Gas in a balloon occupies 3.3 l. what volume will it occupy if the pressure is changed from 100.0 kpa to 90.0 kpa (at constant temperature).

Since the temperature is a constant, we can use Boyle's law to solve this.

Boyle' law says "at a constant temperature, the pressure of a fixed amount of an ideal gas is inversely proportional to its volume.

P α 1/V               ⇒                 PV = k (constant)

Where, P is the pressure of the gas and V is the volume.

Here, we assume that the gas in the balloon is an ideal gas. 

We can use Boyle's law for these two situations as,
            P₁V₁ = P₂V₂

P₁ = 100.0 kPa = 1 x 10⁵ Pa
V₁ = 3.3 L
P₂ = 90.0 x 10³ Pa
V₂ =?

By substitution,
    1 x 10⁵ Pa x 3.3 L = 90 x 10³ Pa x V₂
                            V₂ = 3.7 L
Hence, the volume of gas when pressure is 90.0 kPa is 3.7 L.