Based on your problem, what you are looking for is the quantity of heat. To solve for it, you will need this formula:
Q = mc(T2-T1)
Where: Q = Quantity of heat m = mass of the substance c = Specific heat T2 = Final temperature T1 = Initial temperature
Now the specific heat of water is 4.184 J/(g°C), meaning that is how much energy is required to raise the temperature of 1g of liquid water by 1 degree Celsius.
Since your mass is in kilograms, let us convert that into grams, which will be equal to 50,000 grams. Now we can put our given into the equation:
Q = mc(T2-T1) = 50,000g x 4.184 J/(g°C) x (80°C - 45°C) = 50,000 g x 4.184 J/(g°C) x 35°C = 7,322,000 J or 7,322 kJ or 7.322 MJ
Energy = mass ×specific heat capacity×change in temperature' In this case, the mass of water is 50 kg, the specific heat capacity of water is 4184J/kg the change in temperature is 80-45= 35 degrees celcius Therefore, heat energy = 50 ×35 ×4184=7322000 Joules But 1kJ =1000 Joules therefore, the energy needed will be 7322 kJ
