Chemistry - Topic 5 - Moles

what is Avogadro’s constant and its symbol?

• 6.02×10²³/mol

• L or Nₐ

• gives the number of atoms/molecules/ions in one mole of a given substance

what is the molar mass of a substance?

the mass per mole of a substance in g mol^-1

empirical formula

smallest whole number ratio/simplest ratio of atoms of each element in one molecule of a substance

molecular formula

indicates the number of atoms of each element in one molecule

is Ba(OH)₂ soluble or insoluble?

soluble

what is a precipitation reaction?

two soluble salts reacting to form one insoluble salt i.e. a precipitate

core practical 1: measure the molar volume of a gas - why do we add a large spatula of sodium carbonate to the acid and wait until all of this solid has reacted?

CO₂ is soluble, so some will dissolve in the solution - therefore, we must first saturate the solution with CO₂. then, there will still be some HCl left over for the second reaction, and all of the CO₂ gas produced in the second reaction will be pushed into the gas syringe.

describe how to carry out an experiment to determine the water of crystallisation (x) in CuSO4.xH2O

• weigh a known mass into a crucible

• heat (avoid the formation of black CuO if possible)

• heat and reweigh and repeat until constant mass

• DRAW A DIAGRAM

core practical 1: Measure the molar volume of a gas - safety precaution

ensure the delivery tube does not become blocked with liquid

core practical 2: prepare a standard solution from a solid acid and use it to find the concentration of a solution of sodium hydroxide - procedure

• weigh a sample of a solid in a weighing boat

• transfer to a beaker

• reweigh the boat and subtract to get the mass of the solid transferred

• add around 100ml of distilled water to the beaker and stir to ensure all solid dissolves

• transfer to a volumetric flask . rinse the beaker with distilled water and add the rinsings to the flask

• fill up to the line (disgram of volumetric flask with meniscus on line)

• stopper and invert to thoroughly mix

core practical 3: find the concentration of a solution of hydrochloric acid - the pipette should be rinsed with distilled water and then with some HCl - why?

• rinse with water - to clean burette

• rinse with HCl - to get rid of water which could slightly change concentration of HCl which would change moles of HCl

core practical 3: find the concentration of a solution of hydrochloric acid - why do you only add a few drops on indicator?

indicators are themselves weak acids

core practical 3: find the concentration of a solution of hydrochloric acid - why do you fill a burette below head height?

dangerous otherwise, acid could spill, danger to eyes and face

core practical 3: find the concentration of a solution of hydrochloric acid - why do you use a white tile?

to make the colour change more apparent

how accurately can you read a burette?

+-0.05cm³

core practical 3: find the concentration of a solution of hydrochloric acid - why do you swirl while adding the NaOH?

to evenly distribute/mix NaOH

core practical 3: find the concentration of a solution of hydrochloric acid - what is the end-point of a titration?

when the colour changes

core practical 3: find the concentration of a solution of hydrochloric acid - what is the equivalence point?

• when the moles of NaOH = moles of HCl therefore there is only NaCl + H₂O

• the point at which the acid has been exacly neutralised

core practical 3: find the concentration of a solution of hydrochloric acid - what is a rough titration? what is the purpose of the rough titration?

• open the tap

• when you see the colour change, close

• to get an idea of the end point of the titration so the accurate titration may be carried out more rapidly

core practical 3: find the concentration of a solution of hydrochloric acid - what are concordant results?

at least two results which are within +-0.10cm³ of each other

what is an accurate result?

a result which is close to the real, true value

core practical 3: find the concentration of a solution of hydrochloric acid - what are four random errors when carrying out a titration?

• reading miniscus incorrectly on burette

• misreading miniscus on pipette

• not swirling conical flask frequently enough

• misjudging colour change

core practical 3: find the concentration of a solution of hydrochloric acid - what are two systematic errors when carrying out a titration?

• adding too much indicator

• overshooting the end-point

what are random errors?

• above or below the true value

• cancelled out during average

what are systematic errors?

