Amount of substance

Question 1

Three ways to calculate moles?

Answer 1

1. moles (n) = mass (g) ÷ molar mass
2. moles (n) = concentration (mol/dm3) x volume (dm3)
3. moles (n) = volume (dm3) ÷ 24.0 OR
   moles (n) = volume (cm3) ÷ 24000

Question 2

What is meant by molecular formula?

Answer 2

The number of atoms of each element in a molecule

Question 3

What is meant by empirical formula?

Answer 3

The simplest whole! number ratio of atoms of each element in a compound

Question 4

What assumptions do you make when determining the formula of a hydrated salt?

Answer 4

1) All the water has been lost

2) There was no further decomposition

Question 5

What is the ideal gas equation? (with units)

Answer 5

pV=nRT

p= Pressure (Pa)
V= Volume (m3)
n= Moles of gas (mol)
R= Ideal gas constant (8.31)
T= Temperature (K)

Question 6

What assumptions do you make when using the ideal gas equation?

Answer 6

• There is random motion
• There are elastic collisions
• Negligible size
• no intermolecular forces

Question 7

How do you convert?

• cm3 to m3
• dm3 to m3
• ºC to K
• kPa to Pa

Answer 7

• x 10^-6
• x 10^-3
• +273
• x 10^3

Question 8

How would you carry out an experiment to find a relative molecular mass?

Answer 8

1) Add a sample of the volatile liquid to a small syringe via needle. Weigh the small syringe
2) Inject the sample through the self-sealing rubber cap. Reweigh the small syringe to find the mass of volatile liquid added to the syringe
3) Place the gas syringe in a boiling water bath at 100ºC. The liquid evaporates producing a gas. The pressure is recorded

Question 9

Why may you not get collect the theoretical yield in a reaction?

Answer 9

• The reaction may be incomplete
• Other reactions may have taken place alongside the main reaction
• Purification of the product may result in loss of some of the product

Question 10

How do you calculate percentage yield?

Answer 10

(actual yield/ theoretical yield) x 100

Question 11

What is atom economy?

Answer 11

A measure of how well atoms have been utilised in a chemical reaction.

Question 12

Why is it good to have high atom economy?

Answer 12

• Larger proportion of desired products means less waste products
• Important for sustainability as they make the best use of natural resources

Question 13

What two observations can be seen?

Answer 13

• fizzing / bubbling
• solid dissolving
• common in oxides + carbonates ( solid being reacted )

Question 14

Why may the volume of carbon dioxide produced be less than calculated (in an acid base reaction) ?

Answer 14

Because of the water being produced

Carbon dioxide is slightly soluble in water

Question 15

What to do when given percentage composition by mass of a compound? ( when working out an empirical formula)

Answer 15

Percentage/ Mass number = Moles
Work out moles for each element in the compound
find the ratio

Question 16

What is water of crystallisation?

Answer 16

Water molecules in the structure of coloured crystals

Question 17

How is white anhydrous copper (II) sulfate made?

Answer 17

When blue crystals are heated, bonds break and water is driven off

Question 18

What is the method of finding the formula of a hydrated salt?

Answer 18

(1) Weigh an empty crucible
(2) Add the hydrated salt into the weighed crucible. Weigh them together.
(3) Heat the crucible contents gently for about one minute, then heat strongly for a further three minutes
(4) Leave the crucible to cool and then reweigh it with the anhydrous salt.

1) Calculate the mol of anhydrous salt formed
2) Calculate mass and mol of water
3) Find the smallest whole-number ratio