Topic 2 - Quantitative Analysis

Question 1

What value is represented by a mole?

Answer 1

6.023x10^23 (602300000000000000000000)

Question 2

Why is the value of a mole as it is?

Answer 2

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass (A~r or M~r) for that substance

Question 3

What is the molar mass of a substance?

Answer 3

The mass of one mole in grams

Question 4

What is the formula relating mass, moles and M~r?

Answer 4

mass, g (of element or compound) = number of moles x M~r (of element or compound)
OR
number of moles = mass, g o element or compound) / M~r (of element or compound) ¦¦¦¦

Mass on top of triangle

Question 5

Carbon has A~r = 12. What is the mass in grams of one mole of carbon?

Answer 5

12g

Question 6

Describe the experiment to find out how much of a substance is dissolved in a solution.

Answer 6

1) Weigh clean, dry evaporating basin. Weigh 10g of solution and put into basin.
2) Gently heat basin to evaporate water from solution.
3) When all water seems evaporated, weigh dry evaporating basin and remaining solid again. Reheat and weigh evaporating basin + contents until no further change in mass - know then all water has evaporated.

Question 7

What is meant by concentration?

Answer 7

Concentration is a measure of how much stuff dissolved in a certain amount of solution.

Question 8

What two units can concentration be measured in?

Answer 8

g/dm^3 or mol/dm^3

Question 9

What is the formula to work out the mass concentration?

Answer 9

mass concentration = mass, g / volume, dm^3

Question 10

How do you convert from mass concentration to mole concentration?

Answer 10

mole concentration, mol dm^-3 = mass concentration, g dm^-3 / M~r

Question 11

How do you convert from mole concentration to mass concentration?

Answer 11

mass concentration, g dm^-3 = mole concentration, mol dm^-3 x M~r

Question 12

How does hard water react with soap?

Answer 12

• Won’t easily lather; reacts to form a precipitate of scum

- More soap is needed to get a lather so lots is wasted

Question 13

What ions does hard water contain?

Answer 13

Calcium ions (Ca^2+) and magnesium ions (Mg^2+)

Question 14

What causes temporary hardness and where does it come from?

Answer 14

Calcium hydrogencarbonate Ca(HCO3)2

Calcium carbonate reacts with acid rain

Question 15

How does calcium hydrogencarbonate react with water?

Answer 15

Soluble and dissolves in water, releasing calcium Ca[2+] ions

Question 16

What compounds cause hardness in water and how?

Answer 16

• Magnesium sulphate, MgSO4 - dissolves in water releasing Mg[2+] ions
• Calcium sulphate, CaSO4 - dissolves in water releasing Ca[2+] ions
• Calcium hydrogencarbonate - dissolves in water releasing Ca[2+] ions

Question 17

What are the two types of hardness?

Answer 17

Temporary and permanent

Question 18

What compound causes permanent hardness?

Answer 18

Calcium sulphate

Question 19

Explain how to remove temporary hardness.

Answer 19

• Boiling
• Calcium hydrogencarbonate decomposes forming insoluble CaCO3
• Doesn’t work for permanent hardness as heating a sulphate does nothing

Calcium hydrogencarbonate–>calcium carbonate+water+carbon dioxide

Ca(CHO3)2 (aq)–>CaCO3(s)+H2O(l)+CO2(g)

Question 20

Explain how both temporary hardness and permanent hardness can be removed.

Answer 20

• Ion exchange resin
• Water supply fed through this to remove excess hardness
• Resin contains lots of sodium/hydrogen ions + exchanges them for calcium or magnesium ions in the water that runs through them

Na2Resin(s)+Ca2+->CaResin(s)+2Na+(aq)

Question 21

What type of reaction is a acid-base titration and how does it do this?

Answer 21

Neutralisation reactions

Hydrogen ions H+ from an acid react with hydroxide ions OH- from a soluble base (alkali)

Question 22

What is the ionic equation for an acid-base neutralisation reaction?

Answer 22

H+(aq)+OH-(aq)->H2O(l)

Question 23

What does a titration allow you to find?

Answer 23

Allows you to find exactly how much acid is needed to neutralise a quantity of alkali or vice versa

Question 24

Explain how you would do a titration.

Answer 24

• Using pipette + pipette filler, add some alkali (23cm^3) to conical flask with 2-3 drops of indicator (phenolphthalein, methyl orange, etc.)
• Fill burette with acid
• Using burette, add acid to alkali bit by bit, regularly swirling conical flask. Go slow near ender where near colour change
• Indicator changed colour when alkali neutralised (phenolphthalein goes pink)
• Repeat for reliability

Question 25

What safety precautions must be taken when doing a titration?

Answer 25

Fill burette below eye level so doesn’t tip into eyes

Question 26

What is the formula linking number of moles, concentration and volume?

Answer 26

number of moles, mol = concentration, mol/dm^3 x volume, cm^3

Question 27

How do you work out the concentration of an unknown substance in a titration?

Answer 27

• Work out number of moles of known substance
• Write down balanced equation for the reaction to see the number of moles of unknown you have
• Work out concentration of the unknown

Question 28

How can you make a soluble salt from an acid and an insoluble reactant?

Answer 28

• Add base to acid; solid dissolves in acid as it reacts. Know acid neutralised as excess solid just sinks to flask’s bottom
• Filter out excess solid to get solution containing only salt + water
• Heat gently to evaporate off water + crystalline the salt

e.g.
copper oxide + hydrochloric acid -> copper chloride + water
CuO(s)+HCl(aq)->CuCl2(aq)+H2O(l)

Question 29

How can you make a soluble salt from an acid and a soluble reactant?

Answer 29

• Soluble salts can be made by reacting acid with alkali (soluble base), e.g. Na, K, ammonium hydroxides
• Do not know when reaction finished as no signal acid has been neutralised; as soluble
• Have to add exactly right amount of alkali to neutralise the acid; Must carry out a titration to work out exact amount needed
• Then carry out reaction with right proportions of acid/alkali; no need for indicator as know the volumes needed
• Solution remaining when reaction complete contains only the salt + water; Evaporate water slowly to be left with pure, dry salt

e.g.
sulphuric acid+sodium hydroxis->sodium sulphate and water
H2SO4(aq)->2NaOH(aq)->NaSO4(aq)+2H2O(l)