A hydrocarbon C3Hy burns in air according to the following equation: C3Hy(g) + 5O2(g) → 3CO2(g) + y/2 H2O(g) The value of y is

On the reactant side, there are $5 \times 2 = 10$ oxygen atoms from $5\text{O}_2$.
On the product side, there are $3 \times 2 = 6$ oxygen atoms from $3\text{CO}_2$ and $\frac{y}{2} \times 1 = \frac{y}{2}$ oxygen atoms from $\frac{y}{2}\text{H}_2\text{O}$.

• D. 8.

3. An element X has a valency of +3. The correct formula of its chloride is

Element X has a valency of +3, meaning it forms an ion with a $+3$ charge ($\text{X}^{3+}$).
Chlorine (Cl) is a halogen and typically has a valency of -1, forming an ion with a $-1$ charge ($\text{Cl}^{-}$).

To balance the $+3$ charge of one $\text{X}^{3+}$ ion, three $\text{Cl}^{-}$ ions are required ($3 \times (-1) = -3$).
The charges balance: $(+3) + 3(-1) = 0$.

The formula for the chloride of X is $\text{XCl}_3$.

• B. $\text{XCl}_3$