Unit 5 – Chap 10 & 16Solutions & Empirical Formulas (last but definitely not least!)

April

Solutions and Entropy

What do you know already about solutions?

Solutions are:• Solutions of gases, liquids or

solids are everywhere• Homogenous mixture• Solute and Solvent• Is this a solution:

– AIR– CARBON DIOXIDE GAS– CARBONATED BEVERAGE– SALAD DRESSING– 14 KARAT GOLD– 24 KARAT GOLD

Solutions

A solution is a homogenous mixture of 2 or more substances mixed evenly at a particle level.

One part is regarded as the solvent and the others as solute

Solution Terminology

Solute – substance being dissolved (lesser amt)

Solvent – component whose physical state remain the same (greater amt)

Practice: 14.1 - WHICH IS A SOLUTE AND WHICH IS THE SOLVENT? 2 OUNCES OF OIL & 2 GALLONS OF GASOLINE CARBON DIOXIDE IN A SELTZER WATER

Dissolve – to mix uniformly and completely at the molecular level

Miscible – liquids that can dissolve in one another form solution Immiscible - liquids that don’t dissolve in one anotherAqueous solution (aq)

Solutions – phet simSolubility measures the maximum amount of solute that can be dissolved in a solvent at a given temperature. (measures maximum concentration)

Ionic solutes

Called hydrated ions when the solvent is water

The dissolving process

Watch another version

Watch NaCl dissolve in water

The dissolving process

• When an ionic compound dissolves in water:– ionic bonds are broken– attraction between water

molecules are broken– attraction between water

and ions are formed

What happens during dissolving?

• How does arrangement of particles change?

• How does motion of particles change?

Lattice energy and hydration energy

Why do solutions form? • One major driving force is energy• Is dissolving of an ionic compound endothermic

or exothermic? Why?– Example of exothermic dissolving:

• dissolving LiCl in water

– Example of endothermic dissolving:• dissolving NH4Cl in water

• Which bonds/attractions broken during dissolving?• Which bonds/attractions formed during dissolving?• Which are stronger?

Entropy: A new concept that explains a lot

Entropy =

Due to the laws of probability, the entropy of the universe is constantly increasing and never decreases.

Processes that would lead to a decrease in entropy of the universe are not possible.

The universe can be divided into the system and its surroundings.

Why do things happen?

• There are only two reasons for an event to occur (including a chemical process):– Entropy of the system increases

– Entropy of the surroundings increases (exothermic reactions are an increase in the surroundings’ entropy)

MOLARITY AND DILUTIONSChapter 9

Solution concentration- terms• Dilute

– Small amount of solute• Concentrated

– Large amount of solute

Stock - routinely used solutions prepared in concentrated form.

How do you quantify concentration of a solution?

• Concentration = amount of solute per amount of solution (or solvent)

• Molarity = moles solute / liters solution• Example problems:

– What is the molarity of a solution if 23 g of KCl are dissolved to make 4 L of solution?

– How many grams of KCl should be dissolved to make 500. mL of a 0.10 M solution?

Solute concentration expressions

• There are a few different ways to calculate the concentration of the solution.

• In Biology, we used: • Mass percent:

(mass solute / mass of solution) * 100

• This year in Chemistry, we will also use:• Molarity(M):

moles solute / Liter solution

• Molarity (M) = moles of solute per volume of solution in liters:

• Because volume is temperature dependent, molarity can change with temperature.

mol of soluteL of solutionM =

Molarity (M)

6 moles of HCl2 liters of solution

3 M HCl =

Concentration: molarity exampleIf 0.435 g of KMnO4 is dissolved in enough water to give 250. mL of solution, what is the molarity of KMnO4?

Now that the number of moles of substance is known, this can be combined with the volume of solution — which must be in liters — to give the molarity. Because 250. mL is equivalent to 0.250 L .

As is almost always the case, the first step is to convert the mass of material to moles.

0.435 g KMnO4 • 1 mol KMnO4 = 0.00275 mol KMnO4

158.038g KMnO4

Molarity of KMnO4 = 0.00275 mol KMnO4 = 0.0110 M 0.250 L solution

Molarity problems

1. What is the molarity of a solution if 23 g of KCl are dissolved to make 4 L of solution?

2. How many grams of KCl should be dissolved to make 500. mL of a 0.10 M solution?

Molarity problems

3. A truck carrying 22.5 kL of 6.83 M aqueous hydrochloric acid (HCl) used to clean brick has overturned on the highway, and the acid needs to be neutralized. How many moles of HCl have been spilled?

