Question

Water at 80°C is mixed with water
at 40°C to get 20 kg of water at
50°C. The mass of hot water taken
is
(A)
15 kg
(B)
10 kg
(C)
5 kg
(D) 8 kg​

Answer 1

Answer:

since it is given that 50 gms water at 20 ⁰ C and 50 gms of water at 40⁰ C are mixed.  Since the masses of the liquid at different temperatures are same, the answer is very easy and simple :  average of 20⁰C and 40⁰C.  that is:  30⁰C.

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final temperature of the mixture =  

    = [ m1 * T1 + m2 * T2 ] / (m1 + m2)

    = [ 50 gms * 20⁰ C + 50 gms * 40⁰C ]  / (50+50)

    = 3,000 / 100 = 30⁰C

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another way using specific heats :

  let the final temperature be = T ⁰C

  Amount of heat given out by the hot water = m * s * (40⁰C - T)

          = 50 gms * s* (40 -T)

  Amount of heat taken in by the cold water = m * s * (T - 20⁰C)

      = 50 gms * s * (T - 20 )

  As the amounts are equal, because the heat is transferred from hotwater to the cold water without any loss of heat to any surroundings,

          50 * s * (40 -T) = 50 gm * s  * (T-20)  

                40 - T = T - 20

            2 T = 60    => T = 30⁰C

Explanation: