Solutions

Solutions• A solution is made up of a solute dissolved

in a solvent.

• Example: seawater is a solution.

• It is made up of salt (the solute) dissolved in water (the solvent).

• Solutions can be described as concentrated or dilute.

• A concentrated solution has a large amount of solute per solvent

• A dilute solution has a small amount of solute per solvent.

Concentration of solutions

• The concentration of a solution is the amount of solute dissolved in a certain amount of solvent.

1. Moles per litre

Concentration can be measured in :

• moles of litre (mol L-1 ).

• This is also known as molarity.

• 1M solution =

• 1 mole in 1 litre =

• 1 mole in 1000cm3

2. Grams per litre

Concentration can be measured in :

Grams per litre of litre (g l-1 ).

• 1g l-1 solution =

• 1g in 1 litre =

• 1g in 1000cm3

Example 1A solution contains 10g of Sodium Hydroxide in I litre of solution.What is the concentration expressed in moles per litre.

How many moles in 1 litre of solution?

10g / RMM = moles

10g / 40 = 0.25

There are 0.25 moles of NaOH in one litre.Answer = 0.25 mol L-1

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

Example 2A solution contains 28g of Potassium Hydroxide (KOH) in I litre of solution.What is the concentration in moles per litre.

28g / RMM = moles28 / 56 = 0.5

There are 0.5 moles of KOH in one litre.Answer = 0.5 mol L-1

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

Example 3:A solution contains 48g of MgSO4 in 3 litres of water.What is the concentration in moles per litre.

How many grams in 1L ?

48/ 3 = 16

There are 16g of MgSO4 in one litre.

Given Grams per 3 litres GRAMS PER LITRE Find MOLES PER LITRE

How many moles in 1 litre of solution?

16 / RMM = moles

16 = 0.1333120

There are 0.1333 moles of MgSO4 in one litre.Answer = 0.1333 mol L-1

Given Grams per 3 litres GRAMS PER LITRE Find MOLES PER LITRE

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

Q219• Calculate the molarity of a solution which

contains 65g of HCl in a litre

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

65 / rmm = moles

65 / 36.5 = 1.7808 moles in a litre

Answer : 1.7808M

Q219b• Calculate the molarity of a solution which

contains 65g of HCl in a litre

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

25 / rmm = moles

25 / 56 = 1.7857 moles in 250cm3

1.7857 x 4 = 7.1428 moles in a litre

Answer : 7.1428 M

Q219c• Calculate the molarity of a solution which

contains 22g of H2SO4 per 100cm3 of a solution

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

22 / rmm = moles

22 / 98 = 0.2245 moles in 100cm3

0.2245 x 10 = 2.2448 moles in a litre

Answer : 2.2448 M

Q219d• Calculate the molarity of a solution which

contains 10g of NaOH per 2L solution

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

10 / rmm = moles

10 / 40 = 0.25 moles in 2L

0.25 /2 = 0.125 moles in a litre

Answer : 0.125 M

Q219e• Calculate the molarity of a solution which

contains 12.5g of Na2CO3 per 200cm3 solution

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

12.5 / rmm = moles

12.5 / 106 = 0.1179 moles in 200cm3

0.1179 x 5 = 0.5896 moles in a litre

Answer : 0. 5896 M

221• A chemist dissolves 98.4 g of FeSO4 in

enough water to make up 2L of solution.

• What is the molarity of the solution?

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

98.4 / rmm = moles

98.4 / 152 = 0.6474 moles in 2L

0.6474 / 2 = 0.3237 moles in a litre

Answer : 0. 3237 M

Q220 • 0.080g of ethanol in 100cm3 of blood

• 0.080 x 10 = 0.8g of ethanol in 1L of blood

• 0.8/ RMM= Moles of ethanol

• 0.8/ 46 = 0.0174 moles in 1L

• Answer = 0.0174M

Q223• Calculate the mass of NaOH in 20cm3 of

0.1M NaOH

Moles of XMass of X in

g

X Relative Molecular Mass

÷ Relative Molecular Mass

Moles x rmm = mass

0.1 x 40 = 4.0g in 1L

4.0g /1000 = .004g in 1cm3

0.004 x 20 = 0.08 g in 20cm3

Answer :0.08g

q223

• B) 1.53g

• C)0.0158g

• D) 146g

• E)0.196g

Q224

• A) 40gl-1

• B)112gl-1

• C)12.25gL-1

• D)1.06gl-1

• E)0.49gl-1

Example 4What is the concentration in grams per litre of a 0.025M solution of Ca(OH)2?

