write the equation for the acid-base reactions in Eg. 1
a) write the products for the reaction, including water
b) determine if the reactants are strong or weak. If strong they separate, if weak they stay together.
c) determine if the product is soluble or not, separate it into ions if it is soluble
d) write everything out, if a participant is the exact same on both sides remove it from the equation.
e) balance what's left
Practice: Z Ch4 65to67 see MsO
Notable exceptions
Both carbonic acid & sulphurous acid are unstable weak acids which, when formed in aqueous solutions, break down into a water & soluble gas. The best example of this is the reaction btw vinegar & baking soda.
Eg. 3Write the equation for the reaction between vinegar & baking soda.
Eg. 4Write the products of the reaction between Na2SO3 & HCl.
Titrations
We talk about acids & bases in this chapter because neutralization rxns are a very common rxn that occurs in aqueous solution. Titration procedures are often used in acid-base chemistry.
Titration is an analytical procedure in which an unknown substance is combined slowly with a solution of known concentration & identity (titrant) until an end point is indicated by a change in color.
DEMO
Eg. 525 mL of 0.15M NaOH is added to a 50.0 mL sample of vinegar. What is the concentration of acetic acid in
vinegar?
Eg. 6What is the concentration of 50.0 mL of NaOH solution that is neutralized by 30.0 mL of 0.500 M HCl?
Practice: Z Ch4 69to71 73 75 77 78 (do 77 & 78 after BLB) BLBch4 29 30 32 39dh 40de 52abcd 54
68) many possibilities see MsO, 78) 1.528 x10-2 M NaOH