EL 9 - How salty?

Question 1

What is an acid?

Answer 1

A compound that dissociates in water to produce H+ ions

known as proton donor

Question 2

What is a base?

Answer 2

A compound that reacts with an acid to produce water and a salt
(known as a proton acceptor)

Question 3

What is an alkali?

Answer 3

A base that dissolves in water to produce OH- ions

Question 4

What is the difference between a base + an alkali?

Answer 4

An alkali dissolves in water but a base does not

Question 5

Name the key property of Group 2 oxides + hydroxides?

Answer 5

They are bases - react with acids to form salts

Question 6

Use the example of the reaction HCl + NH3 reacting to make NH4Cl - which of the reactants acts as an acid/base?

Answer 6

HCl acts as acid - donates proton (H+ ion) to NH3

NH3 acts as base - accepts proton

Question 7

What is the Bronsted-Lowry Theory?

Answer 7

States that:

• Any compound that can transfer a proton to any other compound is an acid
• The compound that accepts the proton is a base
• A proton is a nuclear particle with a unit positive electrical charge; it is represented by the symbol H+ because it constitutes the nucleus of a hydrogen atom.

Question 8

What does an the reaction between acid and water (which acts as a base in this reaction) always produce?

Answer 8

Oxonium ion (H3O+ ion)

Question 9

What are the properties of an oxonium ion?

Answer 9

The same as acids (can itself act as an acid, donating H+ and becoming H2O molecule)

Question 10

What can H3O+ (oxonium ion) be shortened to in a formula?

Answer 10

H+

Question 11

Write equation of the dissociation of HCl into ions

Answer 11

HCl(aq) -> H+(aq) + Cl-(aq)

Question 12

What is a neutralisation reaction?

Answer 12

When an acid and an alkali react to form a salt and water

Question 13

Write the ionic equation of neutralisation

Answer 13

H+(aq) + OH- -> H20(l)

Question 14

State 3 things you need to do to find the exact reacting volume at neutralisation

Answer 14

• Dilute a solution
• Make up a standard solution
• Carry out an acid base titration
  (* You need to know at least one of the reactant’s concentration to work out the reacting volume)

Question 15

How do you find out the concentration in mol/dm^3

Answer 15

concentration in g/dm^3 / molar mass in g/mol

Question 16

What 2 things does the concentration of the solution depend on?

Answer 16

• The amount of solute (solute is the substance that will be dissolved in the solvent)
• the final volume of solution

Question 17

How do you find the amount of solute in a particular volume?

Answer 17

amount of solute (mol) = concentration of solution (mol/dm^3) x volume of solution (dm^3)

Question 18

What is the equation for concentration?

Answer 18

c = n / V
Concentration: c (in mol/dm^3)
Volume: V (in dm^3)
Amount: n (in moles)

Question 19

Describe the 4 steps of finding reacting volumes of solutions

Answer 19

1. Write a balanced equation
2. Write down what the equation tells you about the amount in moles of the substances you are interested in
3. Use the known concentrations of the solutions to change amounts in moles to volumes of solutions
4. Scale the volumes of solutions to the ones in the question

Question 20

Describe the steps of making a standard solution

Answer 20

1. Weigh a weighing bottle and record the mass accurately in your results table.
2. Add specific amount of solute to the weighing bottle and weigh it again.
3. Transfer the solid to a 250 cm3 beaker and re-weigh the weighing bottle. Record all balance readings and work out the exact mass of the solute that has been transferred to the beaker.
4. Dissolve the solute in about 100 cm3 of distilled water in the beaker and carefully pour the solution into a 250 cm3 volumetric flask.
5. Rinse the beaker two or three times with small volumes of distilled water, and transfer the washings to the volumetric flask.
6. Make up the solution to 250 cm3 with more distilled water. Use a dropper pipette to add the last few drops of water to ensure that the bottom of the meniscus is exactly level with the mark on the neck of the flask.
7. Stopper the flask and invert it several times.

Question 21

Suggest why it was necessary to weigh the weighing bottle after you had transferred the solid to the beaker

Answer 21

To make sure all the solid has been removed and transferred to the beaker (make sure there is none left or stuck on the weighing bottle)

Question 22

Describe the reason why you should not add the solid sodium carbonate directly to the volumetric flask

Answer 22

It would be difficult to transfer the solid in the volumetric flask as it has a very thin neck - it would be difficult to transfer without some of it missing the flask

Question 23

How did you help the solute dissolve in the water in step 2?

Answer 23

Use the stirring rod to mix and help it break down and dissolve more easily

Question 24

Why is the beaker rinsed several times with distilled water and the washings added to the volumetric flask?

Answer 24

To make sure that all of the dissolved solute is poured into the flask (no more left in the beaker)

Question 25

Why do you use a dropper pipette to add the last few drops of water to the flask?

Answer 25

To make sure that you don’t add too much water and that you add the exact volume needed

Question 26

Why do you invert the flask several times in step 7?

Answer 26

To allow oxygen bubbles in and so that all of the solute has dissolved

Question 27

How would the concentration of solute change if you dropped some of the solid onto the bench while transferring it from the balance to the beaker?

Answer 27

There would be a slightly lower concentration of solute

Question 28

How would the concentration of solute change if you added too much water so that the meniscus was higher than the mark on the flask?

Answer 28

There would be a slightly lower concentration of solute

Question 29

A student decided to wash the volumetric flask in hot water before use to ensure that it was clean. How might this affect the concentration of the standard solution that was made up using the hot flask?

Answer 29

It would lower the concentration

Question 30

Suggest why standard solutions are useful

Answer 30

Standard solutions are useful because they can be used to identify/determine the concentration of a substance with an unknown concentration