2.1.3 Amount of a substance Flashcards
what is meant by one mole of a substance
one mole of a substance is the amount of that substance that contains 6.02x10^23 particles
how are the number of moles (n) and the number of particles (N) linked
N = n x NA
the A is placed below the N like the 2 in H2O
explain what is meant by Molar mass
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
What is the method for limiting and excess
- balanced equation
- mole ratio
- number of moles
- if you have, you need
Then work out the mass using the limiting reactant
- mole ratio
- mass
-
state and explain Avagadro’s law
- 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.
what triangle is used for measuring the volume of a gas at room temperature and pressure
V = n x 24
how do you translate between cm3 and dm3
cm3 -> dm3 = /1000
dm3 -> cm3 = X1000
explain the molecular formula
the molecular formula gives the exact number of each type of element
eg. H2O or C6H12O6
define empirical formula
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
from what can empirical formula be calculated
The mass of elements
OR
The percentage composition
they are the same method, the info is just presented differently
how do you calculate the empirical formula
(method)
- 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
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)
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
define anhydrous
without water
define water of crystallisation
water of crystallisation is the amount of water in a structure.
It is displayed using a dot formula
eg. CuSO4. 5H2O
outline the method to carry out an experiment to determine water of crystallisation
- weigh empty crucible
- add hydrated salt into the weighed crucible and reweigh
- use tripod and gauze, bunsen burner to heat the crucible and contents slowly for about a minute.
- heat strongly for a further 3
- leave to cool
- weigh the crucible and anhydrous salt
Outline how to calculate water of crystallisation from experimental results
- calculate the mass then moles of anhydrous salt from weight of anhydrous salt + crucible - mass of crucible.
- calculate the mass then moles of water in the hydrated salt from mass of crucible + hydrated salt - mass of crucible and anhydrous salt.
- find the smallest whole number ratio between the anhydrous salt and water using moles
- use the ratio to provide x in the hydrated salt formula
How accurate is the experimental formula from water of crystalisation
it is based off of 2 assumptions.
1. all the water has been lost
2. no further decomposition
How do you deal with the assumption that all the water has been lost in water of crystillisation experiments
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)
what triangle connects volume and concentration
number of moles (OR) mass = Volume x Concentration
what is the ideal gas equation
p x V = n x R x T
ideal gas equation words
Pressure x Volume = number of moles x gas constant x temperature
what are the units for the ideal gas equation
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
ideal gas equation relationships
what happens to volume if pressure increases and n and T stay the same
If n and T stay the same but pressure increases then Volume decreases
ideal gas equation relationships
What happens if temperature increases but n and p stay the same
If n and p stay the same but Temperature increases then Volume increases
What happens if Temperature increases but n and v stay the same
If n and v stay the same but temperature increases then pressure increases
ideal gas equation relationships
What happens if number of moles increases but T and p stay the same
If T and p stay the same but number of moles increases then Volume increases
