Moles - TN Flashcards
Calculate the relative formula mass of Fe2O3
Fe = 56
O = 16
56 x 2
16 x 3
=160
what is a mole
The unit for an amount of substance. 1 mol is the amount of substance that contains the same number of particles as there are atoms in 12.0 g of carbon-12
What is Avogadro’s constant
The number of atoms in one mole of a substance. Its value is 6.02 × 10^23
Equation for: how many moles are in this substance?
moles = mass/Mr
What is Relative Atomic Mass
The weighted average mass of all the isotopes of an element
What is Relative Molecular Mass
The total average mass of one molecule of a compound
What is the Empirical Formula
The simplest whole number ratio of atoms in a compound
Calculate the EMPIRICAL FORMULA of a compound which contains 92.3% carbon and 7.7% hydrogen
(would be same calculation if given 7.8g carbon and 3.1g hydrogen)
1) Take the percentage (or mass if you are given it) and divide each by the Ar of the element TO FIND MOLES
92.3/12 = 7.69 7.7/1 = 7.7
2) Determine the ratio of the atoms’ moles by dividing both by the smallest number
7.69/7.69 = 1 7.7/7.69 = 1.0013
CH
What mass if formed when 54.75g of HCl is reacted with sufficient Mg?
REACTING MASSES
Mg + 2HCl ==== MgCl2 + H2
54.75g ?g
1)Find no moles in compound of known mass
54.75/ 1+35.5 ==1.5
Find the ratios
HCl : MgCl2
2 : 1 - from equation
1.5 : 0.75 - moles worked out
3) find mass of MgCl2
0.75 x (24 +(2 x 35.5))
= 71.25g
Percentage Yield equation
Percentage yield = actual yield/theoretical yield x100
Why will percentage yield never be 100%
- mass could be lost to surroundings
- the substance might not be pure
- incomplete reaction (eg. insufficient heat provided)
Describe the experiment to determine how many water of crystallisation CuSO4 has
Weigh crucible
Weigh crucible + CuSO4-xH20
Heat crucible
Weigh crucible + CuSO4
Determine mass of CuSO4 and H2O lost
Find moles of each
Find ratio of H20 to CuSO4
CuSO4 : H2O
1 : x
x = number of waters of crystallisation
eg. CuSO4-5H2O
what is a molecular formula
the actual number of atoms of each element in a compound
How to find which substance is limiting or in excess
10g of Mg mixed with 10g HCl
Mg + 2HCl -> MgCl2 + H2
1) Find the moles of both Mg and HCl by using moles = mass/Mr and the ratios
2) Find mass of Hcl NEEDED by doing moles x Mr
3) This means that to react with 10g Mg we would need 30.4g HCl
BUT we only have 10g HCl so it is NOT ENOUGH so it is LIMITING and
Mg is in EXCESS
LIMITING REAGENTS
S + 3T -> 2W
with 3mol S and 6mol T, how many W can you make?
S should need 9mol T because
S : T
1 : 3
3 : 9
so with only 6mol T, only 2mol S can react
ratio 1: 3 : 2
moles 2:6:4
4MOL W