Solutions

Just because 2 or more substances are mixed together, does not mean they will react chemically (meaning that their atoms rearrange and new substances are formed). Often, the substances are simply physically mixed, resulting in the formation of a mixture. In a mixture, new substances ARE NOT formed, and the properties of the original substances contribute to the properties of the mixture.

Heterogeneous mixtures result when 2 or substances are mixed physically, but does not result in a uniform composition. Examples include salads, oil & water, and sand-shells-etc. on an average beach. While very common, they are not the focus our chemical production unit.

Solutions

Solutions are homogenous mixtures. They contain substances (elements and/or compounds) that have been physically mixed together and that have a uniform composition at the particle level.

A solute is a substance that is dissolved by a solvent to make a solution (a homogenous mixture). The solvent is the substance present in larger amount in the mixture, while the one that is present in a smaller amount is called the solute. When a solute mixes with a solvent, we usually say it is dissolved by the solvent. The result is that the individual particles (atoms or molecules) of the solute are spread evenly around the solvent to make a solution.

A solution where water is the solvent is called an aqueous solutions. In chemical equations you can use the symbol (aq).

Other examples of solutions include regular air, salt water and most drinks.

.

Concentration & Reaction Rate

.

The concentration of a solution is the amount of solute per a certain amount of solution (or per a certain amount of solvent).

A solution with a higher concentration has more solute particles in a given amount of solvent than one with a lower concentration.

When a solution is dilute if it has a relatively low concentration and concentrated it has a relatively high concentration of solute. However, these terms are only useful for giving a vague sense of concentration, and in chemistry is is preferable to express concentration quantitatively, with a unit.

A solution can be diluted by adding more solvent to it. A solution can be more concentrated if more solute is added.

Concentration & Reaction Rate

With reference to Collision Theory, how and why does the concentration of a solution affect the rate of chemical reaction?

.

1 Liter

2 Liters

Quantifying Concentration: MolarityThere are many ways to quantify and express concentration. They generally give some measure of the amount of solute per amount of solution.

As an oversimplified example, use the cartoon image below:

Concentration = 13 dots/ L Concentration = 13 dots/2L = 6.5 dots/L

Molarity (M = mol/L) - The moles of solute particles per liter of solution.

A mole is a unit used in chemistry that represent a very large number of particles. We will learn to do calculations with it later. For now, it is only important to understand that 1 mole of any substance is a certain, set number of particles of that substance.

Examples:

1M sugar solution = 1 mole of sugar molecules / 1 liter solution

2M sugar solution = 2 moles of sugar molecules / 1 liter solution

A 2M sugar solution is twice as concentrated as a 1M solution.

Preparing a Solution

To Prepare a solution of a given molarity:

First - measure out the correct amount of solute.

Second - Add solvent until you get to the total volume you need.

Example:

For a 1M sugar solution, first measure out 1 mole of sugar. Then, add water to dissolve the sugar until the total volume is 1 liter.

You could also measure out 0.5 moles of sugar and add water until you had 0.5 liter of solution.

The important thing is the ratio of solute-to-solution volume.

.

Preparing Solutions: EquipmentMeasuring Mass

A digital balance is used to measure out specific masses of solid solutes.

Graduated/ Measuring Cylinders are useful for measuring lots of different volumes of liquids quickly and with a fairly high degree of precision. Common sizes are 10mL, 50mL, 100mL and 250mL.

Volumetric Flasks are useful for measuring certain volumes of liquid with a very high degree of precision. Common sizes are 50mL, 100mL, 250mL, 500mL and 1L.

*Volumetric flasks cannot be used for any other volume but the one it is marked for.

Graduated Pipettes/Pipettes are useful for measuring lots of different small volumes of liquid with a high degree of precision. Common sizes are 10mL and 25mL.

*A downside is that they are somewhat slow, and the readings are often more challenging to take.

Other Glassware

Beakers are very useful for holding and pouring liquids. They should never be used to measure volumes of liquids as they are very imprecise.

Erlenmeyer/ Conical Flasks are very useful for holding liquids, especially those that need to be sealed off, as they have a narrow neck. They should never be used to measure volumes of liquids as they are very imprecise.

Glassware & Measuring Volume

Concentration PracticePerform the following conversions:

1.) 540 mL = ____ L 2.) 1.20L = ____ mL

3.) A solution is made from 0.5 moles of solute mixed with water to make 500mL of solution.

What is the concentration in molarity (M)?

4.) A student adds 0.20 moles of a salt into 0.250L of solution.

What is the concentration in molarity (M)?

5.) How many moles of sugar are needed to make 400mL of a 2.0M solution of sugar water?

6.) A teacher wants to prepare 1 liter of a 1.0M solution of hydrochloric acid from the most concentrated available in the lab storage room. The most concentrated form has a concentration of 11.65M. Explain how she should make this solution, showing your work.

.

Dilution

Dilution

Dilution Equation

If the changes in volume and concentration aren't that simple, the following equation can be used to solve for specific volumes or concentrations involved in the dilution.

C1V1 = C2V2

C1 = Initial concentration C2 = Final concentration

V1 = volume of initial solution V2 = Final volume of solution after dilution

that must be used [V2 = V1 + volume of water being added]

Practice Using the equations with the amounts in the examples above:

.

In dilution, volume & concentration are inversely proportional.

Examples

Volume is doubled --> Half the initial concentration

50 mL of solution has another 50 mL of water added to it. The new volume is 100mL (which is double the original volume of 50mL). If the original concentration was 2M, the new concentration will be 1M.

Volume is tripled --> One third the initial concentration

30 mL of solution has another 60 mL of water added to it. The new volume is 90mL (which is triple the original volume of 30mL). If the original concentration was 3M, the new concentration will be 1M.

Volume increased 10x --> 1/10th Initial concentration

10mL of solution has another 90 mL of water added to it. The new volume is 100mL (which is ten times the original volume of 10mL). If the original concentration was 1M, the new concentration will be 0.1M.

Dilution is a technique for lowering concentration. More solvent is added to a solution, which lowers the ratio of solute-to-solvent.

Dilution Practice Examples1.) 100mL of a 1M solution has water added until the new volume is 200mL. What is the new concentration (in M)?

2.) A student has 500mL of a 2M solution. She measures out 50mL of this solution and adds water until the final volume is 250mL. What is the new concentration (in M)?

3.) 20 mL of a 0.4M solution has 60 mL of water added to it. What is the new concentration?

4.) A student wants to create 100 mL of a 5M solution. Her starting solution has an initial concentration of 10M.

a) By what factor is she diluting her original solution?

b) What volume of the starting solution should she use?

c) How much water does she need to add?

5.) A student has 200 mL of a 1.0M solution. Explain what he should do to prepare 100 mL of a 0.8M solution.

.