(0.025)(74)

There are 1.85g of Ca(OH)2 in one litre.Answer = 1.85gL-1

Given Moles PER LITRE Find Grams PER LITRE

Example:What is the mass of CuSO4 in 250cm3 of a 3M solution?

How many grams in 1 litre of solution?

There are 478.5g of CuSO4 in one litre.Answer =478.5 gL-1

Given Moles PER LITRE Find Grams PER LITRE Grams per 250cm3

How many grams in 250cm3 litre of solution?

(478.5)/ 4 =

Answer = 119.625grams in 1 litre = 119.625gl-1

Given Moles PER LITRE Find Grams PER LITRE Grams per 250cm3

3. Mass per volume

• Concentration is also measured in % mass of solute(g) per volume of solvent(cm3)

• This is written as % (w/v)

• 1% (w/v) means 1g of solute in 100cm3 of solvent• 20% (w/v) means 20g of solute in 100cm3 of solvent

Answer: w/v means how many grams of KOH are in 100cm3 of water Given mass in 500cm3 Want mass in 100cm3

70g of KOH in 500cm3 of water

You need to divide by 5!

70/5 = 14

14g of KOH in 100cm3 of solutionAnswer = 14w/v%

The solution of KOH has 70g in 500cm3 of water. Express the concentration in w/v%

Answer:w/v means how many grams of NaCl are in 100cm3 of water Given mass in 250cm3 Want mass in 100cm3

75g of NaCl in 250cm3

75 / 250 x 100 = 30

30g of NaCl in 100cm3 of solutionAnswer = 30 w/v%

Q225 .A solution of sodium chloride consists of 75 grams in a 250cm3 volume of water. Express the concentration in w/v%

4. Volume per volume

• Concentration is also sometimes measured in % volume of solute (cm3) per solvent (cm3)

• This is written as % (v/v)

• 5% (v/v) means 1cm3 of solute in 100cm3 of solvent

USE: This is used for expressing the alcohol content in wine

• 11% written on a bottle of wine means 11cm3 of alcohol in 100cm3 of wine!

Answer:40%v/v means 40cm3 of ethanol in 100cm3 of whiskey

Given volume in 100cm3 Want volume in 30cm3

• x cm3 of ethanol = 30cm3 of whiskey 40cm3 of ethanol = 100cm3 of whiskey

(30)(40) = (100)(x)1200 = x100

12 = x

12cm3 of ethanol in 30cm3 of whiskey

The label on a bottle of whiskey says that the ethanol content is 40% (v/v). How many cm3 of ethanol are there is 30cm3 of the whiskey?

Answer:4.8%v/v means 4.8cm3 of ethanol in 100cm3 of beer

Given volume in 100cm3 Want volume in 568cm3

(568)(4.8) / (100) = 27.264

27.264 cm3 of ethanol in 30cm3 of beer

Q226.The label on a bottle of beer says that the ethanol content is 4.8% (v/v). How many cm3 of ethanol are there in one pint 568cm3 of the beer?

5. Mass per mass

• Concentration is also sometimes measured in % mass of solute(g) per mass of solvent(g)

• This is written as % (w/w)

• 36% (w/w) means 36g of solute in 100g of solvent

Answer:35% w/w means 35g of solute in 100g of solvent Given mass in 100g Want mass in 450g

(35)/100 x 450 = ?(35)/100 x 450 = 157.5

157.5g of solute in 450g of solventAnswer: 157.5g

A solution has a w/w% concentration of 35%. What mass of solute would be present in 450g of solvent?

Answer:37.9% w/w means 39.7g of solute in 100g of solvent Given mass in 100g Want mass of solvent with 5g of

solute

37.9/ 100 = 0.379g in 1cm3 of solvent5/0.379 = 13.1926

There would need to be 13.1926g of solution Answer: 13.1936g

Q227. Concentrated hydrochloric acid is a 37.9 (w/w)% solution of HCl in water. What mass of this solution would have to be taken so that it contains 5g of HCl?

