PRELIMINARY STAGE 2025
Our Lady of Grace SHS: 71 points
Benkum SHS: 50 points
Prampram SHS: 31 points
QUESTION
Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of C$_5$H$_8$.
ANSWER: 17
SOLUTION:
Balance C first: 5 carbons $\Rightarrow$ 5CO$_2$.
Balance H: 8 hydrogens $\Rightarrow$ 4H$_2$O.
Balance O: $4+10=14$ O atoms needed $\Rightarrow$ 7O$_2$.
$C_5H_8+7O_2\rightarrow4H_2O+5CO_2$.
$\text{Sum of coefficients} = 1+7+4+5$
$\text{Sum of coefficients} = 17$
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PRACTICE QUESTIONS
1. QUESTION: Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of $C_3H_8$.
ANSWER: 13 or $1.3\times10^{1}$
SOLUTION:
$C_3H_8+5O_2\rightarrow3CO_2+4H_2O$
$\text{sum} = 1+5+3+4$
$\text{sum} = 13$
2. QUESTION: Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of $C_4H_{10}$.
ANSWER: 33 or $3.3\times10^{1}$
SOLUTION:
$2C_4H_{10}+13O_2\rightarrow8CO_2+10H_2O$
$\text{sum} = 2+13+8+10$
$\text{sum} = 33$
3. QUESTION: Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of $C_3H_4$.
ANSWER: 10 or $1\times10^{1}$
SOLUTION:
$C_3H_4+4O_2\rightarrow3CO_2+2H_2O$
$\text{sum} = 1+4+3+2$
$\text{sum} = 10$
4. QUESTION: Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of $C_5H_{12}$.
ANSWER: 20 or $2\times10^{1}$
SOLUTION:
$C_5H_{12}+8O_2\rightarrow5CO_2+6H_2O$
$\text{sum} = 1+8+5+6$
$\text{sum} = 20$
5. QUESTION: Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of $C_6H_{14}$.
ANSWER: 47 or $4.7\times10^{1}$
SOLUTION:
$2C_6H_{14}+19O_2\rightarrow12CO_2+14H_2O$
$\text{sum} = 2+19+12+14$
$\text{sum} = 47$
6. QUESTION: Determine the sum of the coefficients of the reactants and products in a balanced equation for the combustion of $C_4H_8$.
ANSWER: 15 or $1.5\times10^{1}$
SOLUTION:
$C_4H_8+6O_2\rightarrow4CO_2+4H_2O$
$\text{sum} = 1+6+4+4$
$\text{sum} = 15$