amount of substance

Question 1

definition of empirical formula

Answer 1

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

Question 2

what is the molecular formula?

Answer 2

the number of atoms of each element in a molecule

Question 3

what is the Avogadro’s constant?

Answer 3

the number of units in one mole of any substance which is 6.02x10^23

Question 4

what is the mole?

Answer 4

one mole is the amount of substance that contains 6.02x10^23

Question 5

how do you calculate the no. of particles in a substance?

Answer 5

mol x Na (avogrado’s constant)

Question 6

what is the molar mass?

Answer 6

the mass per mole of a substance (g mol-1)

Question 7

definition of relative molecular mass

Answer 7

the weighted mean mass of a molecule of a compound compared with 1/12th of the mass of an atom of carbon-12

Question 8

definition of relative formula mass

Answer 8

the weighted mean mass of the formula unit of a compound compared with 1/12th go the mass of an atom of carbon-12

Question 9

how to work out the empirical formula:

Answer 9

• convert from mass to moles of each element given
• divide each value by the smallest value to find the ratio
• use the ratio of the atoms to write the empirical formula

Question 10

how to work out the molecular formula:

Answer 10

• find the empirical formula first
• then find the Mr of the empirical formula
• divide the Mr by the molecular mass (which is given in the question)
• with the answer, multiply it by each atom in the empirical formula

Question 11

what is the water of crystallisation?

Answer 11

water molecules that are bonded into a crystalline structure of a compound e.g with
CuSO4.8H2O - per every CuSO4 molecule there are 8 water molecules

Question 12

what causes an anhydrous element?

Answer 12

when the crystals are heated the bonds holding the water within the crystal are broken and so the water is driven out

Question 13

definition of anhydrous

Answer 13

containing no water molecules

Question 14

how would you carry out an experiment to determine the water of crystallisation in hydrated crystals?

Answer 14

1) weigh an empty crucible - record
2) add hydrated salt and then weigh - record
3) put the crucible on a tripod over a bunsen burner, using a pipe-clay triangle to support it
4) heat the crucible gently for a minute then strongly for 3 minutes
5) when done leave to cool, then weigh the crucible which should now have anhydrous salt - record

Question 15

what are sources of error during the experiments of hydrated copper salts and determining the water of crystallisation?

Answer 15

• thermal decomposition
• not all the water has been lost

Question 16

equation linking moles, volume and concentration:

Answer 16

moles = concentration x volume

Question 17

what is the molar gas volume?

Answer 17

the volume per mole of gas molecules at a stated room temperature and pressure

Question 18

what is the molar gas volume at RTP?

Answer 18

24.0 dm^3

Question 19

what is the ideal gas equation?

Answer 19

it is used to calculate the number of moles found in a specific volume, temperature, or pressure of a gas

Question 20

what assumptions are made for the molecules that make up an ideal gas?

Answer 20

• random motion
• elastic collisions
• negligible size
• no intermolecular forces

Question 21

formula for ideal gas equation:

Answer 21

p(pressure ) x V(volume) = n(amount of gas molecules) x R(ideal gas constant) x T(temperature)

pV = nRT

Question 22

what are the units for each in ideal gas equation?
- pressure (p)
- volume (V)
- amount of gas molecules (n)
- ideal gas constant (R)
- temperature (T)

Answer 22

• Pa
• m^3
• mol
• 8.31 J mol-1 K-1
• K

Question 23

conversions to make between units if needed in ideal gas equation:

cm3 - m3
dm3 - m3
.C - K
kPa - Pa

Answer 23

cm3 - m3 = x10^-6
dm3 - m3 = x10^-3
.C - K = +273
kPa - Pa = x10^3

Question 24

what is the real gas equation and why was it created?

Answer 24

(p + n2 x a / V2)(V - nb) = nRT
- accepts IF accepts vol of gas molecules

Question 25

what is percentage yield and what is the equation?

Answer 25

expressing the conversion of starting materials into a desired product

actual yield / theoretical yield x 100

Question 26

what is the theoretical yield?

Answer 26

the maximum possible amount of product

Question 27

what affects the theoretical yield?

Answer 27

• reaction may not have been completed
• side reactions may have also taken place
• purification of the product may result in loss of some of the initial product

Question 28

what is the limiting reagent? (definition)

Answer 28

the reactant that is not in excess, which will be used up first and stop the reaction

Question 29

what is the atom economy and why is it used?

Answer 29

the atom economy of a chemical reaction is a measure of how well atoms have been utilised

to avoid waste

Question 30

atom economy equation:

Answer 30

sum of molar masses of desired product / sum of masses of all products

Question 31

benefits of reactions with high atom economies:

Answer 31

• produce large proportions of desired products and less waste products
• important for sustainability as they make the best use of natural resources
• industrial processes more efficient
• preserves raw materials
• reduces waste