CALCULATIONS

What is a mole?

the amount of substance in grams that has the same number of particles as an atom in 12g of carbon-12

What is the relative atomic mass?

average mass of one atom compared to one twelfth of an atom of carbon-12

What is the relative molecular mass?

the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12

How do you calculate moles for pure solids, liquids and gases?

moles = mass/Mr

How do you calculate moles for gases?

PV=nRT

How do you calculate moles for solutions?

concentration = moles/volume

How do you convert mg to g?

divide by 1000

How do you convert kg to g?

multiple by 1000

How do you convert a tonne into gs?

multiple by 1000000

What is avogradros constant

one mole contains 6.022×10²³ atoms

What is the calculation to find the number of particles?

moles of substance x avogradros constant

How do you find the density?

mass/volume

What is the empirical formula?

simpest ratio of atoms of each element in the compound

How would you find the empirical formula of a compound?

• divide each mass by the atomic mass of the element

• divide the answers by the smallest one of those numbers

• multiply to give whole numbers

• the whole numbers are the empirical formula

What is the molecular formula?

the actual number of atoms of each element in the compound

What is a hydrated salt?

contains water of crystalisation

How would you dehydrate using a crucible?

• weigh 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 (ensures the reaction is complete)

Why should large amounts of calcium sulfate not be used in the decomposition experiment?

as decomposition is likely to be incomplete

Why would a wet crucible give inaccurate results?

mass loss would be too large as the water would be lost when heating

Why should small amounts of the solid not be used?

as the percentage uncertainties in weighing will be too high

What is a solution?

a mixture formed when a solute dissolves in a solvent

How do you change cm³ into dm³?

divide by 1000

How do you change cm³ into m³?

divide by 1000000

How do you change dm³ into m³?

divide by 1000

What occurs if ions dissociate?

when soluble ionic solids dissolve in water they will dissociate into separate ions, leading to the concentration of ions differing from the concentration of the solute

How would you make a solution?

1. weigh the sample bottle containing the required mass of solid on a balance

2. transfer to beaker and reweigh sample bottle

3. record the difference in mass

4. add 100cm³ of distilled water to the beaker, using a glass rod to stir

5. heat gently to ensure all the solid has dissolved

6. pour the solution into a 250cm³ graduated flask via a funnel

7. rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask

8. make up to the mark with distilled water using a dropping pipette for the last few drops

9. invert flask several times to ensure uniform solution

Where should the miniscus sit?

the bottom of the miniscus should sit on the line on the neck of the flask

How do you dilute a solution?

• pipette 25cm³ of original solution into a 250cm³ volumetric flask

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

• invert the flask several times to ensure uniform solution

Why is a volumetric pipette more accurate than a measuring cylinder?

it has a smaller uncertainty

Why should you use a teat pipette to make up to the mark in the volumetric flask?

ensures the volume of the solution is accurately measured so it doesn’t go over the line

How does diluting a solution affect the solution?

• doesn’t affect the moles present

• increases the volume of solution so the concentration would be lower

How would you calculate the new diluted concentration?

original concentration x original volume/new diluted volume

What is the new diluted volume equal to?

the original volume of solution added + the volume of water added

What’re some common hazards in chemistry?

• irritants (dilute acids and alkalis), wear goggles

• corrosive (stronger acids and alkalis), wear goggles

• flammable, keep away from naked flames

• toxic, wear gloves, wash hands after use

• oxidising, keep away from flammable materials

What is the ideal gas equation?

PV= nRT

What’re the units for the ideal gas equation?

• P - Pa

• V- m³

• T- K

• n- moles

• R- 8.31JK^-1 mol^-1

How do you convert degrees celcius into kelvin?

add 273

What occurs in an experiment involving a gas syringe?

• volume of gas depends on pressure and temperature so these need to be noted

What’re some potential errors when using a gas syringe?

• gas escapes before bung is inserted

• syringe sticks

• some gases like carbon dioxide and sulfur dioxide are soluble in water so the true amount of gas is not measured

What equation relates 2 of the same amount of gas under different conditions together?

P1V1/T1 = P2V2/T2

What is the property of equal volumes of any gases measured under the same conditions of temperature and pressure?

they contain the same number of moleculesd

What is the equation for the percentage yield?

actual yield/ theoretical yield x 100

What is the equation for the percentage atom economy?

mass of useful products/ mass of all reactants x 100

Why do chemists want a high percentage yield?

there has been an efficient conversion of reactants and products

Why do chemists want a high percentage atom economy?

the maximum mass of reactants ends up in the desired product

What is the method for titrations?

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

• pipette 25cm³ of alkali into a conical flask

• touch surface of alkali with pipette

• add acid solution from burette

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

• add a few drops of indicator

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

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

• note burette reading before and after addition of acid

• repeat titrations until atleast 2 concordant results

What is the colour change with phenolphthalein?

pink (alkali) to colourless (acid)

What is the colour change with methyl orange?

yellow (alkali) to red (acid)

What is a concordant result?

readings within 0.1 of each other

What’re safety precautions for titrations?

• acids and alkalis are corrosive so wear eye protection and wipe up spillages

• wear gloves

What will the jet space not being filled properly prior to commencing the titration lead to?

errors as it will lead to a larger than expected titre reading

Why is the conical flask used instead of a beaker?

as it is easier to swirl the mixture in the conical flask without spilling the contents

How could indicators affect the titration?

they’re weak acids so lots of drops will affect the titration result

What can you assume if there are 2 or 3 concordant results?

that the results are accurate and repeatable and that the titration technique is good and consistent

How should you record results?

• results should be recorded in a table

• results should be recorded in full (initial and final)

• record titre volumes to 2dp

Why is distilled water added to the conical flask during a titration?

washes the sides of the flask so all the acid on the side is washed into the reaction mixture to react with the alkali

Why can distilled water be added to the titration?

as water does not react with the reagents or change the number of moles of acid added

How do you deal with excess acid?

sodium hydrogen carbonate and calcium carbonate are good for neutralising excess acid as they’re not corrosive and will not cause a hazard if used in excess

What is a reading?

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

What is a measurement?

the values taken as the difference between the judgements of 2 values

How do you calculate the percentage uncertainty?

uncertainty/ measurement made on apparatus x100

How can you decrease apparatus uncertainties?

• decrease the sensitivity uncertainty by using an apparatus with a greater resolution

• increase the size of the measurement made

How do you find the percentage uncertainty of a burette?

use 2 readings, as the uncertainty should be taken account at the start and end readings

How can you reduce uncertainties in a titration?

• replace measuring cylinders with pipettes or burettes which have a lower apparatus uncertainty

• use a larger volume titre in the burette

How do you reduce uncertainty in measuring mass?

• use a balance which measures to more decimal places

• use a larger mass

• weigh by difference