Agas has a volume of 800 ml at a temperature of -55.0 degrees celsius. what volume will the gas occupy at 25.0 degrees celsius? i need the formula for this.

Charle's law says "at a constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature".

V α T 

Where V is the volume and T is the temperature in Kelvin of the gas. We can use this for two situations as,
V₁/T₁ = V₂/T₂

V₁ = 800 mL

T₁ = -55.0 ⁰C = 218 K
V₂ = ? 
T₂ = 25.0 ⁰C = 298 K

By applying the formula,
800 mL / 218K = V₂ / 298 K                   
                   V₂ = (800 mL / 218 K) x 298 K
                   V₂ = 1093.58 mL

Hence, the volume of the gas at 25.0 ⁰C  is 1093.58 mL