1.2 Amount of Substance

Question 1

How many particles does 1 mole contain (i.e. what’s Avogadro’s constant)?

Answer 1

6.02 x 10²³ particles

Question 2

What is the formula for converting between no. of particles and no. of moles?

Answer 2

Question 3

What formula do you use to calculate the no. of moles from mass?

Answer 3

Question 4

What formula do you use to calculate the no. of moles from concentration?

Answer 4

Question 5

Write the ionic equation for this reaction:

HNO₃ + NaOH → NaNO₃ + H₂O

Answer 5

Question 6

What does ‘aq’ mean?

Answer 6

Aqueous (solution in water)

Question 7

When you’ve written an ionic equation, what should you do?

Answer 7

Check the charges & atoms are balanced

Question 8

What is the formula for percentage yield?

Answer 8

Question 9

What does percentage yield tell you?

Answer 9

Tells you how wasteful process is = based on how much product is lost

(Doesn’t measure how wasteful reaction itself)

Question 10

What is atom economy?

Answer 10

Measure of proportion of reactant atoms that become part of desired product (rather than by-products)

Question 11

What is the formula for atom economy?

Answer 11

Question 12

Why do companies in the chemical industry try to use processes with high atom economies?

Answer 12

• Better for environment
  
  • ∵ produce less waste
• More efficient use of raw materials
  
  • More sustainable (use natural resources more slowly)
• Less expensive
  
  • Spend less on separating desired product from waste products & less on treating waste

Question 13

What is empirical formula?

Answer 13

Simplest whole number ratio of atoms of each element in a compound

Question 14

What is molecular formula?

Answer 14

Actual no. of atoms of each element in a compound

Question 15

What is a standard (volumetic) solution?

Answer 15

Solution that you know the exact concentration of

Question 16

What does the making of standard solution involve?

Answer 16

Involves dissolving a known amount of solid in known amount of water to create known concentration

Question 17

Describe how you would make 250 cm³ of a 2.00 mol
dm^-3 solution of sodium hydroxide (8 steps)

Answer 17

1. Work no. moles of sodium hydroxide needed (0.5 moles)
2. Work out no. of grams of sodium hydroxide (20.0 g)
3. Place weighing boat on digital balance & weigh out mass of solid
   
   1. Tip it into beaker
   2. Re-weight boat
   3. Subtract mass of boat from mass of boat and solid together = find precise mass of solid used
4. Add distilled water to beaker (e.g. 100 dm³) & stir until sodium hydroxide dissolved
5. Pour solution into large enough volumetric flask, using funnel
6. Rinse beaker & stirring rod with distilled water & add that to flask
7. Top flask up to correct volume with distilled water - make sure bottom of meniscus reaches the line
8. Stopper the flask & turn it upside down a few times = makes sure it’s mixed

Question 18

Making a standard solution:

Why do you re-weigh the boat?

Answer 18

∵ it may contain traces of solid

Question 19

Making a standard solution:

Why do you rinse the beaker & stirring rod with distilled water and add that to the flask?

Answer 19

Makes sure no solute clings to beaker/rod

Question 20

What do titrations allow you to do?

Answer 20

Allow to find out exactly the volume of acid needed to neutralise a measured quantity of alkali (& vice versa)

Use this data to work out concentration of alkali

Question 21

Describe a method for a titration

Answer 21

1. Use pipette to measure set volume of solution that you want to find concentration of
   
   1. Put into flask
2. Add drops of indicator
3. Fill burette with standard solution of acid
4. Use funnel to pour acid into burette
   
   1. Do this below eye level
5. Do rough titration to get idea of end point
   
   1. Add acid to alkali using burette = give flask regular swirls
6. Do accurate titration
   
   1. Take initial reading of acid in burette
   2. Run acid within 2 cm³ of end point
   3. Add acid dropwise until reach end point
7. Work amount of acid used to neutralise alkali
   
   1. (Final reading - initial reading = volume of titre)
8. Repeat titration few times - until have 3 concordant results
9. Use results to calculate mean volume - leaving anomalous results

Question 22

What kind of indicators are used in a titration?

Answer 22

Indicators that change colour quickly over very small pH range = used so you know exactly when reaction has ended

Question 23

Name 2 indicators that could be used in a titration (for for acid/alkali reactions)

Answer 23

• Methyl orange
• Phenolphthalein

Question 24

What colour is methyl orange in acids?

Answer 24

Red

Question 25

What colour is methyl orange in alkalis?

Answer 25

Yellow

Question 26

What colour is phenolphthalein in acids?

Answer 26

Colourless

Question 27

What colour is phenolphthalein in alkalis?

Answer 27

Pink

Question 28

What do pipettes measure?

Answer 28

Only one volume of solution

Question 29

What do burettes measure and let you do?

Answer 29

Measures different volumes & lets you add the solution drop by drop

Question 30

What is the ideal gas equation?

Answer 30

Question 31

What is rule for volume of gases?

Answer 31

At room temperature and pressure, 1 mole of any gas occupies 24 dm³

Question 32

What formula is used to calculate the volume of gases?

Answer 32

Question 33

State the conversions for mg to g & vice versa

Answer 33

mg → g ÷ 1000 mg ← g x 1000

Question 34

What is the mass in grams of a single atom of silver? The Avogadro constant = 6.02 x 10²³ Ag = 107.9

Answer 34

1.79 x 10^-22 g

Question 35

What is the mass in grams of a single molecule of water? The Avogadro constant = 6.02 x 10²³ Mr of water = 18.02

Answer 35

2.99 x 10^-23 g

Question 36

Answer 36

13.3 g

Question 37

Answer 37

2.41 x 10²²

Question 38

Answer 38

Question 39

Answer 39

Question 40

State why it's important to fill the space below the tap in the burette before beginning an accurate titration (1)

Answer 40

(Will fill during titration and cause) titre value to be too high

Question 41

State why rinsing the inside of the conical flask with distilled water during a titration can improve the accuracy of the end point (1)

Answer 41

Returns reagent on the sides of the flask to the reaction mixture (to ensure that all of the acid/alkali reacts)

Question 42

Answer 42

Question 43

Answer 43

Question 44

State why rinsing doesn't change the volume of base solution needed in a titration (1)

Answer 44

no effect on amount/moles of either reactant or water is not a reagent