EL9 - Reacting Masses, Acids and Bases, Neutralisation, and Concentration of Solutions

Question 1

Which practical do you use to calculate quantities or concentrations of substances in solution?

Answer 1

TITRATIONS

Question 2

In titrations a solution of known concentration (standard solution) is reacted with another solution of unknown concentration.

What can you work out from titrations?

Answer 2

The balanced equation for the reaction.

The concentration of the unknown solution
(or the quantities / concentrations of substances in solution).

Question 3

WORKED EXAMPLE:
Calculating masses from balanced equations:

Calculate the mass of calcium oxide and carbon dioxide that would be produced if 100.0g of calcium carbonate is heated.

Ar(Ca) = 40.0
Ar(C) = 12.0
Ar(O) = 16.0

Answer 3

Step 1: Write a balanced equation for the reaction:

CaCO3 (s) –> CaO (s) + CO2 (g)

Step 2: From the equation, work out the moles of reactants and moles of products:

1 mole of CaCO3 produces 1 mole of CaO and 1 mole of CO2.

Step 3: Calculate the molecular mass of the reactants and products from the atomic mass of the atoms that make up the substances.
Ar(Ca) = 40.0
Ar(C) = 12.0
Ar(O) = 16.0

Mr (CaCO3) =
40.0g + 12g + (3 x 16.0g) = 100.0g

Mr (CaO) =
40.0g + 16.0g = 56.0g

Mr (CO2) =
12.0g + (2 x 16.0g) = 44.0g

Step 4: Work out the masses of the products from the mass that was reacted

100.0g of calcium carbonate gives 56.0g of calcium oxide and 44.0g carbon dioxide.

Question 4

What are properties expected of acids?

Answer 4

• Turns litmus paper red
• Reacts with carbonates to give carbon dioxide
• Neutralised by bases

Question 5

What is the general definition of an acid?

Answer 5

A substance that DONATES H+ in a chemical reaction.

Question 6

What is the general definition of a base?

Answer 6

A substance that ACCEPTS H+ in a chemical reaction.

Question 7

NOTE:

H+ is just a proton.

Answer 7

NOTE:

H+ is just a proton.

Question 8

What reaction produces an oxonium ion? (H3O+)

Answer 8

HCl (aq) + H2O (l) –> H3O+ (aq) + Cl- (aq)

(acid + base –> oxonium ion + neg ion)

______

In this reaction, water behaves as a base.
The H3O+ ion is called the oxonium ion.
It’s a very common ion and is present in every solution of an acid in water – it occurs in every ACIDIC solution.
The acid donates H+ to H2O to form H3O+.

_______

The bonding in the H3O+ ion:

A lone pair on the oxygen atom forms a dative covalent bond to H+.

The H3O+ ion can act itself as an acid, donating H+ and becoming an H2O molecule.

H3O+ will often be shortened to H+ (aq) and the dissociation of HCl (aq) into ions represented by:

HCl (aq) –> H+ (aq) + Cl- (aq)

When an acid dissolves in water, H+ (aq) ions form in solution.

Question 9

What is an alkali?

Answer 9

A base that dissolves in water to produce hydroxide ions
(OH- (aq)).

NOTE:
NOT ALL BASES ARE ALKALIS

Question 10

What does a neutralisation reaction involve?

Answer 10

An alkali reacting with an acid, forming a salt.

The reaction involves the hydrogen ions in the acidic solution and the hydroxide ions in the alkali, so the ionic equation for a neutralisation reaction is:

H+ (aq) + OH- –> H2O (l)

Question 11

By using a known concentration of either an acid or base, the exact reacting volume at neutralisation can be found out.

In order to successfully do this, what three things do you need to do?

Answer 11

• dilute a solution
• make up a standard solution
• carry out an acid-base titration

Question 12

How do you convert g dm-3 to mol dm-3?

Answer 12

Divide it by molar mass.
________
i.e.

conc (mol dm-3)=
conc (g dm-3) / molar mass (g mol-1)

________
e.g. The molar mass of sodium hydroxide (NaOH) is 40.0gmol-1, so a solution containing 80gdm-3 has a concentration of:

80gdm-3 / 40.0gdm-3 = 2.0moldm-3

Question 13

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

Answer 13

amount (mol) =

conc of solution (mol dm-3) x volume of solution (dm3)

Question 14

WORKED EXAMPLE: The amount of a solute in a solution:

Calculate how many moles of sodium hydroxide are in a 250cm3 solution of sodium hydroxide with a concentration of 2 mol dm-3.

Answer 14

Step 1: Convert 250cm3 to dm3.

250 / 1000 = 0.25dm3

Step 2: Calculate the moles in 0.25dm3 of solution.

2 mol dm-3 x 0.25dm3 = 0.5mol of sodium hydroxide

Question 15

When carrying out a chemical reaction in solution, knowing the equation for the reaction, how can you use the concentrations of the reacting solutions to predict the volumes you will need?

Answer 15

concentration (v) (mol dm-3) =
amount (n) (moles) / volume (V) (dm3)

STEPS:

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.
(volume = amount x conc)

4) Scale the volumes of solutions to the ones in the question.