Chapter 5 A Matter of Concentration

Ionic Phenomena = Things that happen to ions, which can be observed

What is matter? STUFF!

Matter can be found in 3 states:

Matter can be categorized 5 ways!

Matter Element Compound Pure substancesMixtures Homogeneous mixtures Suspensions Heterogeneous mixtures

Element Substances on the periodic table Made up of identical atoms 100% copper wire (Cu)Pure gold (Au)

More elements Chlorine gas (Cl 2 )

Compound A substance which contains different atoms Water (H 2 O)

Sugar (C 12 H 22 O 11 ) Salt (NaCl)

Mixtures Contain two or more different elements or compounds, physically mixed together

Homogenous Mixtures To the naked eye, looks like one substance (completely uniform) Salt water – looks clear like pure water

Vinegar is a mixture of acetic acid (CH 3 COOH) and water (H 2 O) Brass – looks like a pure metal, but it’s a mixture of copper and zinc Solution = Liquid homogenous mixture

Sterling silver – mixture of silver and copper Plumber’s solder – mixture of lead and tin

Stainless steel – mixture of iron, carbon, chromium, and nickel Air!

Heterogeneous Mixtures To the naked eye, it looks like it’s made of two or more substances – an obvious mixture These substances can be taken apart into their original components

Asphalt Dirt Concrete

The dreaded fruitcake!

Suspensions Between homogenous and heterogenous mixtures – tiny solid bits floating around in liquid Murky water

Tomato juice – miniscule tomato particles floating in water Strawberry milkshake

And now, onto Solutions

Milk is a suspension The percentage (1%, 2%, 3.25%) means that it contains that much milk fat, and the rest is water

Most of the milk is made of water Water = solvent (what the milk fat is dissolved into) Milk fat = solute (particles dissolved into solvent) Milk in carton = solution (total volume = solvent + solute)

Most of the milk is made of water Highest percentage = Highest concentration of milk fat

Most of the milk is made of water The higher the percentage of milk fat (solute), the less water is needed as a solvent

Most of the milk is made of water In 1L of 1% milk: 10ml milk fat, 990ml water In 1L of 2% milk: 20ml milk fat, 980ml water In 1L of 3.25% milk: 32.5ml milk fat, 967.5ml water

Concentration Concentration of milk = amount of milk fat amount of milk

Concentration Concentration of milk = amount of milk fat amount of milk

Concentration equation Concentration (c) = Solute mass (m) solution volume (V) C = m V C  g/L m  g V  L

Making salt water If you add 1g of NaCl to 1 L of water, you get: C = 1 g = 1 g/L 1 L 1 g/L aqueous solution of NaCl

Making salt water If you add 10g of NaCl to 1 L of water, you get: C = 10 g = 10 g/L 1 L 10 g/L aqueous solution of NaCl

Making salt water If you add 10g of NaCl to 500 mL of water, you get: C = 10 g = 20 g/L 0.5 L 20 g/L aqueous solution of NaCl Remember: 1000ml = 1L

Arrange these solutions from lowest to highest concentration Solution# grams of sugar # Litres of solution Concentration (g/L) A3 g1 L B5 g0.5 L C8 g4 L D7 g3 L

Arrange these solutions from lowest to highest concentration Solution# grams of sugar # Litres of solution Concentration (g/L) A3 g1 L3 g/L B5 g0.5 L10 g/L C8 g4 L2 g/L D7 g3 L2.3 g/L Solution C  Solution D  Solution A  Solution B

Which salt solution is most concentrated? Solution# grams of salt # Litres of solution Concentration (g/L) A10 g400 ml B0.5 kg1.5 L C0.2 kg800 ml

Which salt solution is most concentrated? Solution# grams of salt # Litres of solution Concentration (g/L) A10 g400 ml25g/L B0.5 kg1.5 L333.3g/L C0.2 kg800 ml250g/L C = m ÷ V = 10g ÷ 0.4L = 25g/L C = m ÷ V = 0.5kg ÷ 1.5L = 500g ÷ 1.5L = 333.3g/L C = m ÷ V = 0.2kg ÷ 0.8L = 200g ÷ 0.8L = 250g/L

