solubility curves - tsfx - the school for · pdf file · 2012-09-06solubility...

The School For Excellence 2011 The Essentials – Unit 2 Chemistry – Book 1 Page 7

SOLUBILITY CURVES

The solubility of a solute is the maximum amount of solute that can be dissolved in a given amount of solvent at a given temperature. A saturated solution is a solution in which no more solute can be dissolved at a given

temperature. An unsaturated solution is a solution into which more solute can be dissolved. A supersaturated solution is an unstable solution as it contains more dissolved solute

than a saturated solution. One way of measuring the solubility is to determine the maximum mass of solute that can be dissolved in 100 g of solvent at a given temperature. This is given in grams of solute per 100 g of solvent or g per 100 g. Solubility curves show the relationship between solubility and temperature. An example of a solubility curve is given below.


Each point on the curve represents a saturated solution, any point below the curve an unsaturated solution and any point above the curve a supersaturated solution.

We can use solubility curves to: Determine how much solid will dissolve at a given temperature. Obtain a general idea about the ability of a solute to dissolve in water, at different

temperatures. Compare the different solubilities of different solutes. Determine the mass of crystals deposited, when a saturated solution so cooled from a

higher to a lower temperature. Note: When a solution is cooled, the solute tends to become less soluble and no longer

remains dissolved. The solute comes out of the solution as crystals and this process is known as crystallisation.

Unlike most solids, gases become less soluble as the temperature increases. They are,

however, more soluble as pressure increases.


QUESTION 8 (a) What is the solubility of 3KNO at 60°C?

(b) A solution of 3KNO contains 27 g of 3KNO in 50 g of water at 40C. Give a name to

this type of solution. (c) 110 g of 3KNO was dissolved in 100 ml of water at 60°C. If this solution was cooled to

20°C, what mass of 3KNO would crystallise out of this solution upon cooling?


QUESTION 9 75 g of 3NaNO was dissolved in 50 ml of water at 80°C. If this solution was cooled to 40°C,

what mass of crystals would appear? Solution


CONCENTRATIONS OF SOLUTIONS The concentration of a solution describes the relative amounts of solute and solvent present. 1. A high Solute:Solvent ratio gives a concentrated solution. 2. A low Solute:Solvent ratio gives a dilute solution. The term concentration also describes the extent to which particles are spread apart from each other. One can express concentration in a variety of ways.

PERCENTAGE BY MASS (W/W)

Percentage by mass (w/w) indicates that the % is based on the masses of both the solute and solution. It describes the mass of solute (g) present in 100 g of solution. For example: 0.8% w/w indicates that there are 0.8 g of solute dissolved in 100 g of solution.

100)(

)()/( gsolutionofmassgsoluteofmasswwionConcentrat

When using this formula, it is important that the masses of both species are in the same units.

QUESTION 10 Twenty grams of a salt solution contains 4.0 grams of salt. What is the concentration (w/w) of salt in this solution? Solution

wwionConcentrat /%20100204


QUESTION 11 How much salt is required to prepare 500 g of a 10% w/w salt solution? Solution

QUESTION 12 The label on a can of tomato soup states that the salt content is 300 mg per 100 g of soup. What is the percentage by mass of salt in the soup? Solution

%10%100500

x

1.0500

x

gx 50


PERCENTAGE BY VOLUME (V/V)

Percentage by volume (v/v) indicates that the % is based on the volumes of both the solute and solution. It describes the volume of solute (ml) present in 100 ml of solution. For example: 11% alcohol v/v indicates that there are 11 ml of alcohol dissolved in 100 ml of solution.

100)(

)()/( mlsolutionofvolumemlsoluteofvolumevvionConcentrat

When using this formula, it is important that the masses of both species are in the same units. This formula is best used when the solute is a liquid.

QUESTION 13 A 170 ml glass of fruit drink contains 15.0% (v/v) of pure orange juice. What volume of pure orange juice is present in this solution? Solution

QUESTION 14 A bottle of wine contains alcohol at a concentration of 14% (v/v). What volume of alcohol is present in 250 ml of wine? Solution

%15100170

x

mlx 5.25


PERCENTAGE BY MASS/VOLUME (W/V)

Percentage mass per volume (w/v) describes the mass of solute (g) dissolved in 100 ml of solution. For example: 2% (w/v) indicates that there are 2 g of solute dissolved in 100 ml of solution.

100)(

)()/( mlsolutionofvolumegsoluteofmassvwionConcentrat

When using this formula it is important that mass and volume are of the same order, i.e. g and ml, kg and L.