• consistently above or below true value

• not cancelled out during average

what are zero errors?

zero errors are a particular form of systematic error caused by measuring instruments that have a false zero

how to reduce percentage uncertainty in temperature rise recorded with a thermometer?

• use a higher resolution thermometer

• graduated every smaller interval

• increase temperature rise

how to calculate the size of the error in a titre volume and how to reduce error?

• 2 measurements - initial reading and final reading

• 2×0.05/actual value x 100

• increase actual value to reduce error by:

  • diluting the solution in the burette

  • (increasing volume of HCl)

  • (increasing concentration of HCl)

how to calculate the error in the pipette volume?

• 0.03 (written on pipette)/25 × 100

how to calculate the error in a mass reading using a 2d.p. balance and how to reduce error?

• 2×0.005 (every time you use the mass balance, you tear it)/actual value x 100

• improve by using a greater mass

• improve by using a 3d.p. mass balance

how to calculate % yield and 3 reasons why you may not get all the expected amount of product?

• mass of product obtained/maximum theoretical mass of product x 100

• the reaction may be reversible (both the forwards and backwards reaction can take place)

• some of the product may be lost when it is separated from the reaction mixture

• some of the reactants may react in other reactions

what is atom economy?

a measure of what proportion of the products of a reaction are the desired product and how much is waste. the higher the atom economy, the less waste that is produced.

how to calculate % atom economy?

• mass of desired product as shown in equation/total mass of products as shown in equation x 100

• alternatively Mr of desired product/Mr of all products x 100 - Mr includes balancing numbers

burning ethanethiol balanced equation (CH₃CH₂SH)

ethanethiol + oxygen → carbon dioxide + sulphur dioxide + water

lithium and water reaction

lithium + water → lithium hydroxide + hydrogen

ammonia and nitric acid reaction

ammonium nitrate

the student deduced that M was sodium. Comment on the value for Ar of the metal M by calculating the range of values of Ar.

• experimental difference = 23.23 × 0.0168 = 0.390

• range = 23.23 +-0.39 = 22.84-23.62

• the Ar of sodium (23.0 lies within this range)

the burette is the largest source of experimental uncertainty. explain how the the percentage uncertainty of the mean titre could be reduced without changing the apparatus or simply repeating the experiment.

• dilute the solution of HCl in the burette

• gives a larger titration volume so smaller percentage

what is the mass of one silicon atom?

Mr/Avogadro's constant

what is the molar mass?

mass is the mass of one mole, which has the same value as the relative atomic mass or relative formula mass. So for glucose (relative formula mass = 180.0), the molar mass is 180.0 g/mol.

suggest how a chemical process could have a high percentage yield, but a low atom economy. (2)

• (has a high % yield) if there is a good conversion of reactants to products

• (has a low atom economy) if there are lots of waste products made

a student suggested that the same salt could also be made by reacting potassium metal with sulphuric acid.

suggest why this reaction is not the preferred method. (1)

potassium is a very reactive metal, so the reaction will be dangerously vigorous.

a student adds deionised water above the mark in a 250.0 cm³ volumetric flask.

state why the procedure has to be restarted rather than using a teat pipette to remove the excess water. (1)

• removal of the excess solution will remove some of the dissolved sodium hydroxide (so that the exact concentration will be unknown)

  OR

• the concentration won’t be known because the total volume will be more than 250.0 cm³

describe how the student should prepare the 250.0 cm³ of ethanedioic acid solution from ethanedioic acid crystals. (4)

• weigh the ethanedioic acid crystals in a weighted container e.g. beaker and record the exact mass

• dissolve crystals in around 100 cm³ of distilled water

• transfer solution into volumetric flask, rinse beaker and add rinsings to volumetric flask

• fill volumetric flask with distilled water up to the mark, add a stopper and invert 5 times to thoroughly mix (draw a diagram)

what is the equation related to gas volumes?

what is the equation relating pressure, volume, number of moles, the gas constant and temperature?

• pV = nRT

• p in Pa

• V in m³ (dm³ × 10^-3)

• T in K