Moles 1 Flashcards
What is relative formula mass?
relative mass of one formula unit of a substance
What is relative atomic mass?
average mass of all isotopes of an element taking into account their abundance
What is percentage by mass?
The percentage by mass of an element in a compound is simply a measure of how much of the mass of a substance is made from a particular element
How do you calculate percentage by mass?
(RAM of an element x atoms / RFM) x100
Calculate the percentage by mass of C in C₃H₈
C x 3 = 12x3 = 36
H x 8 = 1x8 = 8
RFM = 44
RAM x atoms = 12 x 3 = 36
36/44 x100 = 81.8%
How do you calculate moles?
mass/RFM
What is empirical formula?
The simplest whole number ratio of elements in a compound
What is molecular formula?
The number and type of atoms
Calculate the empirical formula of C₃H₆
mole ration = 3:6 -> 1:2
CH₂
How do you calculate empirical formula when given mass?
table of mass, RFM, moles, ratio
A compound has empirical formula CH₂ and the RFM is 70 - What is molecular formula?
RFM of molecular/ RFM of empirical
RFM CH₂ = 14
multiple = 70/14 = 5
molecular = CH₂ x 5 = C₅H₁₂
Describe the practical on how to determine empirical formula
Weigh empty crucible
put metal in
re-weigh metal + crucible
heat until no further change
re-weigh crucible
What is water of crystallisation?
when a salt forms crystals, a particular amount of water gets trapped inside the crystals
Describe the practical of determining the water of crystallisation
1) find mass of empty crucible
2) add hydrated slat, weigh again
3) heat until no further change
4) weigh again
5) heat again for 2 mins
6) reweigh crucible
7) repeat steps 5 and 6 until constant mass ( heating to constant mass)
Why would x be higher?
not all water evaporates
Why would x be lower?
some solid escapes
safety flame added soot or total mass increases
What is a balanced equation?
how many moles are used and made in a reaction
How do you determine molar ratio?
2CA + O₂ - > 2CaO
2:1:2
2 mol of CA
1 mol of O₂
2 mol of CaO
How do you calculate percentage yield?
actual/theoretical
What is the theoretical yield?
maximum amount of product possible
What is the actual yield?
the actual mass of product obtained
What are the reasons for not reaching theoretical yield?
loss of product
incomplete reaction
impure reactant
side reaction (unwanted reactions)
What mass of calcium hydroxide is formed when 10g of calcium oxide reacts with 10g of water?
CaO + H₂ O -> Ca(OH)₂
moles of CaO = 0.17857 (limiting reagent as more H₂ O needed)
moles of H₂ O = 0.5555
CaO:H2O
1:1
0.17857:0.17857
LR: Ca(OH)₂
1:1
0.17857:0.17857
mass = moles x RFM = 0.17857 x 74 = 13g
How do you determine which substance is the limiting reagent?
whichever is not in excess
