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Chemistry 30

Unit 5:

Solutions and Solubility

TEACHER

Components of a Solution

· A solution is composed of two or more pure substances mixed together, one of which is a solvent and the other a solute.

· A cup of instant coffee is the solution, the hot water is the solvent, and the instant coffee is the solute.

· A saturated solution is a solution that contains the maximum amount of solute dissolved in a solvent. A supersaturated solution is a solution where conditions have been changed to allow more solute to dissolve than would at room temperature (this is done by heating the solvent or solution).

· When the solvent and solutes are liquids we can use the terms miscible and immiscible. Miscible means that the two liquids will mix together, and immiscible means that the two liquids will not dissolve in one another.

· Remember, the solvent and solute can be solid, liquid OR gas. There are special names for solutions where the solute and solvent are both solids and when the solute is a liquid, and the solvent is a solid.

· Solid-in-solid – alloys (e.g., brass = zinc + copper, sterling silver = silver + copper).

· Liquid-in-solid – amalgams (e.g., older dental fillings = mercury + silver).

· Factors that may affect solubility are temperature and pressure:

· Temperature: Gases: Temp ⬆, solubility ⬇

Solids and liquids: Temp ⬆, solubility ⬆

· Pressure: Gases: Pressure ⬆, solubility ⬆

Solids and liquids: unaffected

· Like dissolves like. That is polar covalent molecules usually dissolve in other polar covalent molecules and non-polar covalent molecules dissolve in non-polar covalent molecules.

RECALL: Polarity can be determined by subtracting electronegativities. A polar molecule is one where one (or more) atom in the molecule has a stronger pull on the electrons than another atom, so the electrons move closer to the atom with the stronger pull (higher electronegativity). This creates a dipole, where the atom with the stronger pull has a slightly negative charge and the atom with the lower electronegativity (less pull) has a slightly positive charge.

Water as a Solvent:

· Solutions in which water is the solvent are called aqueous solutions.

· Most covalent liquids are only able to dissolve other covalent compounds; water can dissolve both ionic and covalent compounds.

The attraction of water dipoles for ions pulls ions out of a crystalline lattice and into aqueous solution.

Aqueous Solutions:

· Sometimes when we mix two dissolved salts together, we end up with a precipitate. This is because a new ionic compound is formed that is insoluble in water.

We can use solubility charts or solubility tables to determine if a solid will be formed.

Ion

Solubility

Exceptions

NO3–

soluble

none

ClO4–

soluble

none

Cl–

soluble

 except Ag+, Hg22+, *Pb2+

I–

soluble

except Ag+, Hg22+, *Pb2+

Br-

soluble

except Ag+, Hg22+, *Pb2+

SO42-

soluble

except Ca2+, Ba2+, Sr2+, Hg2+, Pb2+, Ag+

CO32-

insoluble

except Group IA and NH4+

PO43-

insoluble

except Group IA and NH4+

OH–

insoluble

except Group IA, *Ca2+, Ba2+, Sr2+

S2-

insoluble

except Group IA, IIA and NH4+

Na+

soluble

none

K+

soluble

none

NH4+

soluble

none

* = slightly soluble

Ex. If we mix sodium ions with hydroxide ions will a precipitate form?

No

Ex. If we dissolved sodium hydroxide in calcium chloride, will a precipitate form?

2NaOH + CaCl2 2NaCl + Ca(OH)2

NaCl is soluble, Ca(OH)2 is not soluble so a precipitate will form.

Ex. Calcium nitrate with sodium carbonate

Ca(NO3)2 + Na2CO3 CaCO3 + 2NaNO3

Calcium Carbonate is insoluble so a precipitate will form, sodium nitrate is soluble

See Components of Solutions Assign

Concentration

· Concentration refers to the amount of solute dissolved in a specific amount of solvent.

· Concentrated and dilute are qualitative terms we use to describe concentration. Dilute means there is not a lot of solute in the solution and concentrated means there is a lot of solute in the solution.

· Common concentrations are molarity, molality, ppm, and ppb.

Molarity

This is the number of moles of solute dissolved in one litre of solution.

The formula for molarity is as follows:

c =

n = # moles of solute

V = volume of solution in litres

c = concentration in moles per litre (M)

Ex. If a teaspoon (5.0mL) of a 0.50 M solution of NaCl was evaporated, how many moles of sodium chloride would be left? What mass of NaCl would be left?

