5.1 - Empirical and Molecular Formula Flashcards
What is Empirical formula?
The Empirical formula shows the smallest whole number ratio of atoms of each element in a compound.
How can the Empirical formula be calculated?
1) Mass or % composition of mass/ Relative atomic mass (Mr)
2) Divide by smallest number of the answers
3) This should give a whole number ratio like 1:2
4) These whole number are used to write the Empirical formula.
Give an example of calculating using masses?
Mass of copper oxide = 4.29g
Mass of coper = 3.43
Mass of oxygen removed = 4.28 - 3.43 = 0.85g
Copper Oxygen
3.43/63.5 0.85/16
0.0540 0.0531
0.0531 0.0531
1 1
CUO ratio is 1:1
How can the Empirical formula be calculated when the oxygen value is not provided?
Analysis of results for some compounds does not include values for oxygen so sometimes need to calculate the percentage of oxygen.
Q) A compound has a % composition by mass of Na = 29.1% S = 40.5% with the rest being oxygen.
% of oxygen = 100 - (29.1+ 40.5%) = 30.4%
Then calculate the Empirical formula
How calculate using combustion analysis?
Many Organic compounds contain carbon and hydrogen, or carbon hydrogen and oxygen. When known mass is completely burned. It is possible to collet and measures the masses of CO2 and H20 formed.
A 1.87g sample of organic compound was completely burned forming 2.65g of CO2 and 1.63g of water.
Mass of carbon = 2.65 x 12/ 44 = 0.723g
Mass of hydrogen = 1.63 x 2/18 = 0.181g
0.181 + 0.723 = 0.904
Mass of oxygen = 1.87 - 0.904 = 0.966
Then calculate empirical formula
What is molecular formula?
The Molecular formula of a compound shows the actual number of the atoms of each element in a compound
How are moles calculated?
Amount of moles = Mass of substance in grams/ Relative atomic/Molecular mass
N = Mass / Mr
The mole is the unit of measurement for substances. It always contains the same number of particles.
How is molar mass calculated?
N = M/Mr
O2 mass = 5.26
5.26/ 32 = 0.164 molar mass
How can the molecular formula be calculated from the empirical formula
A compound has the empirical formula CH and a relative molecular mass of 104
The ‘formula mass’ or just mass of the empirical formula is 13 so 104/13 = 8 so molecular formula of compound = C8H8
What is the ideal gas equation?
Pressure Volume =Number of moles x Temperature x Gas constant
PV = NRT
What are the units for for the ideal gas equation?
p = Pressure is pascals (Pa)
V = Volume in cubic meters (m3)
T = Temperature in kelvin (K)
n = amount of moles (mol)
R = the gas constant (8.31 Jmol)
What is the conversion for the Ideal gas equation?
Kpa > Pa : multiply by 1000
cm3 > m3 : divide by 1000000
dm3 . m3 : divide by 1000
oC > K : add 273
What is a mole?
A mole is the amount of substances that contain the same number of particles as the number of carbon atoms in exactly 12g of 12C isotope.
What is Avogadro’s constant?
Avogadro’s constant allows the number of particles present in a sample of a substance with a known mass
6.02 x 1023
What is the moles equation triangle?
