Water and Solutions

Concentration

2 9.1 Solutes, Solvents, and Water

concentration: the amount of each solute compared to the total solution.

Concentration

3 9.1 Solutes, Solvents, and Water

Concentration

More solute Less solute

How can we express concentration quantitatively (with numbers)?

4 9.1 Solutes, Solvents, and Water

( )( )

( )

( )( / )

( )

( )(%) 100

( )

,

mass of solute gconcentration g L

volume of solution L

mass of solute gconcentration

m

mol

ass of solution

es of solute mmol

oleconcentration

volume of solarity M

n

g

utio L

Concentration

There are several ways to express concentration

5 9.1 Solutes, Solvents, and Water

Suppose you dissolve 10.0 g of sugar in 90.0 g of water. What is the mass percent concentration of sugar in the solution?

6 9.1 Solutes, Solvents, and Water

Concentration

Calculate the molarity of a salt solution made by adding 6.0 g of NaCl to 100 mL of distilled water.

7 9.1 Solutes, Solvents, and Water

What happens when you add 10 g of sugar to 100 mL of water?

sugar

100 mLH2O

10 g

Conc. (%) = 10 g/110 g

Solubility

Water molecules dissolve sugar molecules …

8 9.1 Solutes, Solvents, and Water

Then when two sugar molecules find each other, they will become “undissolved” (solid) again…

… then, they become redissolved in water again.

Solubility

9 9.1 Solutes, Solvents, and Water

This is an aqueous equilibrium! The solution is “saturated

Equilibrium

Solubility

saturation: situation that occurs when the amount of dissolved solute in a solution gets high enough that the rate of “undissolving” matches the rate of dissolving.

10 9.1 Solutes, Solvents, and Water

Temperature has an effect on solubility

Temperature and solubility

210 gsugar

100 mLH2O

20oC

Undissolved sugar

210 gsugar

100 mLH2O

30oC

All the sugar is dissolved

11 9.1 Solutes, Solvents, and Water

solubility: the amount of a solute that will dissolve in a particular solvent at a particular temperature and pressure.

12 9.1 Solutes, Solvents, and Water

Temperature and solubility

You can dissolve (a lot) more sugar at higher temperatures

13 9.1 Solutes, Solvents, and Water

Temperature and solubility

Sugar becomes “undissolved” (solid) as the temperature goes down

14 9.1 Solutes, Solvents, and Water

Temperature and solubility

Temperature does not have the same effect on the solubility of all solutes

15 9.1 Solutes, Solvents, and Water

Temperature affects:

- the solubility of solutes how much

- the rate of solubility how fast

16 9.1 Solutes, Solvents, and Water

Dissolving is a collision process

Slow (cold) molecules are not as effective as fast (hot) molecules

Salt dissolves faster in hot water

Dissolving rate

17 9.1 Solutes, Solvents, and Water

Dissolving rate

Substances are often ground up into powder to make them dissolve faster

A 1 cm cube has a surface area of 6 cm2

The same volume has a surface area of 9 cm2 when

divided up into smaller cubes

18 9.1 Solutes, Solvents, and Water

The rate of solubility increases:

- with an increase in temperature

- with an increase in surface area of the solute

19 9.1 Solutes, Solvents, and Water

Solubility of common gases in water at 25oC

At higher temperatures:

- solid solutes (like salt and sugar) are more soluble

- gases are less soluble

20 9.1 Solutes, Solvents, and Water

Seltzer water is a supersaturated solution of CO2 in water

supersaturation: term used to describe when a solution contains more dissolved solute than it can hold.

This solution is unstable, and the gas “undissolves” rapidly (bubbles escaping)

Hand warmer is a supersaturated solution of sodium acetate

This solution is unstable, and the particles “undissolve” rapidly turning into a solid and giving off heat