Dilution

• What does it mean to dilute a solution? What is added?– Solvent is added to lower the concentration.

• When a solution is diluted, what is conserved? – The amount of solute remains constant before and after

the dilution.

moles BEFORE = moles AFTER

M1V1 = M2V2

Common Terms of Solution Concentration

Stock - routinely used solutions prepared in concentrated form.

Concentrated - relatively large ratio of solute to solvent. (5.0 M NaCl)

Dilute - relatively small ratio of solute to solvent. (0.01 M NaCl): (MV)initial=(MV)Final

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Dilution problems 1. Suppose you have 0.500 M NaCl stock solution. How do you

prepare 250. mL of 0.348 M NaCl solution ?

Concentration 0.500 M NaCl

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Figure 4.10: Steps involved in the preparation of a standard aqueous

solution.

Making a solution staring with a solid

How to make solutions

• Starting with a solid:– Does volume of solute = volume of solution?– What type of glassware is best for accurate

measurement of a certain volume of liquid AND mixing a solution?

• Starting with a concentrated solution:– How does the number of moles of solute change

when a solution is diluted?– What type of glassware is best for accurate delivery

of a liquid with a certain volume?

Dilution problems

2. What is the molarity of a solution of ammonium chloride prepared by diluting 50.00 mL of a 3.79 M solution to 2.00 L?

3. To what volume should 1.19 mL of an 8.00 M solution be diluted in order to obtain a final solution that is 1.50 M?

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Figure 4.12: Dilution Procedure (a) A measuring pipet is used to transfer 28.7mL of 17.4 M acetic acid solution to a volumetric flask. (b) Water is added to the flask to the calibration mark. (c) The resulting solution is 1.00 M acetic acid.

Dilution

When a solution is diluted, _____________ are conserved.

Since M = _________, therefore moles = _______

Example problemsWhat is the molarity of a solution of ammonium chloride

prepared by diluting 50.00 mL of a 3.79 M solution to 2.00 L?To what volume should 1.19 mL of an 8.00 M solution be

diluted in order to obtain a final solution that is 1.50 M?

Composition Calculations

Describing the composition of compounds

• Percent composition (by mass) used to describe what elements a compound is made of, in what proportions.

• Experimentally measured

• % mass of element =

– According to formula OR– According to experimental results

Finding experimental percent composition

• A sample of iron oxide with a mass of 4.76 g is found to contain 3.70 g of iron. What is its percent composition?

Calculating theoretical percent composition from formula

• What is the percent composition of aluminum oxide?

Using percent composition to calculate mass

• Calculate the mass of zinc in a 30.00g sample of zinc nitrate.

50.0g of SF6

• How many grams of sulfur is that?

Empirical formula

• You can use % composition to find out the formula of an unknown compound!

Empirical = determined experimentally Empirical formula = simplest mole ratio

• Examples of empirical formulas:

Empirical formula example #1

• A sample of an iron oxide is 69.94% Fe and 30.06% O. What is the empirical formula?

Empirical formula example #2

• In lab, a student analyzes a sample of nickel chloride and finds that it contains 35% Ni and 65% Cl. What is the empirical formula?

Empirical formula example #3

• A piece of copper ore with a mass of 0.858 grams is found to contain 0.537 g of copper. The remainder of the mass is fluorine. What is the empirical formula of this ore?

Practice problems

Try these problems in small groups:

• What is the percent composition of tin(IV) oxide?• A sample of silver(I) sulfide has a mass of 62.4 g. What

mass of each element could be obtained by decomposing this sample?

• A sample of copper chloride is found to contain 5.46 g of copper and 6.10 g of chloride. What is its percent composition?

Lab procedures

• Iodine vapor is toxic – keep the beakers covered and don’t breathe fumes.

• Decant:

• Gentle heating:

• Mass of product:

PERCENT YIELD• Theoretical Yield – the “calculated” amount. It is

the maximum amount that can be produced by the given reaction.– Reasons for less than 100% yield:

• Incomplete rxn of the limiting reagent• Less than ideal reacting conditions• Reversible rxns• Formation of unwanted reaction products• Loss of product in transferring from one vessel to another

• Actual Yield (experimental) - the quantity of the product that is actually obtained from the reaction.