C5 - Formulae, equations and amounts of substances

What is the definition of a mole

The amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon 12

What is the equation for moles with Mr and mass?

Moles = Mass/Mr

What is the formula for gas volume and moles?

Gas volume = mol. * 24

What is the formula for concentration with mol. and volume

Concentration = mol. / Volume

What is the definition for empirical formula

The simplest ratio of atoms of each element in the compound

What is the Definition of molecular formula

The actual number of atoms of each element in the compound

What are assumptions made for ideal gas

Identical particles in rapid random motion

Particles do not gain or lose kinetic energy when they collide

No intermolecular forces

Particles are hard spheres of negligible size

What is the ideal gas formula?

P V = n R T

P = pressure (Pa)

V = volume (m³)

n = moles

R = Gas constant (8.31 JK^-1 mol^-1)

T = Temperature (K)

What are potential errors of using a gas syringe

Gas escapes before the bunk is inserted

Syringe sticks

Some gasses like carbon dioxide or sulfur dioxide are soluble in water so the true amount of gas is not measured

What are assumptions for ideal gas?

Identical particles in rapid random motion

Particles do not gain or lose kinetic energy when they collide

No intermolecular forces

Particles are hard spheres of negligible size

What are conditions for spectator ions

They can move at room temperature

They don’t change state

They don’t change oxidation number

What is dissociation

Particles in a substance spread out

What is formed when a solid is dissolved in water?

A solution, the ions dissociate

What is formed when sodium chloride is dissolved in water?

Sodium chloride_(s) + aq → sodium ions + Chloride ions

Do polyatomic ions split into individual elements?

No

Ionic equation for when aluminium ions are dissolved in water?

Aluminium sulfate + aq → aluminium ions + sulfate ions

When happens when acid is dissolved in water?

Ions completely dissociate

Ionic equation for nitric acid dissolved in water

Nitric acid + aq → hydrogen acid + nitrate ions

Ionic equation for neutralisation of acid and alkali?

Hydroxide + hydrogen → water

OH^-_(aq) + 2H⁺_(aq) → H₂O

Ionic equation for neutralisation of acid and carbonate?

Carbonate + hydrogen → water + carbon dioxide

CO₃^{2- ₊} 2H⁺ → H₂O_(l) + CO_2(g)

What type of molecules do precipitation reactions have?

• Two solutions

• A solid product

• No elements

Chemical equation of lead nitrate and sodium iodide

Lead nitrate + Sodium iodide → Sodium nitrate + lead iodide

Ionic equation for lead nitrate and sodium iodide

Lead ions_(aq) + iodine ions_(aq) → lead iodide_(s)

_Pb²⁺_(aq) + 2I^-_(aq) → PbI₂

What happens after insoluble salt is made?

The salt is removed by filtration, washed with distilled water to remove soluble impurities, then dried on filter paper

How is salt formed?

When the H+ ions of an acid is replaced by a metal ion or an ammonium ion

What is an alkali?

Soluble base that releases OH- ions in aqueous solutions

How to find the volume of gas evolved from a reaction?

• Gas syringe

• Displacing water in a measuring cylinder

How to find molar mass of an unknown volatile liquid?

• Put a known mass of liquid in a flask and attach it to a gas syringe

• Gently warm the apparatus in a water bath until the liquid completely evaporates

• Record the volume of gas in the syringe and temperature of the water bath\Use ideal gas equation to work out mol. of the liquid and use Mr = mass/mol. to calculate the molar mass

What is water crystallization?

Water molecules that form an essential part of the crystalline structure

What is the formula for removing water from the crystallization in calcium sulfate crystals?

CaSO₄.xH₂O_(s) → CaSO_4(s) + xH₂O_(s)

What is the method for removing water from the crystallization in calcium sulfate crystals?

• Weighing an empty clean dry crucible and lid

• Add 2g of hydrated calcium sulfate to the crucible and weigh again

• Heat strongly with a Bunsen for a couple of minutes

• Allow to cool

• Weigh the crucible and contents again

• Heat crucible again and reweigh until you reach a constant mass (to ensure reaction is complete)

What are errors in the removing water from the crystallization in calcium sulfate crystals experiment?

• Large amounts of hydrated calcium sulfate (50g) should not be used as the decomposition is likely to be incomplete

• Small amounts of hydrated calcium sulfate (0.1g) should not be used as errors in weighing are too high

• The crucible should be dry otherwise a wet crucible would give inaccurate results, it would cause mass loss to be too large as water would be lost when heating

What is the method for carrying out titration?

•rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)

•pipette 25 cm3 of alkali into conical flask •touch surface of alkali with pipette ( to ensure correct amount is added)

•adds acid solution from burette •make sure the jet space in the burette is filled with acid

•add a few drops of indicator and refer to colour change at end point

•phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH is used]

•methyl orange [yellow (alkali) to red (acid): end point orange] [use if HCl is used]

•use a white tile underneath the flask to help observe the colour change

•add acid to alkali whilst swirling the mixture and add acid dropwise at end point

•note burette reading before and after addition of acid •repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other

Safety precautions of titrations

Acids and alkalis are corrosive (at low concentrations acids are irritants)

Wear eye protection and gloves

If spilled immediately wash affected parts after spillage

If substance is unknown treat it as potentially toxic and wear gloves.

Errors of titrations

A conical flask is used in preference to a beaker because it is easier to swirl the mixture in a conical flask without spilling the contents.

Distilled water can be added to the conical flask during a titration to wash the sides of the flask so that all the acid on the side is washed into the reaction mixture to react with the alkali. It does not affect the titration reading as water does not react with the reagents or change the number of moles of acid added.

There will be a small amount of the liquid left in the pipette when it has been emptied. Do not force this out. The pipette is calibrated to allow for it.

