Chapter 13Solutions

Liquids

• Miscible means that two liquids can dissolve in each other–water and antifreeze, water and ethanol

• Partially miscible- slightly–water and ether

• Immiscible means they can’t–oil and vinegar

• In general the principle concerning whether substances will mix is "Like dissolves Like";

that is, polar (and ionic) substances will dissolve in polar solvents and non-polar substances will dissolve in non-polar solvents,

while non-polar and polar substances will not mix.

Solubility in Water :The simple rule to remember is ‘like dissolves like

+

2-

+

Water is a polar solvent and will dissolve polar molecules and substances that contain charged particles.

Substances that dissolve in H2O are said to be solubleE.g. Sugar, ethanol which are polarmost ionic compounds

sucrose

Site of polarity can form H-bonding

Water is polar, the different ends are attracted to the charged ions.

2- +

+

Cl-

Na+

Substances that don’t dissolve are called insolubleE.g. Petroleum (crude oil), which are non-polar

So if you want to dissolve grease which is non-polar, you need to use a non-polar solvent.

Petroleum in a non-polar organic molecule

Solution formation

• Nature of the solute and the solvent–Whether a substance will dissolve

–How much will dissolve

• Factors determining rate of solution...–stirred or shaken (agitation)

–particles are made smaller

–temperature is increased

Temperature and Solutions

• Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often.–Speeds up dissolving.

• Usually increases the amount that will dissolve (exception is gases)

How Much?• Solubility- The maximum amount of

substance that will dissolve at a specific temperature (g solute/100 g solvent)

• Saturated solution- Contains the maximum amount of solute dissolved

• Unsaturated solution- Can still dissolve more solute

• Supersaturated- solution that is holding more than it theoretically can; seed crystal will make it come out;

Concentration is...

• a measure of the amount of solute dissolved in a given quantity of solvent

• A concentrated solution has a large amount of solute

• A dilute solution has a small amount of solute–thus, only qualitative descriptions

• But, there are ways to express solution concentration quantitatively

• Concentration

= amount of solute

amount of solvent (or solution)

Molarity

(M) = amount of solute in moles

amount of solution in liters

What is the molarity of a solution with 2.0 moles of NaCl in 250 mL

of solution?

• Molarity= moles/liter

• = 2.0 mol/ .250 L

• = 8.0 M

• ( = 8.0 mol/l; = 8.0 molar; are other units that can be used.)

Calculating molarity

• Calculate the molarity of Na2SO4 (s) if you have 4.2 moles in 500. L of solution.

• Molarity = moles/liter

• = 4.2 mol/ 500. L

• = 8.4 x 10-3 M

• Could also have mol/l or molar as units

• How many moles of NaCl are needed to make 6.0 L of a 0.75 M NaCl solution?

• Moles = (0.75 mol/l)(6.0 l)

• = 4.5 mol

Calculate the mass of Na2 SO4 (s) in

1.6 L of a 2.5 M solution.

• Molarity = moles/volume• Moles = (molarity)(volume)• = ( 2.5 mol/l) ( 1.6 l)• = 4.00 mol

molesThen Find mass

Mole x molar mass

M x V mass

• Need molar mass: 142.02 g/mol

Mass = 568.08g = 570 g

Making solutions

• 10.3 g of NaCl are dissolved in a small amount of water, then diluted to 250 mL. What is the concentration?

• How many grams of sugar are needed to make 125 mL of a 0.50 M C6H12O6 solution?

Alternate Measures of Concentration

• Molar concentration is the accepted method of determining the amount of substance dissolved in a solvent.

• parts per million (ppm),

• parts per billion (ppb)

• parts per trillion (ppt).

• Part per million

• ppm = mass of solute x 106

mass of solution

• Part per billion

• ppb = mass of solute x 10 9

mass of solution

• Part per trillion

• ppt = mass of solute x 10 12

mass of solution

solvent ofkg

solute of moles(m)molality

Molality

(independent of temperature)

1 kg of water = 1 liter of water

Molality = moles of solute kg of solvent

Molality

• - What is the molality when 0.750 mol is dissolved in 2.50 L of solvent?

– Molality = 0.750 mol

2.50 kg

= 0.300m

Molality

• Ex- Suppose you had 58.44 grams of NaCl and you dissolved it in exactly 2.00 kg of pure water (the solvent). What would be the molality of the solution?

kg

molm

• Step One: convert grams to moles.• Step Two: divide moles by kg of solvent

to get molality.• 58.44 grams/mol is the molecular weight

of NaCl• 58.44g x 1 mol = 1.00 mol.• 58.44 g

Molality = 1.00 mol

2.0 kg

=0.500 mol/kg (or 0.500 m). or 0.500-molal.

• 1) Calculate the molality of 25.0 grams of KBr dissolved in 750.0 mL pure water. (0.280m)

• 2) 80.0 grams of glucose (C6 H12 O6, mol. wt = 180. g/mol) is dissolved in1.00 kg of water. Calculate the molality. (0.444m)

• 3) Calculate the molality when 75.0 grams of MgCl2 is dissolved in 500.0 g of solvent.(1.58m)

1. We can describe the concentration of a solution by using the mass percent.

Mass percent is the mass of solute present in a given mass of solution.

Mass Percent = mass of solute 100%Mass of solution

= grams of solute 100%(grams of solute) + (grams of solvent)

Example 1:We make a solution by dissolving 1.0g of sodium chloride in 48g of water. What is the mass percent of the solute?

Mass Percent = 1 .0 g solute x100% 49 g solution

= 2.0% NaCl

Example 2:A solution is prepared by dissolving 1.00g of ethanol, C2H5OH, with 100.0g of water. What is the mass percent of ethanol in this solution?Mass Percent = 1 .00 g ethanol 100%

101 g solution= 0.990% C2H5OH

Example 3:Cow’s milk typically contains 4.5% by mass of the sugar lactose, C12H22O11. Calculate the mass of lactose present in 175 g of milk.

Mass of solution (milk) = 175g

Mass percent of solute (lactose) = 4.5%

Mass Percent = grams of solute 100%grams of solution

4.5% = grams of solute 100% 175 g

Grams of solute = (4.5%)(175 g) = 0.045 175g 100%

Grams of solute = 7.9g lactose