HIGHER CHEMISTRY REVISION.
Unit 3 :- Acids and Bases
1. A student added 50 cm3 of 4.0 mol l-1 hydrochloric acid to 4.0 g of
magnesium ribbon.
(a) The balanced equation for the reaction is:
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2 (g)
Show by calculation which reactant is in excess.
(a)
4 g of Mg = 4/24.3 = 0.165 mol.
50 cm3 of 4.0 mol l-1 HCL =
50/
1000 x
0.4
= 0.2 mol
From equation 0.165 mol of Mg
would need 2 x 0.165 = 0.33
mol of HCl.
We only have 0.2 mol HCl
so the Mg is in excess.
(b) The experiment was repeated under the same conditions using
ethanoic acid instead of hydrochloric acid.
Circle the correct words in the table to show the results for
ethanoic acid.
Ethanoic acid
Rate of reaction
Volume of gas produced
slower
faster / same / slower
more / same
same/ less
2. Potassium cyanide, KCN, can be made by the reaction of an acid with an alkali.
A solution of the salt has a pH of 8.
(a) What is the concentration of H+(aq), in mol l-1, in the solution?
(b) What can be concluded about the strengths of the acid and the alkali used
in the reaction?
(c) Write the formula for the acid used in the reaction.
(a)
1 x 10-8 mol l-1.
(b)
Weak acid and strong base.
(c)
KCN
3. Compounds can have different structures and properties.
NH4NO3
SiO2
BaSO4
K2O
Na2CO3
P2O5
Identify the salt which dissolves in water to form an
alkaline solution.
Na2CO3
3. Two flasks contained equal volumes of 0.1 mol l-1 hydrochloric acid and 0.1 mol l
ethanoic acid.
Identify the two true statements about both solutions.
A
They give the same colour with Universal Indicator solution..
B
They have a pH less than 7.
C
They conduct electricity equally well.
D
The have equal concentrations of hydrogen ions.
E
They react at the same rate with magnesium.
F
They neutralise the same number of moles of sodium hydroxide.
B and F
4. A solution of 0.0010 mol l-1 hydrochloric acid can be described as a
dilute solution of a strong acid.
(a)
What is meant by a strong acid?
(b)
What is the pH of the 0.0010 mol l-1 hydrochloric acid used in the
experiment?
(a)
A strong acid is fully ionised in aqueous solution.
(b)
[H+(aq)] = 1 x 10-3 mol l-1.
pH = 3.
-1
5. Calcium hydroxyapatite makes up 95% of tooth enamel.
Tooth decay is caused when tooth enamel is attacked by acid in the mouth.
(a) One of the acids which attacks tooth enamel is 2-hydroxypropanoic acid, which has the
molecular formula C3H6O3.
Draw the structural formula for this acid.
(b) Calcium hydroxyapatite reacts with acid in the mouth as shown by the following
balanced equation.
Ca10(PO4) 6(OH) 2
+ 8H+

10Ca2+
+ 2 H 2O +
6HPO4x-
calcium hydroxyapatite
What is the value of x?
(c) The pH of a solution in the mouth is 5.
What is the concentration of hydrogen ions, in mol l-1, in this solution?
(a)
H H
OH
H-C –C - C =O
H H
(b)
(c)
x=2
1 x 10-5 mol l-1.
6. Ammonium chloride (NH4Cl) is soluble in water.
How does the pH of a solution of ammonium chloride compare with the pH of
water?
Ammonium chloride has a lower pH as it is an acidic solution – salt of strong acid
and weak base.