Chemistry Atoms and Moles Topic- Key Knowledge.

Question 1

Formula for number of moles in a substance?

Answer 1

N= Mass (g)/Mr (relative molecular mass)

Question 2

How is the Mr or Relative Molecular Mass of a substance found?

Answer 2

The relative atomic masses (Ar) of all the atoms in a molecule are added.
E.g H20’s Mr is (2x1)+16
=18

Question 3

How is the Ar or Relative atomic mass found? + formula!

Answer 3

Ar is the weighted mean mass of all isotopes of an atom, compared to 1/12 of a carbon-12 atom.
Ar= % abundance x mass no (-> sum of all isotopes)/100

E.g: Clorine isotope abundance = 75% 35Cl and 25% 37Cl, (75x35) + (25x37)/100, = 35.5

Question 4

Atom

Answer 4

The smallest distinct particle of a substance

Question 5

Molecule

Answer 5

2 or more atoms covalently bonded together

Question 6

Atomic number

Answer 6

The number of protons in an atom’s nucleus

Question 7

Mass number

Answer 7

The number of protons and neutrons for a single isotope

Question 8

Isotopes

Answer 8

Atoms with the same number of protons, but a different number of neutrons

Question 9

Formula for percentage yield?

Answer 9

Percentage yield = actual yield/maximum mass x 100

Question 10

Empirical formula

Answer 10

The simplest whole number ratio of atoms in a compound

Question 11

Molecular formula

Answer 11

Number + type of atoms in a molecule
E.g H20 -> 2 hydrogen atoms and 1 oxygen atom