Avogadro's constant and moles

Question 1

what is a ‘mole’

Answer 1

the unit for amount of substance/number of particles

shown as mol in calculation/values

Question 2

one mole/mol of particles of a substance is…

Answer 2

Avogadro’s constant number of particles of that substance

atoms, ions or molecules

Question 3

1 mol of particles in grams

Answer 3

mass of 1 mol of particles is the ‘relative particles mass’ in grams

Question 4

Avogadro’s constant

Answer 4

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

Question 5

calculation for amount of substance from a given mass

Answer 5

amount (mol) = mass9g)/Ar or Mr

Question 6

calculating number of particles

Answer 6

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.

Question 7

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

Answer 7

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]

Question 8

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

Answer 8

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