Answer
$7.0$ moles of $CO_2$ are produced when 3.5 moles of $C_2H_2$ reacts.
Work Step by Step
1. Find the mole-mole conversion factors:
According to the coefficients: 2 moles $C_2H_2$ = 4 moles $CO_2$
$\frac{ 2 \space moles \space C_2H_2}{ 4 \space moles \space CO_2}$ and $\frac{ 4 \space moles \space CO_2}{ 2 \space moles \space C_2H_2}$
2. Use the mole-mole conversion factors to calculate the amount of equivalent $CO_2$ in moles:
$ 3.5$ moles $C_2H_2 \times \frac{ 4 \space moles \space CO_2}{ 2 \space moles \space C_2H_2} = 7.0$ moles $CO_2$