topic 9- calculations

Question 1

The relative formula mass is equal to…

Answer 1

The sum of the relative atomic masses of all the atoms in the compound

Question 2

The relative atomic mass is the…

Answer 2

average mass of the atom

Question 3

How do we calculate relative formula mass?

Answer 3

We find the relative atomic masses (Ar) of all the elements that make the compound. Then we multiply the Ar values by the number of atoms of that element in the compound. Then we add these numbers together.

Question 4

Calculate the Mr of sulphuric acid (H₂SO₄):

Answer 4

Ar of hydrogen= 1
Ar of sulphur= 32
Ar of oxygen= 16
Mr of H₂SO₄= (2x1)+(1x32)+(4x16)=98

Question 5

What is the empirical formula of an ionic compound?

Answer 5

The simplest ratio of ions possible

Question 6

Molecular formula

Answer 6

Shows the actual amount of atoms which make up a molecule

Question 7

Calculating empirical formula:

Answer 7

• mass (given in question)
• divide by Ar
• divide by smallest
• formula

Question 8

The law of conservation of mass:

Answer 8

Total mass of reactants= total mass of products

Question 9

In a chemical reaction atoms cannot be…

Answer 9

Destroyed or created

Question 10

Equation for number of moles:

Answer 10

n(mol)= m (g) moles= mass
——— ———
Mr (g/mol) relative formula mass

Question 11

Examples of reactions in which mass changes:

Answer 11

• corrosion of metal
• zinc + hydrochloric acid
• burning of wood

Question 12

Triangle for mass, concentration and volume:

Answer 12

Mass

  Volume   x    Concentration

Question 13

Equations for concentration:

Answer 13

• Concentration (g/dm³) = mass (g) / volume (dm³)

- Concentration (mol/dm³)= moles (mol)/ volume (dm³)

Question 14

What is Avogadro’s number?

Answer 14

6.02x 10²³

Question 15

what is a mole?

Answer 15

1 mole of any substance is 6.02x 10²³ atoms of that substance

Question 16

how do we know what 1 mole is of a substance?

Answer 16

• 1 mole= the relative formula mass (Mr) of a substance

- for elements this is the same as the relative atomic mass

Question 17

balance the equation: H₂ + O₂ —> H₂O

Answer 17

2H₂ + O₂ —> 2H₂O

Question 18

limiting reactant

Answer 18

the reactant that is used up is called the limiting reactant because the reaction stops when it is used up

Question 19

How do we make sure that no acid remains after the reaction?

Answer 19

By using an excess (‘more than necessary’) amount of the other reactant

Question 20

How do we reduce the uncertainty of results?

Answer 20

We can take multiple readings and calculate an average from them

Question 21

Method of empirical formula:

Answer 21

Find the mass of oxygen
Divide the masses of all elements 
Divide by the smallest number
Make the numbers whole numbers
State the empirical formula

Question 22

Equation for the percentage atoms economy:

Answer 22

Mr (desired product)
———
Sum of Mr (for all reactants)