EL 6 - What are we made of?

Question 1

What is the relative atomic mass?

Answer 1

The relative atomic mass of an element is the mass of its atom relative to carbon 12

Question 2

What is the formula used to work out moles or the no. of moles of formula units?

Answer 2

no. of moles (mol) or no. of moles of formula units = mass (g) / molar mass (g/mol)

Question 3

How many moles does 32g of oxygen have?

Answer 3

1 mol (oxygen gas is O2 not O)

Question 4

What is Avogadro’s constant and what is its value?

Answer 4

The no. of formula units in 1 mole of a substance is a constant: 6.02x10^23 per mol

Question 5

What is the difference between the molecular formula and the empirical formula?

Answer 5

Molecular formula: actual no. of atoms in a molecule

Empirical formula: the simplest ratio of atoms in a compound (can be the same as molecular f. but not always)

Question 6

What is water of crystallisation?

Answer 6

The crystals of some ionic lattices include molecules of water.
These water molecules are are fitted within the lattice in a regular manner (and are called water of crystallisation)

Question 7

What happens when hydrated crystals are heated?

Answer 7

The water of crystallisation is removed as steam leaving the anhydrous solid

Question 8

What is theoretical yield?

Answer 8

The expected amount of product in a reaction carried out under ideal conditions

Question 9

Name at least 3 factors that can reduce the amount of products produced (yield)

Answer 9

• Loss of products from the reaction vessels, particularly if there are several stages to the reaction
• Side-reactions occurring, producing unwanted by-products
• Impurities in the reactants
• Changes in temp. and pressure
• If the reaction is an equilibrium system

Question 10

How do you calculate the percentage yield?

Answer 10

experimental yield/theoretical yield x 100