Topic 5 - Amount of substance

Question 1

Define a mole

Answer 1

Any substance which contains exactly 6.022 x 10^23 elementary entities.

Question 2

What is the formula for atom economy?

Answer 2

(Mr desired products / Mr of all products) * 100

Make sure to include the stoichiometry in this calculation!

Question 3

What steps would you follow to write an ionic equation?

Answer 3

1. Write out the full balanced equation with state symbols
2. Split any aqueous ionic species into their ions. Leave everything else as it is.
3. Cancel any species which appears on both sides (spectator ions)

Question 4

What is the formula for percentage yield?

Answer 4

(actual yield / theoretical yield) * 100

Question 5

What is an empirical formula?

Answer 5

The simplest whole number ratio of elements in a compound

Question 6

How would you calculate empirical formula

Answer 6

1. Take the mass (or percentage) of each element and divide it by its Ar to get moles
2. Divide each of the moles by the smallest number of moles
3. This should give you whole numbers, if not, multiply (by 2 or 3 etc) until you have a whole number ratio
4. Write out the empirical formula

Remember not to round your values!

Question 7

How to you calculate empirical formula using combustion data?

Answer 7

1. Use the mass of CO2 to calculate mass of C
2. Use the mass of H2O to calculate mass of H
3. Calculate mass of O by subtracting the mass of C and H from the total mass of the unknown compound
4. Do a normal empirical formula calculation

Question 8

What formula links mass, moles and Mr?

Answer 8

mass = moles x Mr

Question 9

What formula links concentration, moles and volume?

Answer 9

moles = concentration x volume

Question 10

What is the volume of one mole of gas at RTP (20 C in this case!)

Answer 10

24 dm3

Question 11

What formula links Avogadro’s number, moles and number of particles?

Answer 11

no. particles = moles x Avogadro’s no.

Question 12

What is the formula of the ideal gas equation, and what are the units for each component?

Answer 12

PV = nRT
P in Pa
V in m3
T in K

Question 13

Name two common indicators and their colour changes

Answer 13

Phenolphthalein
colourless in acid –> pink in alkali

Methyl orange
red in acid –> orange at equivalence point!! –> yellow in alkali

Question 14

What is the formula to calculate percentage uncertainty?

Answer 14

(% error of apparatus / measurement made) * 100

Question 15

What steps should you follow to calculate an unknown concentration?

Answer 15

1. Calculate the moles of the solution of known concentration
2. Write a balanced equation and use it to work out moles of the solution of unknown concentration
3. Use this moles and the given volume to calculate concentration