If the jet space in the burette is not filled properly prior to commencing the titration it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading.

Only distilled water should be used to wash out conical flasks between titrations because it does not add and extra moles of reagents

What are the things you should record in titration?

•Results should be clearly recorded in a table

•Result should be recorded in full (i.e. both initial and final readings)

•Record titre volumes to 2dp (0.05 cm3 )

How to do quality control in titration?

In quality control it will be necessary to do titrations/testing on several samples as the amount/concentration of the chemical being tested may vary between samples.

How do you know if results are accurate and reproducible?

lf 2 or 3 values are within 0.10cm3 and therefore concordant or close then we can say results are accurate and reproducible and the titration technique is good/ consistent

What indicators change colour quickly over a small pH range?

Indicators:

Methyl orange - Turns yellow to red when adding acid to alkali

Phenophthalien - turns red to colourless

What are readings in uncertainty?

the values found from a single judgement when using a piece of equipment

What are measurements in titrations?

the values taken as the difference between the judgements of two values (e.g. using a burette in a titration)

What are the uncertainties of readings and measurements?

The uncertainty of a reading (one judgement) is at least ±0.5 of the smallest scale reading. The uncertainty of a measurement (two judgements) is at least ±1 of the smallest scale reading.

How to decrease the apparatus uncertainties?

To decrease the apparatus uncertainties you can either decrease the sensitivity uncertainty by using apparatus with a greater resolution (finer scale divisions ) or you can increase the size of the measurement made.

What are uncertainties of a measurement using a burette?

If the burette used in the titration had an uncertainty for each reading of ± 0.05 cm3 then during a titration two readings would be taken so the uncertainty on the titre volume would be ± 0.10 cm3 .

How to reduce uncertainties in a titration?

Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty

To reduce the uncertainty in a burette reading it is necessary to make the titre a larger volume. This could be done by: increasing the volume and concentration of the substance in the conical flask or by decreasing the concentration of the substance in the burette.

How to reduce uncertainties in measuring mass?

Using a balance that measures to more decimal places or using a larger mass will reduce the % uncertainty in weighing a solid.

Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added.

How do you know which measurement has the highest experiment uncertainty when looking at a series of measurements

the experiments with the smallest readings will have the highest experimental uncertainties.

How do you know if the difference in results due to other errors or sensitivity of the experiment using the percentage uncertainty due to the apparatus and percentage difference between actual and calculate value

If the %uncertainty due to the apparatus < percentage difference between the actual value and the calculated value then there is a discrepancy in the result due to other errors.

If the %uncertainty due to the apparatus > percentage difference between the actual value and the calculated value then there is no discrepancy and any difference in the results can be explained by the sensitivity of the equipment.

How to help deal with random errors?

Repeating experiments and finding the mean

What is percentage yield?

Actual amount of product you collect

How can percentage error be reduced?

% yield in a process can be lowered through incomplete reactions, side reactions, losses during transfers of substances, losses during purification stages.

What is percentage atom economy?

Measures the percentage of desired product and how much is wasted

What are substitution reactions?

Some atoms from one reactant is replaced with atoms from another reactant

What does sustainable chemistry require chemists to design?

Sustainable chemistry requires chemists to design processes with high atom economy that minimise production of waste products.

When is there 100% atom economy

Reactions where there is only one product where all atoms are used making product are ideal and have 100% atom economy.

Why percentage yield is not 100%?

Incomplete reaction

Side reactions may be involved

Reversible reactions

Purification causes loss in products

What is a hazard?

A hazard is a substance or procedure that can has the potential to do harm

What is a risk?

This is the probability or chance that harm will result from the use of a hazardous substance or a procedure

How do reduce the risk of irritants?

dilute acid and alkalis- wear googles

How to reduce risk of corrosive substances?

stronger acids and alkalis wear goggles

How to reduce risk of flammable substances?

keep away from naked flames

How to reduce risk of toxic substances?

wear gloves- avoid skin contact- wash hands after use

How to reduce risk of oxidising substances?

Keep away from flammable / easily oxidized materials

How to neutralise excess acid in the stomach?

Sodium hydrogen carbonate (NaHCO3 ) and calcium carbonate (CaCO3 ) are good for neutralizing excess acid in the stomach or acid spills because they are not corrosive and will not cause a hazard if used in excess. They also have no toxicity if used for indigestion remedies but the CO2 produced can cause wind. Magnesium hydroxide is also suitable for dealing with excess stomach acid as it has low solubility in water and is only weakly alkaline so not corrosive or dangerous to drink (unlike the strong alkali sodium hydroxide). It will also not produce any carbon dioxide gas.

How to convert moles concentration to mass concentration?

Moles concentration x Mr = Mass concentration

What is the molar concentration with moles and volume?

Concentration = Moles / volume

What is the concentration of gas in parts per million?

Volume of gas x 1,000,000 / Volume of air

What is the concentration of mass in parts per million in solutions?

Mass of solute x 1,000,000 / Mass of solution

What is the formula for mass concentration with mass and volume?

Mass concentration = mass / volume

How to make a solution?

• Weigh the sample bottle containing the required mass of solid on a 2dp balance

• Transfer to beaker and reweigh sample bottle

• Wash any solid left in sample bottle and transfer

• Record the difference in mass

• Add 100cm³ of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid

• Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved

• Pour solution into a 250cm³ graduated flask via a funnel

• Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask

• Make up to the mark with distilled water using a dropping pipette for last few drops

• Invert flask several times to ensure uniform solution

What is a monotropic acid?

Acid with one H⁺ ion per unit

What is a diprotic acid?

Two H⁺ ions per unit

What is a triprotic acid?

Three H⁺ ions per unit