Chemistry: Structured Paper C3 - Topic 3

Chemical Analysis is very useful.

- Used for crime investigations and how much alcohol someone carries.

- What substances are present in foods?

- Purity of water.

- Quality Control.

QUALITATIVE - what is present

QUANTATIVE - how much is present

Flame Tests -SPOT THE COLOUR

SODIUM (Na) gives orange/yellow flame.

POTASSIUM (K) gives lilac flame.

CALCIUM (Ca) gives brick red flame.

COPPER (Cu) gives blue/green flame.

Add Sodium Hydroxide and Look for coloured precipitates.

Calcium (Ca) > White

Copper (Cu) > Blue

Iron (Fe) (II) > Sludgy Green

Iron (Fe) (III) > Reddish Brown

Aluminium (Al) > White > Clear

Ammonium (NH4) > Smell

Hyrdrochloric acid - can help detect Carbonates and Sulphites

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Carbonates give off CO2 with HCl !!

You can test for carbon dioxide using lime water.

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Sulphites give off SO2 with HCl

You can test for sulphur dioxide using damp potassium dichromate paper. The paper turns from orange to green

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Test for Sulphates with HCl and Barium Chloride

Sulphate Ions produce a white precipitate.

> The origional compound was a sulphate.

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Test for Halides with Nitric Acid and Silver Nitrate
(Chloride, Bromide, Iodide ions)

A chloride gives a white precipitate of silver chloride. 

A bromide gives a cream precipitate of silver bromide.

An iodide gives a yellow precipitate of silver iodide.

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An Indicator - a dye that changes colour.

LITMUS: Blue litmus paper turns red if the solution is an acid.
*lots of H+ ions are present.

Red litmus paper turns blue if the solution is an alkali.
*lots of OH- ions are present. 

WHEN AN ACID REACTS WITH A METAL, IT GIVES OFF HYDROGEN GAS.

To Test for alkalis: - Heat the substance with an Ammonium Salt.

The smell is quite distinctive.

A MOLE = 6.023 x 10(23)

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass of that substance.

NUMBER OF MOLES = Mass in grams (OF ELEMENT OF COMPOUND)
Divided by 
Relative atomic mass (OF ELEMENT OR COMPOUND)

Eg. How many moles are there in 66g of carbon dioxide?

Mr of CO2 = 12 + (16 x 2) = 44
No of moles = Mass (g) / Mr = 66/44 = 1.5 moles

Avogadros Law - "One mole of any gas occupies 24dm(cubed)"

VOLUME OF GAS = MASS OF GAS divided by Mr of Gas

                                 all times by 24.

Example: whats the volume of 4.5 moles of chlorine at RTP?

1 mole = 24dm(cubed), so 4.5 moles = 4.5 x 24 = 108. 

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You can calculate volumes in reactions if you know the masses.
2.7g of C gives 9.9g of CO2.
Volume = Mass divided by Mr (all times by 24)

so, (9.9/44) x 24 = 5.40dm(cubed)

• Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali.
• Using a pipette, add some alkali to a conical flask, along with two or three drops of indicator.
• Fill a burette with acid (DO THIS AT EYE LEVEL)
• Add the acid to the alkali a bit at a time. - giving the conical flask a regular swirl - this will help the indicator change colour.
• Record the volume of acid used to neutralise the alkali.
• REPEAT.

Step 1; work out how many moles of the known substance you have.
No. of moles = concentration x volume = 0.1 x (25/1000) = 0.0025 moles.

Step 2; write down the equation for the reaction
2NaOH + H2SO4 > Na2SO4 + 2H2O .. and work out how many moles of the unknown stuff you must have had...

Every 2 moles of sodium hydroxide > there was 1 mole of sulphuric acid.

So, if you had 0.0025 moles of sodium hydroxide, you must have had half that amount of sulphuric acid: 0.0025 ÷ 2.

Step 3; Finally, work out the concentration of the 'unknown' stuff:
(Concentration = number of moles  ÷ volume)

= 0.00125 ÷ (30/1000) = 0.0417 moles per dm(cubed)

Drinking water = needs to be good quality!

• microorganisms in drinking water can cause cholera and dysentery.
• filtered to take these out.
• good water = good life.

Water is a really useful Solvent.

• water dissolves most ionic compounds.