Topic 1.4- Key concepts/ calculations

Question 1

How to calculate relative atomic mass? Ar

Answer 1

(Mass x abundance)+
(Mass x abundance)
——————————
total abundance

Question 2

What is the relative formula mass (Mr)?

Answer 2

Average mass for one set of a molecule

Question 3

What is Ar (Relative atomic mass)?

Answer 3

Average mass of the atom

Question 4

How to calculate Mr (Relative formula mass)?

Answer 4

-Add the mass of all the atoms in the molecule.
-Ignore any large numbers before the atom (if there are two of a molecule don’t include it)

Question 5

How to calculate the percentage of mass an atom is in a molecule using the Mr?

Answer 5

Ar (Relative atomic mass)
———————————- x100
Mr ( Relative formula mass)

Question 6

What is the empirical formula?

Answer 6

-simplest whole number ratio of molecular formula

Question 7

How to work out the empirical formula?

Answer 7

-Divide all numbers of atoms by the smallest to make it one and the others in the same ratio

Question 8

What is the empirical formula of C2H6

Answer 8

-2/2=1
-6/2=3
-CH3

Question 9

How to work out moles?

Answer 9

Mass in grams of atom/
Ar (Relative atomic mass) of the atom

Question 10

How to work out empirical formula from mass (grams)

Answer 10

-Find moles (mass/Ar or Mr)
-Divide by smallest mole
-Multiply to make them all whole numbers

Question 11

Experiment to determine the empirical formula of a simple compound e.g magnesium oxide?

Answer 11

• Remove impurities from magnesium rub it or cut it.
  -Find mass of magnesium
  -Heat, occasionally removing lid to allow oxygen.
• When all is white powder, reweigh it.
• With weight of oxygen and magnesium use formula to find empirical formula

Question 12

What is the law of conservation of mass?

Answer 12

Mass is neither created or destroyed in a chemical reaction

Question 13

What happens to mass in precipitation reaction in a closed system?

Answer 13

• It stays the same

Question 14

What happens to mass in a non-closed system in which a gas is formed?

Answer 14

Gas will escape and thus mass will seem smaler.

Question 15

How to find the concentration in dm3-

Answer 15

mass g / volume dm3 (cm^3/1000)

Question 16

How to convert cm3 to dm3

Answer 16

divide by 1000
1000cm3= 1dm3

Question 17

What is a mole

Answer 17

Turns the Ar of an atom into the same number of Ar as grams.
C=12 Ar
one mole of carbon is 12g
O=16 Ar
One mole of oxygen =16g

Question 18

How to find the number of moles for a molecule?

Answer 18

Mass g/
Relative formula mass Mr

Question 18

What is the value of a mole?

Answer 18

6.02x10^23

Question 18

Calculate the mass of calcium sulfate that could be produced from 80g of calcuim.

Answer 18

80g/40(Ar of calcium)=2 moles
136 (mr of calcium sulfate) x 2 moles= 272 grams

Question 18

How to calculate the mass of reactants and products, given the mass of one product?

Answer 18

• Find how many moles of the product we have
• Use the equation with moles and Mr to find the mass.

product 1 mass/ mr = moles
moles in ratio to other product (e.g H+ 2H2O means 1 H mole for every 2 H2O moles)
Product 2 moles x Mr= Mass

Question 19

How to calculate atom economy?

Answer 19

All useful products/ All reactants

Question 20

How to calculate theoretical yield?

Answer 20

calculate maximum mass of product if there was no loss

Question 21

What is the actual yield?

Answer 21

Mass of product