= 0.0025 mol

Ex. Antifreeze is a solution of ethylene glycol, C2H6O2, in water. If 4.50 L of antifreeze contains 2.00 kg of ethylene glycol, what is the concentration of the solution?

Moles of C2H6O2 = 2.00 × 103 g C2H6O2 × = 32.2 mol C2H6O2

c= = = 7.16 mol/L

Standard solution:

· A standard solution is a solution of a known concentration. This means it has a precise mass of solute in a specific volume of solution.

· Standard solutions are used in experiments where the concentration of a solution must be known.

· We use volumetric flasks to prepare standard solutions as they have a small margin of error when compared to other pieces of lab equipment. Volumetric flasks come in a variety of sizes (volumes).

To prepare a standard solution we follow the following steps:

1. Calculate the required mass of solute needed using the volume and concentration you want to end up with.

2. Weigh out the mass of the solute needed and add it to a volumetric flask of the appropriate size.

3. Dissolve the solid in pure water using less than half of the final solution volume.

4. Once the solute is dissolved, add the rest of the water. Be sure to use a medicine dropper for the final few milliliters of water. Use the calibration line to set the meniscus in the appropriate spot.

Ex. Describe the preparation of 2.000 L of a standard aqueous solution containing 0.1000 mol/L potassium nitrate.

n = Vc

= 2.000 L × 0.1000 mol/L = 0.2000 mol

mass of KNO3 = 0.2000 mol KNO3 × = 20.22 g KNO3

To prepare the solution we place 20.22 g of KNO3 in a 2.000 L volumetric flask. About half the required amount of water is added. When all the potassium nitrate is dissolved, the solution is diluted with the remaining water.

Dilution Calculations:

· When making a solution in chemistry laboratories you usually only have access to substances with high concentration solutions (called stock solutions) and are then required to dilute the stock solutions. A calculation needs to be completed in order to determine the amount of distilled water that needs to be added to a certain volume of stock solution in order to create the desired concentration.

· Since the number of moles of solute in a solution does not change when you dilute it, the equation for dilution is as follows:

V1 C1 = V2 C2

before after

Ex. Water is added to 200.mL of 2.40M ammonia cleaning solution (NH3), until the final volume is 1.00L. Find the molar concentration of the final diluted solution.

Vi Ci = Vf Cf

(0.200L)(2.40M) = (1.00L)(Cf)

Cf = 0.480M

Ex . What volume of concentrated sulphuric acid (containing 18.0 M H2SO4) is required to prepare 5.00 L of 0.150 M aqueous sulphuric acid solution by dilution with water?

V1 = ? V2 = 5.00 L

c1 = 18.0 mol/L c2 = 0.150 mol/L

V1 =

= = 0.0417 L = 41.7 mL

Ion Concentration:

Consider a 0.20M aqueous solution of sodium carbonate. The sodium carbonate will be completely dissociated into ions:

Na2CO3(s) 2 Na+(aq) + CO32-(aq)

The concentration of the sodium ions can be calculated using the conversion factors from the balanced equation (similar to mol- mol stoichiometry). Remember: concentration is represented by using square brackets.

[Na+] = 0.20 mol/L Na2CO3 × = 0.40 mol/L [Na+]

[CO32-] = 0.20 mol/L Na2CO3 × = 0.20 mol/L [CO32-]

Ex. What are the concentrations of the ions in an aqueous solution containing 0.15 mol/L iron(III) nitrate?

Fe(NO3)3(s) Fe3+(aq) + 3 NO3−(aq)

[Fe3+] = 0.15 mol/L Fe(NO3)3 × = 0.15 mol/L Fe3+

[NO3−] = 0.15 mol/L Fe(NO3)3 × = 0.45 mol/L NO3−

Ex. 250mL of 0.30M K2SO4 and 250mL of 0.80M MgCl2 are mixed and no reaction results. What is the concentration of each substance in the final solution, and the concentration of each individual ion?

[K2SO4]:

Cf = 0.15M

[MgCl2]:

Cf = 0.40M

[K+]=0.30M [SO42-]=0.15M [Mg2+]=0.40M [Cl-]= 0.80M

Ppm & ppb:

· Usually in chemistry we work with molarity when discussing concentration. When working with very dilute solutions (solutions with low concentrations) it is more convenient to work with ppm or ppb.

· A “part” refers to a gram.

· A concentration of 1ppb is equivalent to 1g of solute dissolved in 1,000,000,000g of solvent.

· The allowable concentration of toxic chemicals is usually measured in ppm or ppb.

· For example the allowable concentration of