Answer
a. The pressure increases.
b. The pressure decreases.
c. The pressure increases.
Work Step by Step
a. Volume is inversely proportional to the Pressure. Thus, when the volume decreases (compression) at constant temperature and amount of gas, the pressure increases.
According to the Boyle's law:
$$P_1V_1 = P_2V_2$$ $$P_1(4.0 \space L) = P_2(2.0 \space L)$$ $$P_1 \frac{4.0 \space L}{2.0 \space L} = P_2$$ $$2P_1 = P_2$$
Since they must be positive values, $P_2 \gt P_1$
b. When the volume increases (expansion) at constant temperature and amount of gas, the pressure decreases.
c. When the volume decreases (compression) at constant temperature and amount of gas, the pressure increases.
