Structure 1.4 - Counting Particles by Mass - the Mole

Question 1

What is Avogadro’s constant?

Answer 1

the number of particles equivalent to the relative atomic mass or molecular mass of a substance in grams
6.02 x 1023 g mol-1

Question 2

What is molar mass?

Answer 2

The mass of a substance with a number of particles equivalent to Avogadro’s constant.

Question 3

mole

Answer 3

the mass of a substance which contains the same number of particles as there are atoms in 12 grams of carbon-12

Question 4

relative atomic mass (Ar)

Answer 4

of an element is the average
of the masses of the isotopes in a
naturally occurring sample of the
element relative to the mass of 1/12
of an atom of carbon-12.

Question 5

relative molecular mass (Mr)

Answer 5

of a compound is the mass of
a molecule of that compound
relative to the mass of 1/12 of an
atom of carbon-12.

the sum of the relative atomic
masses of the individual atoms making up a molecule.

Question 6

relative formula mass

Answer 6

mass of one formula unit relative
to the mass of 1/12 of an atom of
carbon-12.

Question 7

molar mass

Question 8

1 mole of a substance equals

Answer 8

6.02 x 10 ^23 atoms

Question 9

number of moles equation

Answer 9

number of moles (n) = mass of substance / molar mass(Mr)

Question 10

mass of one molecule equation

Answer 10

mass of one molecule = molar mass / Avogadro’s constant

Question 11

one mole of O2 molecules equals is made up of

Answer 11

2 moles of oxygen atoms
2 x 6.02 x 10 ^23

Question 12

percentage composition by mass

Question 13

how to use percentage composition by mass to work out empirical formula

Answer 13

1. assume you have 100g of the substance
2. divide amount of each substance by Ar
3. divide by smallest number

Question 14

how to work out empirical formula

Answer 14

Step 1. work out no. of moles (mass / Mr)
Step 2. to find out the smallest whole number ratio, divide by the smallest whole number

Question 15

empirical formula

Answer 15

the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound

Question 16

how to work out molecular formula

Answer 16

Step 1. find empirical formula weight by finding Mr
Step 2. divide relative molecular weight by Mr of empirical formula weight
Step 3. multiply empirical formula by this amount

Question 17

molecular formula

Answer 17

the formula that shows the number and type of each atom in a molecule

Question 18

what is the concentration

Answer 18

the amount of solute dissolved in a solvent to make 1 dm3 of solution

Question 19

how to find out percentage by mass from empirical formulas

Answer 19

1. calculate the molar mass of the compound
2. for each element, total the mass of its atoms, then divide by the molar mass
3. times each value by 100 to give percentage