Avogadro's constant and moles Flashcards
what is a ‘mole’
the unit for amount of substance/number of particles
shown as mol in calculation/values
one mole/mol of particles of a substance is…
Avogadro’s constant number of particles of that substance
atoms, ions or molecules
1 mol of particles in grams
mass of 1 mol of particles is the ‘relative particles mass’ in grams
Avogadro’s constant
6.02 x 10[23] mol-1
the number of carbon atoms in exactly 12 g of carbon-12, 12C
- the formula of methane is CH4
- the relative atomic mass (Ar) of C = 12, the Ar of H = 1
- so the relative formula mass (Mr) of CH4 = 12 + (4 × 1) = 16
- so 1 mole of methane = 16 g of methane and contains 6.02 × 10[23] molecules
- 16 g of methane = 6.02 × 10[23] as there are 5 atoms in each formulae (molecule)
- 16 g of methane = 5 × 6.02 × 10[23] = 3.01 × 1024 atoms
or 1g of methane would be the constant/16
calculation for amount of substance from a given mass
amount (mol) = mass9g)/Ar or Mr
calculating number of particles
The number of particles of a substance can be calculated using:
- the Avogadro constant number (6.02 × 10[23])
- the amount of substance in mole
Number of particles = 6.02 × 10[23] × mol (amount)
e.g.
number of water molecules in 0.5 mol of water.
Number of water molecules = 6.02 10[23] × 0.5
= 3.01 × 10[23]
It is important to be clear about the particles involved. For example, 3.01 × 10[23] water molecules contain 9.03 × 10[23] atoms. This is because a water molecule, H2O, contains three atoms.
a) Calculate the amount, in mol, of molecules in 22.5g of water
(Mr of H2O = 18)
b) Calculate the amount, in mol, of atoms in 22.5g of water
c) Use your answer to b to calculate the number of atoms in 22.5g of water
a) 22.5/18 = 1.25 mol
b) 3 x 1.25 = 3.75 mol
c) 3.75 x 6 x 10[23] = 2.25 x 10[24]
a) Calculate the number of atoms in 6g of diamond
Give your answer to two significant figures
(Ar of C = 12)
b) Calculate the mass in grams of 1.00 x 1012 carbon atoms
Give your answer to two significant figures
a) amount = 6/12 = 0.5 mol
number = 0.5 x 6.02 x 10[23] = 3.0 x 10[23] atoms
b) amount = 1x10[12]/6.02x10[23] = 1.66 x 10[-12] mol
mass = 12 x 1.66 x 10[-12] = 2 x 10[-11]g