3) Amount of substance

Question 1

Define amount of substance

Answer 1

the quantity whose unit of the mole, is used as a means of counting any species such as atoms, ions and molecules

Question 2

Define Avogadro constant

Answer 2

the number of atoms per mole of the carbon-12 isotope (6.02 s 10^23 mol^-1)

Question 3

What does the mass of 1 mole of atoms of an element equal?

Answer 3

the relative atomic mass in grams

Question 4

Define molar mass, M

Answer 4

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

Question 5

State the equation linking n, M and m

Answer 5

n = m / M

Question 6

Define molecular formula

Answer 6

the number of atoms of each element in a molecule

Question 7

Define empirical formula

Answer 7

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

Question 8

What type of substance is the empirical formula especially important for?

Answer 8

those forming giant crystalline structures

Question 9

Define relative molecular mass Mr

Answer 9

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

Question 10

Define relative formula mass

Answer 10

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

Question 11

How do you calculate relative molecular mass Mr?

Answer 11

add together relative atomic masses of the elements making up a molecule

Question 12

How do you calculate relative formula mass?

Answer 12

add together relative atomic masses of the elements in the empirical formula

Question 13

Define analysis

Answer 13

investigating the chemical composition of a substance

Question 14

Give the 2 steps for determining empirical formula from mass

Answer 14

1. convert mass to moles using n = m / M

2. divide by the smallest whole number

Question 15

Give the 2 steps for determining molecular formula

Answer 15

1. work out empirical formula
2. find the number of units in one molecule by dividing the relative molecular mass of the compound by the relative mass of the empirical formula

Question 16

Many coloured salts are _ - which means they have water molecules / _ as part of their crystalline structure.

Answer 16

hydrated

water of crystallisation

Question 17

Give 3 measurements and 5 steps for calculating the formula of a hydrated salt

Answer 17

measurements: mass of crucible, mass of crucible + hydrated salt, mass of crucible + anhydrous salt
1. heat the crucible + contents gently (1min) strongly (3min)
2. leave to cool
3. calculate the moles of anhydrous salt, mass and moles of water
4. find the smallest whole-number ratio of the moles calculated
5. write the value of x + the formula of the hydrated salt

Question 18

State 2 assumptions taken when calculating the formula of a hydrated salt and how one can be corrected

Answer 18

all water has been lost -> heat to a constant mass

no further decomposition when heated

Question 19

1cm^3 = _ml?

Answer 19

1

Question 20

1dm^3 = _cm^3?

Answer 20

1000

Question 21

_ ml = 1 litre?

Answer 21

1000

Question 22

Give the equation linking moles, concentration and volume

Answer 22

n = concentration (mol dm^-3) x volume (dm^3)

Question 23

Define standard solution

Answer 23

a solution of known concentration

Question 24

Define molar gas volume Vm

Answer 24

the volume per mole of gas molecules at a stated temperature and pressure (24.0 dm^3 mol^-1 at RTP)

Question 25

Give the equation linking moles, volume and molar gas volume

Answer 25

n = volume / molar gas volume

Question 26

Give the ideal gas equation with units

Answer 26

pV = nRT

pressure (Pa) x volume (m^3) = amount of gas molecules (mol) x ideal gas constant (8.31Jmol^-1 k^-1) x temperature (K)

Question 27

cm^3 -> m^3?

Answer 27

x 10^-6

Question 28

dm^3 -> m^3?

Answer 28

x 10^-3

Question 29

°C-> K?

Answer 29

+ 273

Question 30

kPa -> Pa?

Answer 30

x 10^3

Question 31

Define stoichiometry

Answer 31

the ratio of the amount, in moles, of each substance in a chemical equation

Question 32

Give 1 key piece of apparatus and 3 steps for identifying an unknown group 2 metal

Answer 32

gas syringe
1. calculate moles of H2 collected
2. calculate amount of metal X using the equation:
X(s) + 2HCl(aq) -> XCl2(aq) + H2(g)
1 : 1
3) calculate the relative atomic mass and compare to the periodic table

Question 33

Define theoretical yield

Answer 33

the yield resulting from complete conversion of reactants into products

Question 34

Give 3 reasons why the theoretical yield is difficult to achieve

Answer 34

reaction not going to completion
other side reactions
purification resulting in the loss of some product

Question 35

Define actual yield

Answer 35

the amount of product obtained from a reaction

Question 36

Give the equation for percentage yield

Answer 36

percentage yield = (actual yield / theoretical yield) x 100%

Question 37

Define limiting reagent

Answer 37

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

Question 38

How can you identify the limiting reagent?

Answer 38

calculate moles of each reactant and compare with the equation

Question 39

Give the equation for atom economy

Answer 39

atom economy = (sum of molar masses of desired products / sum of molar masses of all products) x 100%

Question 40

Reactions with high atom economies will produce _ and few _ - making _ use of natural resources. However, sustainability / efficiency will also depend on _.

Answer 40

large proportion of desired products
unwanted waste products
best
percentage yield