Unit 7: Solution Chemistry

Section 1: Molarity and Molality

SolutionsAre a combination of at least two ingredients, such

as water and saltThe substance in the larger quantity is the solventThe substance in the smaller quantity is the solute

Are homogeneous mixturesDon’t have to be liquids

Air is a solution composed of nitrogen (the solvent), oxygen, water vapor, carbon dioxide, and other gases

Brass is a solution composed of copper and zinc

ConcentrationThe higher the ratio of solute to solvent, the more

concentrated the solutionExample: Kool-Aid can be as diluted or concentrated

as you want it to be, just add more water to dilute the drink

The wrong concentration can cause an object to work improperly or be harmful to living thingsExample: Eye drops for humans can be harmful for a

canine because of the different concentration requirements

SaturationWhen the solute and solvent are put together,

there is a limit as to how much solute can be dissolvedSaturated solution: no more solute will dissolve at a

specific temperatureUnsaturated solution: more of the solute can be

dissolved at the same temperatureSupersaturated solution: an unstable and temporary

situation where the solution contains more dissolved solute than it normally would Occurs when there is a change in temperature, volume, or

pressure

Molarity (M)The number of moles of solute per liter of

solutionM = moles

literDeals with concentration

Molarity Example 1What is the molarity of a solution made by

dissolving 0.3 mole of Ca(NO3)2 in enough water to make 500 mL of solution?M = moles

liter= 0.3 mol 0.5 liter= 0.6 M Ca(NO3)2

Molarity Example 2How much 0.5 M NaCl can be made from 0.1

mole of NaCl?M = moles

liter0.5 M = 0.1 mole

x liters

0.5x = 0.1x = 0.2 L NaCl

Molarity Example 3What is the molarity of an Epsom salt (MgSO4)

solution made by dissolving 6 grams of it in enough water to make 2 liters of solution?Step 1: convert grams to moles 6 g MgSO4 x 1 mol MgSO4 = 0.05 mol

MgSO4

120.366 g MgSO4

Step 2: M = moles liter

M = 0.05 mol = 0.03 M MgSO4

2 liters

Molarity Example 4How many grams of KOH are needed in order to

prepare 2 liters of a 0.25 M solution?Step 1: M = moles

liter 0.25 M = x mol 2 L

x = 0.5 mol KOHStep 2: convert moles to grams

0.5 mol KOH x 56.1049 g KOH = 30 g KOH 1 mol KOH

Molality (m)The number of moles of solute per kilogram of

solventm = moles of solute

kilogram of solventAlso deals with concentration, but used less

frequently

Molality Example 1Calculate the molality of an antifreeze solution

made by dissolving 186 g of antifreeze (C2H6O2) in 2 kg of water.Step 1: Convert grams to moles

186 g C2H6O2 x 1 mol C2H6O2 = 3.00 mol C2H6O2

62.0674 g C2H6O2

Step 2: m = moles of solute kilogram of solvent

m = 3.00 mol = 2 m C2H6O2

2 kg

Molality Example 2How many grams of lye (NaOH) should be added to

800 g of water in order to make a 0.5 m solution? Step 1: Convert g of solvent to kg of solvent 800 g x 1 kg = 0.8 kg 1000 gStep 2: Solve for grams of solute m = moles of solute

kilogram of solvent 0.5 m = x mol = 0.4 mol NaOH 0.8 kgStep 3: Convert mol NaOH to g NaOH 0.4 mol NaOH x 39.9959 g NaOH = 20 g NaOH 1 mol NaOH