Convert the following capacities: a) ml in 6 L, b) ml in 0.25 L, c) L in 300 ml, d) L in 4000 ml, e) L in 2 kL

$$
6 \text{ L} \times \frac{1000 \text{ ml}}{1 \text{ L}} = 6000 \text{ ml}
$$
The capacity is $\boxed{\text{6000 ml}}$.

$$
0.25 \text{ L} \times \frac{1000 \text{ ml}}{1 \text{ L}} = 250 \text{ ml}
$$
The capacity is $\boxed{\text{250 ml}}$.

$$
300 \text{ ml} \times \frac{0.001 \text{ L}}{1 \text{ ml}} = 0.3 \text{ L}
$$
The capacity is $\boxed{\text{0.3 L}}$.

$$
4000 \text{ ml} \times \frac{0.001 \text{ L}}{1 \text{ ml}} = 4 \text{ L}
$$
The capacity is $\boxed{\text{4 L}}$.

$$
2 \text{ kL} \times \frac{1000 \text{ L}}{1 \text{ kL}} = 2000 \text{ L}
$$
The capacity is $\boxed{\text{2000 L}}$.

$$
0.0025 \text{ kL} \times \frac{1000 \text{ L}}{1 \text{ kL}} = 2.5 \text{ L}
$$
The capacity is $\boxed{\text{2.5 L}}$.