Standard solutions

• A standard solution is a solution whose concentration is accurately known

Primary Standard solution

• A primary standard is a water soluble substance that is stable and available in pure form.

• They are used to make standard solutions.

Examples of Primary Standards

Sodium Chloride

Anhydrous Sodium

CarbonatePotassium Dichromate

Common substances that are not primary standards include:– Sulfuric acid – absorbs water vapour from the

air.– Sodium hydroxide – absorbs water vapour

from the air.– Iodine – sublimes.

Preparation of a standard solution of sodium carbonate.

We want to make up 250cm3 of a 0.1M solution of Na2CO3

What mass of sodium carbonate is needed??

How to make a standard solution• A special technique is used so that the

concentration of the solution can be accurately known!

1.Find the mass of the solute accurately using a mass balance, and a clock glass.

2. The solute is transferred to a clean beaker (filled with some deionised

water) using a stirring rod

3. A wash bottle (filled with deionised water) is used to rinse the clock glass and stirring rod into the beaker.

The solution is stirred to dissolve the solute completely.

5. The solution from the beaker is poured into a volumetric flask

6. Any solution remaining in the beaker is washed into the volumetric flask,

using a wash bottle

• 7. The flask is filled up to close to the calibration mark with a wash bottle, and then drop wise until the bottom of the meniscus is at the calibration mark.

• 8. The flask is stoppered, and inverted 20 times to sure the solution is mixed properly!

6. Expressing concentration in parts per million (ppm)

• ppm (parts per million) = milligrams in a litre

Convert the following to ppm0.62g / 500cm3

Given grams per 500cm3 Grams per 1000cm3 Need: mg per 1000cm3

0.62g in 500cm3

(0.62) / 500 x 1000 = 1.24g

1.24g in 1000cm3

To convert grams to milligrams

1.24 x 1000 = 1240

1240 mg in 1000cm3

Answer = 1240ppm

228(a)Convert the following to ppm0.54g / 1L

Grams per 1000cm3 Need: mg per 1000cm3

To convert grams to milligrams multiply by 1000

0.54g x 1000 = 540

540 mg in 1000cm3

Answer = 540ppm

228(b)Convert the following to ppm0.18g / 1L

Grams per 1000cm3 Need: mg per 1000cm3

To convert grams to milligrams

0.18g x 1000 = 180

180 mg in 1000cm3

Answer = 180ppm

228c)Convert the following to ppm

0.077g / 100cm3

Given grams per 100cm3 Grams per 1000cm3 Need: mg per 1000cm3

(0.077) x 10 = 0.77

0.77g in 1000cm3

To convert grams to milligrams

(1000)(0.77) = 770

770 mg in 1000cm3

Answer = 770ppm

228d)Convert the following to ppm

0.0009g / 100cm3

Given grams per 100cm3 Grams per 1000cm3 Need: mg per 1000cm3

(0.0009) x 10 = 0.009

0.009g in 1000cm3

To convert grams to milligrams

0. 009g x 1000 = 9

9 mg in 1000cm3

Answer = 9ppm

228e)Convert the following to ppm

0.0017g / 100cm3

Given grams per 100cm3 Grams per 1000cm3 Need: mg per 1000cm3

(0.0017) x 10 = 0.017

0.017g in 1000cm3

To convert grams to milligrams

0. 017g x 1000 = 17

17 mg in 1000cm3

Answer = 17ppm

228f)Convert the following to ppm

0.0003g / 100cm3

Given grams per 100cm3 Grams per 1000cm3 Need: mg per 1000cm3

(0.0003) x 10 = 0.003

0.003g in 1000cm3

To convert grams to milligrams

0. 003g x 1000 = 3

3 mg in 1000cm3

Answer = 3ppm

• If the solution is coloured then the more concentrated a solution the more intense the colour will be!!

Colorimetry is when colour is used to measure the concentration of solutions

Effect of Dilution on Concentration

• When a solution is diluted, more solvent is added.

• The quantity of solute remains unchanged.