4.36 kg rubidium.
That is; Q = mcΔT
We are given;
Quantity of heat = 40.0 kJ or 40,000 Joules
Mass of rubidium = 0.360 J/g°C
Change in temperature, ΔT = 38.0°C - 12.5°C
= 25.5°C
We are required to calculate the mass of rubidium
From the formula;Q = mcΔT
mass = Q ÷ cΔT
= 40,000 ÷(0.360 J/g°C× 25.5°C)
= 4,357.30 g
= 4.36 kg
Therefore, the mass of 4.36 kg rubidium.