Different ways to say numbers of stuff 12 eggs = 1 dozen eggs A score of years = 20 years Avogadro’s number = 602, 000, 000, 000, 000, 000, 000, 000 = 6.02 x molecules = One mole

Mole! Quick lesson on the mole and Avogadro’s number: g

Molar mass An element’s atomic mass (in grams) has one mole of that element’s atoms Atomic mass of Fluorine = U Molar mass of Fluorine = g (round to 19g) = 1 mole of Fluorine atoms Symbol for mole = mol

Molar mass An molecule’s atomic mass (in grams) has one mole of that molecule’s atoms Atomic mass of water (H 2 0) = (2 x 1.008) = U Molar mass of H 2 0 = g (round to 18 g) = 1 mole of water molecules

The Mole equation Number of moles (n) = mass of solute (m) Molar mass (mm) n = m mm n  mol m  g mm  g (per mol)

How many moles of atoms are in 444g of radon (Rn)?

Step 1. Find the atomic mass of Radon Rn 86 Radon Atomic mass = U

Step 2. Make the atomic mass into grams per mole (molar mass) of Rn Rn 86 Radon Molar mass = g  222 g

Step 3. Find out the number of moles using the mole equation Mass (m) = 444 g Molar mass (mm) = g  222 g mm n = m 222 g/mol n = 444 g n = 2 moles of radon

How many moles are in 591g of gold (Au)?

Step 1. Find the atomic mass of Gold Au 79 Gold Atomic mass = U 2.4

Step 2. Make the atomic mass into grams per mole (molar mass) of Au Au 79 Gold Molar mass = g  197 g 2.4

Step 3. Find out the number of moles using the mole equation Mass (m) = 571 g Molar mass (mm) = g  197 g mm n = m 197 g/mol n = 571 g n = 3 moles of gold

Steps to find the number of moles of an element/compound Step 1. Find the atomic mass of the element/compound Step 2. Make the atomic mass into grams per mole (molar mass) Step 3. Find out how many moles are in the given weight of the element/compound using the mole equation

These calculations also work with compounds!

How many moles are in 16.3 g of magnesium bromide (MgBr 2 )?

Step 1. Find the atomic mass of MgBr 2 Mg 12 Magnesium Atomic mass = (79.904) = U Br 35 Bromine

Step 2. Make the atomic mass into grams per mole (molar mass) of MgBr 2 Mg 12 Magnesium Molar mass = g  g Br 35 Bromine

Step 3. Find out the number of moles using the mole equation Mass (m) = 16.3g Molar mass (mm) = g  g mm n = m g/mol n = 16.3 g n = moles of MgBr 2

Molar concentration (Molarity) Molar concentration (C) = # moles of solute (n) volume of solution (V) C = n V C  mol/L n  mol V  L

A solution of 800ml contains 2 moles of NaCl. What is the molar concentration of the solution?

There is only one step: Calculate M! V C = n = 2 mol = 2.5 mol/L = 2.5 M 0.800L

A chemist wants to prepare a 1.25M aqueous solution of Ca(NO 3 ) 2. She has 82g of calcium nitrate. Calculate the maximum volume that the chemist can prepare.

Molarity = Molar concentration Compound: Can use to get molar mass Mass of soluteWhat we’re looking for: Volume (L)

Step 1. Write down what you have, and what you want We have: C = 1.25 M = 1.25 mol/L mass of solute = 82 g solute = Ca(NO 3 ) 2 We want: Volume of solution (L) = ?

