2.1.3 Amount of a substance

Question 1

what is meant by one mole of a substance

Answer 1

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

Question 2

how are the number of moles (n) and the number of particles (N) linked

Answer 2

N = n x NA

the A is placed below the N like the 2 in H2O

Question 3

explain what is meant by Molar mass

Answer 3

the molar mass is the mass of any substance in grams

It is numerically equal to the Mr but has units of g mol-1

Question 4

What is the method for limiting and excess

Answer 4

• balanced equation
• mole ratio
• number of moles
• if you have, you need

Then work out the mass using the limiting reactant
- mole ratio
- mass
-

Question 5

state and explain Avagadro’s law

Answer 5

• Equal volumes of different gases at the same temperature and pressure contain the same number of particles.
• This is because the molecules or atoms occupy an insignificant proportion of the total volume.

Question 6

what triangle is used for measuring the volume of a gas at room temperature and pressure

Answer 6

V = n x 24

Question 7

how do you translate between cm3 and dm3

Answer 7

cm3 -> dm3 = /1000
dm3 -> cm3 = X1000

Question 8

explain the molecular formula

Answer 8

the molecular formula gives the exact number of each type of element

eg. H2O or C6H12O6

Question 9

define empirical formula

Answer 9

the emperical formula is the simplest whole number ratio of the atoms present

eg. KVO3 -> KVO3. BUT C6H12O6 -> CH2O

Always given for substances with a giant structure

Question 10

from what can empirical formula be calculated

Answer 10

The mass of elements
OR
The percentage composition

they are the same method, the info is just presented differently

Question 11

how do you calculate the empirical formula
(method)

Answer 11

• use the mass (if there is a percentage treat it as that many grams (pretend its from 100))
• divide by Ar
• divide by smallest

The ratio needs to be a whole number eg. if the ratio is 1:1.5 -> 2:3

Question 12

how do you go from the empirical formula to the molecular formula
(example with CH being the empirical formula and should have an Mr of 78)

Answer 12

If the empirical formula is CH and it should have an Mr of 78
- find the Mr of CH (13)
- 78/13 = 6
- ratio = 1:1
- times by 6
- molecular formula = C6H6

Question 13

define anhydrous

Answer 13

without water

Question 14

define water of crystallisation

Answer 14

water of crystallisation is the amount of water in a structure.
It is displayed using a dot formula

eg. CuSO4. 5H2O

Question 15

outline the method to carry out an experiment to determine water of crystallisation

Answer 15

1. weigh empty crucible
2. add hydrated salt into the weighed crucible and reweigh
3. use tripod and gauze, bunsen burner to heat the crucible and contents slowly for about a minute.
4. heat strongly for a further 3
5. leave to cool
6. weigh the crucible and anhydrous salt

Question 16

Outline how to calculate water of crystallisation from experimental results

Answer 16

1. calculate the mass then moles of anhydrous salt from weight of anhydrous salt + crucible - mass of crucible.
2. calculate the mass then moles of water in the hydrated salt from mass of crucible + hydrated salt - mass of crucible and anhydrous salt.
3. find the smallest whole number ratio between the anhydrous salt and water using moles
4. use the ratio to provide x in the hydrated salt formula

Question 17

How accurate is the experimental formula from water of crystalisation

Answer 17

it is based off of 2 assumptions.
1. all the water has been lost
2. no further decomposition

Question 18

How do you deal with the assumption that all the water has been lost in water of crystillisation experiments

Answer 18

Heat to a constant mass.

(this is where you heat the crystals repeatedly until the mass no longer changes suggesting that all the water has been removed)

Question 19

what triangle connects volume and concentration

Answer 19

number of moles (OR) mass = Volume x Concentration

Question 20

what is the ideal gas equation

Answer 20

p x V = n x R x T

Question 21

ideal gas equation words

Answer 21

Pressure x Volume = number of moles x gas constant x temperature

Question 22

what are the units for the ideal gas equation

Answer 22

Pressure = pa, 101Kpa = 1atm
Volume = m3 ,1m3 = 1000dm3 or 1x10^6 cm3
number of moles
gas constant = 8.314 Jmol-1K-1, Joules per mole per kelvin
temperature = Kelvin, c -> K + 273 but K -> c -273

Question 23

ideal gas equation relationships

what happens to volume if pressure increases and n and T stay the same

Answer 23

If n and T stay the same but pressure increases then Volume decreases

Question 24

ideal gas equation relationships

What happens if temperature increases but n and p stay the same

Answer 24

If n and p stay the same but Temperature increases then Volume increases

Question 25

What happens if **Temperature increases** but **n** and **v** stay the same

Answer 25

If n and v stay the same but temperature increases then **pressure increases**

Question 26

# ideal gas equation relationships What happens if **number of moles increases** but T and p stay the same

Answer 26

If T and p stay the same but number of moles increases then **Volume increases**