PRELIMINARY STAGE 2026
Contest 22
Wesley Grammar School: 46 points
Kofi Agyei SHTS: 41 points
Yilo Krobo SHS: 27 points
Contest 23
Adiembra SHS: 54 points
T.I. AMASS, Kumasi: 46 points
PRESEC, Bechem: 29 points
ROUND 2 - SPEED RACE
QUESTION
What is the molecular formula of the 6th member of the alkyne homologous series?
NOTE: C6H10 (hexyne) is the 5th member, not the 6th.
Counting from ethyne (C$_2$H$_2$) as the 1st member — since a carbon-carbon triple bond needs at least 2 carbons — the 6th member is heptyne, C$_7$H$_{12}$.
Corrected here.
ANSWER: C$_7$H$_{12}$
SOLUTION:
Alkynes follow $C_nH_{2n-2}$, starting at $n=2$ (ethyne) as the 1st member.
So the "kth member" always has $n=k+1$ carbons.
For the 6th member: $n=7$, giving $C_7H_{2(7)-2}=C_7H_{12}$.
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PRACTICE QUESTIONS
1. QUESTION: What is the molecular formula of the fourth member of the alkyne homologous series?
ANSWER: $C_{5}H_{8}$
SOLUTION:
Alkynes follow $C_nH_{2n-2}$, starting at ethyne (n = 2) as the first member
The fourth member has $n = 5$
Molecular formula: $C_{5}H_{8}$
2. QUESTION: What is the molecular formula of the tenth member of the alkene homologous series?
ANSWER: $C_{11}H_{22}$
SOLUTION:
Alkenes follow $C_nH_{2n}$, starting at ethene (n = 2) as the first member
The tenth member has $n = 11$
Molecular formula: $C_{11}H_{22}$
3. QUESTION: What is the molecular formula of the seventh member of the alkane homologous series?
ANSWER: $C_{7}H_{16}$
SOLUTION:
Alkanes follow $C_nH_{2n+2}$, starting at methane (n = 1) as the first member
The seventh member has $n = 7$
Molecular formula: $C_{7}H_{16}$
4. QUESTION: What is the molecular formula of the sixth member of the alkane homologous series?
ANSWER: $C_{6}H_{14}$
SOLUTION:
Alkanes follow $C_nH_{2n+2}$, starting at methane (n = 1) as the first member
The sixth member has $n = 6$
Molecular formula: $C_{6}H_{14}$
5. QUESTION: What is the molecular formula of the third member of the alkyne homologous series?
ANSWER: $C_{4}H_{6}$
SOLUTION:
Alkynes follow $C_nH_{2n-2}$, starting at ethyne (n = 2) as the first member
The third member has $n = 4$
Molecular formula: $C_{4}H_{6}$
6. QUESTION: What is the molecular formula of the fifth member of the alkene homologous series?
ANSWER: $C_{6}H_{12}$
SOLUTION:
Alkenes follow $C_nH_{2n}$, starting at ethene (n = 2) as the first member
The fifth member has $n = 6$
Molecular formula: $C_{6}H_{12}$