If a gas sample has a pressure of 30.7 kPa at 0.00*C, by how much does the temperature have to decrease to lower the pressure to 28.4 kPa? (volume is constant)

                      252.68 K  or   -20.46 °C

Explanation:

                    According to Gay-Lussac's Law, "Pressure and Temperature at given volume are directly proportional to each other".

Mathematically,

                                              P₁ / T₁  =  P₂ / T₂   (1)

Data Given:

                  P₁  =  30.7 kPa

                  T₁  =  0.00 °C  =  273.15 K

                  P₂  =  28.4 kPa

                  T₂  =  ???

Solving equation for T₂,

                  T₂  =  P₂ T₁ / P₁

Putting values,

                  T₂  =  28.4 kPa × 273.15 K / 30.7 kPa

                  T₂  =  252.68 K  or   -20.46 °C