C4 - Chemical Calculations

Question 1

formula moles

mol

Answer 1

moles = mass / molar mass
mol = g / M_r

Question 2

What are the three types of yields?
How are each type of yield found?

Answer 2

1. percentage yield - how much product was made compared with the amount of product expected
2. theoretical yield - maximum mass of product possible
3. actual yield - given in the question

Question 3

How do you find the percentage yield?

Answer 3

percentage yield = actual yield / theoretical yield

Question 4

How is the theoretical yield found?

Answer 4

The M_r

Question 5

Why will the percentage yield never be 100%?

Answer 5

1. Chemical reactions will never fully react
2. Residues - some chemicals lost through physical transfers
3. Sideways reactions

Question 6

define atom economy

Answer 6

The amount of reactants that end up as useful products. It is a way of measuring efficiency.

Question 7

formula atom economy

Answer 7

atom economy (%) = M_r of useful product / sum of M_r of all products

Question 8

Why does industrial manufacturing require a high atom economy for chemical reactions?

Answer 8

• reduces production of unwanted products
• makes the process more sustainable

Question 9

What are the factors that should be taken into consideration for carrying out chemical reactions?

Answer 9

• % yield
• atom economy (%)
• energy requirements
• toxicity of waste
• rate of reaction - the speed of the reaction
• availability of reactants

Question 10

How many atoms are in 1 mole?
What is the name of this constant?

Answer 10

6.02 x 10 ²³
Avogadro’s constant

Question 11

concentration formula

Answer 11

concentration = mass or moles / volume
concentration = (g | mol) / dm³

Question 12

1dm³ to cm³

Answer 12

1dm³ = 1000cm³

Question 13

define titration

Answer 13

Technique that can be used to analyse the concentration of substances in a solution accurately.

Question 14

define in excess

Answer 14

More than enough of a substance to react with another reactant

Question 15

define limiting factor

Answer 15

Not enough of a substance to react with another reactant

Question 16

how do you work out the unknown concentration of a solution using the known concentration of a solution?

Answer 16

(volume of known * concentration of known) / volume of unknown
only applies if the mole ratio is 1:1, otherwise use a table

Question 17

List the steps to find the concentration of an alkali NaOH, using an acid HCl with a known concentration.

Answer 17

1. fill the burette up to the 0cm³ mark with the HCl
2. use a pipette filler to fill a pipette with NaOH, and write down the volume
3. pour the precisely measured volume of NaOH into a round bottom beaker
4. use 1 or 2 drops of phenolphthalein to change the colour to pink, not purple
5. open the burette and let the HCl drain into the round bottom beaker. Swirl the solution while doing this.
6. close the burette when the solution turns colourless. record the volume of HCl used to neutralise the NaOH.
7. repeat 3 more times

Question 18

What indicator is used in acid-base titrations?

Answer 18

phenolphthalein

FEE + NOL + THAY + LEE + IN

Question 19

What colour is phenolphthalein in:
a. acidic solutions
b. basic solutions

Answer 19

a. colourless
b. pink

Question 20

volume of 1 mole of any gas at standard room temperature and pressure

Answer 20

24dm³

Question 20

masses to balanced equations steps

Answer 20

1. find M_r
2. find mol
3. divide mols by the smallest amount to get integers