Page 11 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 7

An aqueous solution of a substituted ethanoic acid, X 3 CCOOH contains
16.0 g dm of the acid. 25.0 cm of the acid solution exactly neutralises
–3
3
24.50 cm of 0.10 mol dm sodium hydroxide.
–3
3
Penerbitan Pelangi Sdn Bhd. All Rights Reserved.
(a) (i) Write a chemical equation for the reaction between the acid and sodium
hydroxide.
(ii) Write an ionic equation for the reaction.
(b) (i) Determine the relative molecular mass of the acid, X 3 CCOOH.
(ii) Calculate the relative atomic mass of X.
(c) Suggest an identity for X.
Term
1
Answer:
Exam Tips
Exam Tips
The chemical formula of the acid indicates that it is a monoprotic acid. Hence,
one mole of acid reacts with one mole of the alkali.
(a) (i) X 3 CCOOH + NaOH : X 3 CCOONa + H 2 O
(ii) H + OH : H 2 O
–
+
a
(b) (i) ——– = — where M a , M b = molarity of acid and base respectively
M a V a
M b V b b a, b = number of moles of acid and base in the equation
24.50(0.10)
–3
Molarity of acid = ————–– = 0.098 mol dm
25.0
Mass
Relative molecular mass of acid = ——–————–
Number of moles
16.0
= ——–
0.098
= 163.3
(ii) X 3 CCOOH = 163.3
3X + 2(12) + 2(16) + 1 = 163.3
X = 35.4
Relative atomic mass of X = 35.4
(c) Chlorine

6

01 STPM Q&A Chem T1 layout.indd 6 4/19/19 9:28 AM