C3

Question 1

What does the conservation of mass state?

Answer 1

no atoms are lost or made during a chemical reaction so the mass of the products = mass of the reactants

Question 2

How do you calculate the percentage mass of an element in a compound?

Answer 2

(Ar x number of atoms of that element/ Mr of the compound) x 100

Question 3

Why may some reactions involve a change in mass?

Answer 3

a reactant/product is a gas so its mass is not taken into account

Question 4

What equation links mass, Mr and moles?

Answer 4

mass= moles x Mr

rearranged=
- moles= mass/Mr
- Mr= mass/moles

Question 5

What is Avogadro’s constant?

Answer 5

6.02 x 10²³= the number of particles/atoms/molecules in one mole of a substance

Question 6

What is the mass of one mole of a substance in grams equal to?

Answer 6

It is numerically equal to the substance’s relative formula mass

e.g. Sodium= 23g/mol

Question 7

What is a limiting reactant?

Answer 7

The reactant that is completely used up is called the limiting reactant because it limits the amount of products

Question 8

What are the two ways of calculating the concentration of a solution?

Answer 8

• mass of solute(g)/ volume of the solvent(dm³)
• moles of solute(mol)/ volume of solvent(dm³)

Question 9

If you had the concentration measured in g/dm3 how can you change it to mol/dm3?

Answer 9

divide the mass by the relative formula(Mr)

• if the concentration was already in mol/dm3 then multiply by the Mr

Question 10

How do you calculate the molar volume of gases?

Answer 10

volume= moles x 24dm³

at the same temperature and pressure, equal number of moles of any gas will occupy the same volume.

Question 11

How do you convert from dm³ to cm³

Answer 11

X by 1000 (1dm ³ = 1000cm ³)

Question 12

Why is it not always possible to obtain the calculated amount of a product?

Answer 12

• some of the product may be lost when it is separated from the reaction mixture
• the reaction may not go to completion because it is reversible

Question 13

percentage yield equation?

Answer 13

(mass of actual product/ theoretical mass) x 100

Question 14

What is the percentage yield for?

Answer 14

using the most efficient reaction condition and apparatus to reduce energy use, costs and conserve raw materials

Question 15

atom economy equation?

Answer 15

(total Mr of desired product/ total Mr of all reactants) x 100

Question 16

What is atom economy?

Answer 16

Choosing the most efficient reaction to make the product in order to reduce waste and pollution (important for sustainable development)

Question 17

Why are reactions with high atom economy good?

Answer 17

• cost effective
• low waste

Question 18

How do you carry out a titration?

Answer 18

1. use a pipette to measure 25cm3 of the alkali/acid into a conical flask
2. place a white tile beneath the conical flask and add the indicator
3. fill burette with an acid/alkali( of a known concentration) below the 0 mark
4. take the initial reading on the burette
5. gradually empty the burette (using the tap) into the flask and swirl the conical flask
6. when there is a permanent colour change, record the final volume of the acid/ alkali
7. to find the volume added: final volume-initial volume
8. repeat (for consistent results) discard any anomalies and calculate a mean

Question 19

Titration practical

What are the different indicators and what would be seen if an acid/alkali is present?

Answer 19

• strong acid + strong alkali= any indicator
• weak acid+ strong alkali= phenophthalein
• weak acid + weak alkali= methyl orange

results:
- phenolphthalein- colourless (in acid) → pink(alkali)
- methyl orange- red/pink(in acid) → orange(alkali)