Topic 5 - Formulae, Equations and Amounts of Substances

Question 1

What is a mole?

Answer 1

the number of atoms found in exactly 12g of carbon-12

-unit for amount of substance

Question 2

What is Avogadro’s constant?

Answer 2

the number of particles in one mole

6.02 x10^23

Question 3

What is molar mass (M)?

Answer 3

the mass per mole of something

this is the same as relative molecular mass Mr

Question 4

mass =

Answer 4

Mr x no.moles

Question 5

no.moles =

Answer 5

concentration x volume

Question 6

concentration (in g dm-3) =

Answer 6

mass/volume

Question 7

no. particles =

Answer 7

mols x 6.023x10^23 (Avogadro’s constant)

Question 8

What is the emperical formula?

Answer 8

the smallest whole number ratio of atoms of each element in a compound

Question 9

What is the molecular formula?

Answer 9

the formula that shows the actual number of atoms of each type in a molecule

Question 10

How do you calculate emperical formula?

Answer 10

• calc mols of each element (mass/Mr)

- divide by lowest no. mols (to get simplest ratio)

Question 11

percentage of an element in a compound =

Answer 11

Ar of element x no.atoms in formula
____________________________ x100
Mr

Question 12

What actions should be taken to ensure the volume of acid at the end point of a titration is accurate?

Answer 12

• add acid drop by drop
• swirl conical flask
• rinse sides of flask with distilled water between additions
• use white tile
• compare colour of end point with previous titrations
• rinse end of jet of burette with distilled water

Question 13

Why should the acid be added drop by drop at the end of the titration?

Answer 13

to avoid overshooting the end point (ensure not too much acid was added)

Question 14

Why should the conical flask be swirled during a titration?

Answer 14

to ensure all the acid and alkali react (create a homogeneous mixture)

Question 15

Why should the sides of the conical flask be rinsed with distilled water between additions in a titration?

Answer 15

to rinse all the reactants into the solution so they can all react

Question 16

Why should a white tile be used in a titration?

Answer 16

so the end-point can clearly be seen

Question 17

Why should the colour of end point be compared with previous titrations?

Answer 17

to ensure consistency of the end point

Question 18

Why should the jet of the burette be rinsed with distilled water in a titration?

Answer 18

to ensure all the acid is in the conical flask

Question 19

percentage yield (%) =

Answer 19

obtained mass
_____________ x100
theoretical mass

Question 20

Why is 100% yield not obtained?

Answer 20

• incomplete rxn
• loss in transfer
• impurities formed
• reversible rxn
• incorrect measurements

Question 21

What is the ideal gas equation?

Answer 21

pV = nRT

Question 22

What are the units for pressure in the ideal gas equation?

Answer 22

Pa

Question 23

What are the units for volume in the ideal gas equation?

Answer 23

m3

Question 24

How do you convert from cm3 to m3?

Answer 24

÷1000000

Question 25

What are the units for temperature in the ideal gas equation?

Answer 25

K

Question 26

How do you convert from °C to K?

Answer 26

+273

Question 27

How do you make a standard solution (with known conc)?

Answer 27

• weigh out mass of container and solute combined
• dissolve solute in distilled water
• reweigh container (to calc the exact mass that was added to calc exact conc of standard soln)
• tip soln into a volumetric flask
• rinse beaker with distilled water and add washings to volumetric flask (to make sure none is left behind)
• top flask up to correct volume with more distilled water
• place stopper on volumetric flask and turn upside down a few times (to thoroughly mix)

Question 28

What are concordant results?

Answer 28

results within ±0.1cm3 of eachother

Question 29

percentage uncertainty =

Answer 29

uncertainty x no. readings
_____________________ x100
reading

Question 30

How can you minimise percentage error/uncertainty?

Answer 30

• use more precise equipment

- using larger volumes

Question 31

What is a hydrated salt?

Answer 31

a salt produced when enough water is left for crystallisation so that water molecules surround the metal ion in the salt

Question 32

How is a hydrated salt produced?

Answer 32

• small portions of base/metal are added to acid until no more reacts (can be done over heat to increase rate of rxn/how much can be dissolved)
• filter to remove residue
• heat evaporating basin containing saturated solution of salt (heat reduces vol and crystals begin to form)
• transfer solution to crystallising dish (to allow water to evaporate so that hydrated salt is produced)