Question

How much energy is need to raise 50 kg of water from 45 c to 80c?

Answer 1

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

Answer 2

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