Step 2. Find the molar mass of Ca(NO 3 ) 2 N 7 Nitrogen Molar mass = (14) + 6(16) = g  g/mol O 8 Oxygen Ca 20 Calcium CaN 2 O 6

Step 3. Find out the number of moles using the mole equation Mass (m) = 82 g Molar mass (mm) = g  g mm n = m g/mol n = 82 g n = 0.5 moles of Ca(NO 3 ) 2

Step 4. Plug in values to calculate the volume of the solution V C = n

Step 4. Plug in values to calculate the volume of the solution V C = n V 1.25 mol/L = 0.50 mol

Step 4. Plug in values to calculate the volume of the solution V C = n V 1.25 mol/L = 0.50 mol cross-multiply

Step 4. Plug in values to calculate the volume of the solution V C = n V 1.25 mol/L = 0.50 mol cross-multiply 1.25 mol/L (V) = 0.50 mol

Step 4. Plug in values to calculate the volume of the solution V C = n V 1.25 mol/L = 0.50 mol cross-multiply 1.25 mol/L (V) = 0.50 mol 1.25 mol/L

Step 4. Plug in values to calculate the volume of the solution V C = n V 1.25 mol/L = 0.50 mol cross-multiply 1.25 mol/L (V) = 0.50 mol 1.25 mol/L V = 0.40 L

How to solve Molarity problems Step 1. Write down what you have, and what you want Step 2. Find the molar mass of the element/compound Step 3. Find out the number of moles using the mole equation Step 4. Plug values into the molarity equation

What if you need another concentration than what you have? They have to dilute it themselves

This also saves $$ – chemicals are expensive! They mostly buy the really concentrated ones

Back to the milk example In 1L of 1% milk: 10ml milk fat, 990ml water In 1L of 2% milk: 20ml milk fat, 980ml water In 1L of 3.25% milk: 32.5ml milk fat, 967.5ml water

If you really wanted to, you could buy 3.25% milk and add water until it had a concentration of 1% If only you knew the dilution equation...

Dilution equation Higher Concentration sol’n (C 1 ) x Its Volume (V 1 ) Lower Concentration sol’n (C 2 ) x Its Volume (V 2 ) C 1 V 1 = C 2 V 2 C 1, C 2  mol/L V 1, V 2  L = sol’n = solution

Using a stock solution of 17.5 mol/L, you want to prepare 500 ml of a 1.00 mol/L acetic acid solution (CH 3 COOH). What volume of stock solution will you need?

Higher concentration solution (C 1 ) Higher concentration sol’n = stock solution Lower concentration solution (C 2 ) What we’re looking for: V 1 = Volume of C 1 Solute V 2 = Volume of C 2

Step 1. Write down what you have, and what you want We have: C 1 = 17.5 mol/L V 2 = 500 ml = 0.5 L C 2 = 1.00 mol/L solute = CH 3 COOH We want: V 1 = ?

Step 2. Plug values into dilution equation C 1 V 1 = C 2 V 2

Step 2. Plug values into dilution equation C 1 V 1 = C 2 V 2 (17.5 mol/L) (V 1 ) = (1.00 mol/L) (0.5 L)

Step 2. Plug values into dilution equation C 1 V 1 = C 2 V 2 (17.5 mol/L) (V 1 ) = (1.00 mol/L) (0.5 L) 17.5 (V 1 ) = 0.5

Step 2. Plug values into dilution equation C 1 V 1 = C 2 V 2 (17.5 mol/L) (V 1 ) = (1.00 mol/L) (0.5 L) 17.5 (V 1 ) =

Step 2. Plug values into dilution equation C 1 V 1 = C 2 V 2 (17.5 mol/L) (V 1 ) = (1.00 mol/L) (0.5 L) 17.5 (V 1 ) = V 1 = L = 28.6 mL

Steps to solving a dilution problem Step 1. Write down what you have, and what you want Step 2. Plug values into dilution equation

pH scale Used to test the strength of an acid or base (Back to the)

Acid: pH less than 7 Neutral: pH exactly 7 Base: pH more than 7

The pH of various substances SubstancepH (approximate) Sulphuric acid (H 2 SO 4 )0 Hydrochloric acid (HCl), 0.1 M1.0 Vinegar (CH 3 COOH)2.2 Soft drinks, wine, beer3.0 Apples3.1 Black coffee5.0 Soap10.0 Household ammonia11.1 Depilatory cream12.0 Sodium hydroxide (NaOH), 0.1 M13.0

The pH of various substances SubstancepH (approximate) Sulphuric acid (H 2 SO 4 )0 Hydrochloric acid (HCl), 0.1 M1.0 Vinegar (CH 3 COOH)2.2 Soft drinks, wine, beer3.0 Apples3.1 Black coffee5.0 Soap10.0 Household ammonia11.1 Depilatory cream12.0 Sodium hydroxide (NaOH), 0.1 M13.0

How can HCl be only twice as acidic than vinegar? The way pH is calculated is not linear

pH Scale – How it works The pH scale works by powers of 10. That means that an acid with pH 1 has 10 times more H + ions than pH 2. (10 times stronger acid) The left side shows how many times more H + one pH has compared to another.

What does pH actually mean? What does it measure? pH = potential of Hydrogen = H + ion concentration of a solution (M or mol/L)

H + ion concentration (M)Exponential notation (M)pH 1.01 x x x x x x x x x x x x x x x

So if the pH of a solution is 2, it has an H + concentration of 1 x mol/L = 0.01 mol/L

How do chemists find the actual pH of a solution? Litmus paper is not that accurate It just tells you if it’s an acid or a base

Acid-base indicators Different substances change colour at a precise pH Used together, they can give the approximate pH of a substance

bromophenol blue methyl red universal resazurin bromocresol purple phenolphthalein Turning point = when the indicator changes colour

Turning Point – Methyl Red Let’s see what happens when we use Methyl Red. MR is a liquid. If we put a few drops into each beaker, we’ll see what happens. pH 0 pH 1 pH 2 pH 3 pH 4 pH 5 pH 6 pH 8 pH 9 pH 10 pH 11 pH 12 pH 13 pH 14 pH 7

Turning Point – Methyl Red So what is the turning point of Methyl Red? pH 0 pH 1 pH 2 pH 3 pH 4 pH 5 pH 6 pH 8 pH 9 pH 10 pH 11 pH 12 pH 13 pH 14 pH 7

Turning Point – phenolphthalein Let’s see what happens when we use phenolphthalein If we put a few drops into each beaker, we’ll see what happens. pH 0 pH 1 pH 2 pH 3 pH 4 pH 5 pH 6 pH 8 pH 9 pH 10 pH 11 pH 12 pH 13 pH 14 pH 7

Turning Point – phenolphthalein So what is the turning point of phenolphthalein? pH 0 pH 1 pH 2 pH 3 pH 4 pH 5 pH 6 pH 8 pH 9 pH 10 pH 11 pH 12 pH 13 pH 14 pH 7

pH paper = Universal indicator Here, the universal indicator paper is dipped in the solution and compared to the colours

What is the pH range of this solution? IndicatorColour changeTurning point Phenol redyellow  red6.4 – 8.2 Bromophenol blueyellow  violet3.0 – 4.6 Indigo carmineblue  yellow12.0 – 14.0 Bromothymol blueyellow  blue6.0 – 7.6 This solution is yellow in phenol red, violet in bromophenol blue, blue in indigo carmine and yellow in bromothymol blue

What is the pH range of this solution? IndicatorColour changeTurning point Phenol redyellow  red6.4 – 8.2 Bromophenol blueyellow  violet3.0 – 4.6 Indigo carmineblue  yellow12.0 – 14.0 Bromothymol blueyellow  blue6.0 – 7.6 This solution is yellow in phenol red (pH < 6.4) violet in bromophenol blue (pH > 4.6) blue in indigo carmine (pH < 12.0) and yellow in bromothymol blue (pH < 6.0) pH of solution: 4.6 – 6.0