QUESTION 15

The concentration of in water is 10% (w/v). Calculate the mass of in 2.00 L. Solution

QUESTION 16 A solution contains sugar at a concentration of 5% (m/v). What mass of sugar is present in 500 ml? Solution

2Mg 2Mg

mlgvw 100/10/%10 22.0 10x g


PPM AND PPB

For species in an aqueous environment, ppm (parts per million) and ppb (parts per billion) describe the relative masses of materials. For example: A 5 ppm solution of 3Fe describes a solution containing: 5 g of 3Fe in 61 10 g of solution.

5 kg of 3Fe in 61 10 kg of solution. An 8 ppb solution of 3Fe describes a solution containing: 8 g of 3Fe in 91 10 g of solution.

8 kg of 3Fe in 91 10 kg of solution.

For species in a gaseous phase, ppm and ppb describes the relative volumes of gas. For example: A 2 ppm 2H sample contains 2 ml of hydrogen gas in a total volume of 61 10 ml.

When concentration units are stated as mg/kg, mg/L, µg/g, µg/ml, ng/mg or g/tonne these units are also ppm. i.e. 40 mg/kg = 40 ppm or 1.5 µg/ml = 1.5 ppm. For example: 2.6 mg of salt in a 200 ml solution is how many ppm?

2.6 mg in 200 ml X = 13 mg in 1L 13 mg/L = 13 ppm X mg in 1000 ml


QUESTION 17 A solution contains iron at a concentration of 12 ppm. Calculate the mass of iron in 250 g of this solution. Solution

QUESTION 18 Water contains copper ions at a concentration of 6 ppm. What mass of copper would be

present in g8104 ? Solution


OTHER MEANS OF EXPRESSING CONCENTRATION

MASS OF SOLUTE PER LITRE OF SOLUTION Units: Lg / or Lmg / This measure involves expressing concentration in terms of mass of solute ( g or mg ) per

litre ( L ) of solution.

MASS OF SOLUTE PER GRAM OF SOLUTION Units: gmg / or gg / This measure involves expressing concentration in terms of mass of solute (mg or g ) per

gram ( g ) of solution.

QUESTION 19 A solution contains sugar at a concentration of 30 g/L. What mass of sugar is present in 200 ml? Solution


QUESTION 20 Brine is a concentrated salt solution. If a sample of brine contains 90 g of salt in 115 ml of water, what is its concentration in: (a) )/%( vw (b) Lg / (c) Lmg /


(d) gmg / (e) gg /

Note: The density ( d ) of all aqueous solutions is mlg /1 . Given that Vmd , we can say

that Vm for such solutions.


CONVERTING CONCENTRATION UNITS The following relationships will be required to convert concentration units:

Mass Conversions

Volume Conversions

IMPORTANT UNITS

Mass Units Volume Units

311 1 10100

ml L L

1000 1000 1000 1000ng g mg g kg

91 10 g 61 10 g 31 10 g 31 10 g

91 10 g 61 10 g 31 10 g 31 10 gng g mg g kg

1000 1000 1000 1000

1000 1000l ml L

61 10 L 31 10 L

61 10 L 31 10 Ll ml L

1000 1000

gkg 31011

gngg 69 1011011 mg3101

ggmg 31011000

11

ggg 6101000,000,1

11

ggng 9101000,000,000,1

11

3100010001 cmmlL l6101

LLl 6101000,000,1

11


METHOD: Step 1: Draw a flow diagram to illustrate the unit changes required. Consider one unit change at a time. For example: Converting to .

Step 2: For each unit change, assess whether the quantity has been altered in terms of size, or whether it has been simply expressed in a different form. If the change has altered the size of the quantity, correct the other side of the expression by the same factor.

If the change has simply expressed the quantity in a different form (of identical size), do not alter the other side of the expression.

Lmol / ppm

Lmol /

mlmol 1000/mlmol /

gmol /

gmol 6101/

gg 6101/

Lmol /10mlmolx 100/

Lmol /10mlmolx 1000/

Quantity size has changed i.e. decreased 10 fold.

Quantity on left must also decrease 10 fold.

Quantity size has not changed. It has simply been expressed in a different form.

Do not alter the left hand side.


Steps:

/mol L

/1000mol ml

/ 80mol ml

/ 80g ml

QUESTION 21A

The concentration of glucose ( ) in water is . Calculate the mass of

glucose in a solution. Solution

Answer is

6 12 6C H O 20.50 10 M80.00ml

/mol L / 80g ml

20.50 10 /mol L

20.50 10 /1000mol ml

44.00 10 / 80mol ml

/ 80x g ml

44.00 10 180 / 80g ml 27.2 10 / 80g ml

27.2 10 g

/80 ml

Volume has decreased

1000 12.580

fold, therefore, divide

the other side by 12.5 as well. 20.50 10 / 80

12.5mol ml

Convert mol

to mass

solubility curves - tsfx - the school for · pdf file · 2012-09-06solubility...

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