Calculations Flashcards
Calculating mass
mass = molar mass x moles m = M x n
Calculating moles
moles = mass/molar mass n = m/M
Units for mass, moles and molar mass
mass = g
moles = mol
molar mass = gmol^-1 or g/mol
RAM
Relative Atomic Mass
The mass of an atom is so small, it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass.
Avogadro’s number
6.022 x 10^23 is Avogadro’s number (N(subscript)A).
1 mole of any substance = 6.022 x 10^23 units of that substance.
Moles
A mole is the amount of any substance that contains the same number of units as the number of atoms in exactly 12 grams of carbon-12.
Molar Mass
Molar mass is the mass in grams of one mole of a substance.
N (particles)
n x N(subscript)A
Avogadro’s Number
6.02 x 10^23 = N(subscript)A
Mass Percentage equation
Percentage by mass = molar mass of element/molar mass of compound x 100
Mass percentage example
What is the mass percentage of Sulfur in Sulfur dioxide?
Molar mass (SO2) = 64.1 g/mol Molar mass (S) = 32.1
Percentage by mass = 32.1/64.1 x 100 = 50.1%
Purity percentage
Mass of pure substance/ mass of impure substance x 100
Definition of empirical formula
Shows the simplest whole number ratio of elements present in a compound.
Definition of molecular formula
Shows the actual number of atoms in a molecule.
How to work out empirical formula (e.g. 10g of C and 1.6g of H)
- rewrite mass of each element (same with percentage)
- divide mass of each element by RAM
- divide each answer by smallest number
- whole numbers are now the ratio
e.g.
C = 10g H = 1.6g
10/12 = 0.83 1.6/1 = 1.6
0.83/0.83 1.6/0.83
= 1 = 2
–> CH2
How to work out molecular formula (e.g. EF: CH2 and molar mass = 84g/mol)
- find the molar mass of the empirical formula
- divide the molecular formula molar mass by the empirical formula molar mass
- use this number to multiply the empirical formula ratio
e.g. CH2 = 12 + 1 + 1 = 14g/mol 84/14 = 6 1:2 x 6 = 6:12 C6H12
Molar Volume of a gas
V = Vm x n
value of V
Volume in L
volume of a gas
V = Vm x n
Molar volume value
Vm = 24.8
Definition of molar volume of gas
“1 mole of any gas at room temperature (25*C) and room pressure (1 atm) occupies a volume of 24.8L”
Quantity unit conversions
1000 ml = 1L = 1000cm3
1cm3 = 1 mL
24800 cm3 = 24800 mL = 24.8 L
Theoretical Yield
Calculated mass (g) or volume (L).
Actual Yield
Mass (g) or volume (L) actually produced in an experiment.
Percent Yield
How much of the theoretical yield was actually produced.