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Question

How many grams of a liquid of specific heat 0.2 cal/g C at a temperature 40C must be mixed with 100 gm of a liquid of specific heat 0.5 cal/gC at a temperature 20C, so that the final temperature of the mixture becomes 32C?

A
175 gm
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B
300 gm
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C
295 gm
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D
375 gm
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Solution

The correct option is D 375 gm
Given that,
Mass of liquid 2 (m2)=100 gm
Specific heat of liquid 1 (c1)=0.2 cal/gC
Specific heat of liquid 2 (c2)=0.5 cal/gC
Temperature of liquid 1 (θ1)=40C
Temperature of liquid 2 (θ2)=20C
Let m1 be the required mass of liquid 1.

When both the liquids are mixed, heat transfers between them until they reach thermal equilibrium at θ=32C
Using principle of calorimetry, we can say that.
Heat lost by liquid 1 = Heat gained by liquid 2
i.e m1c1(θ1θ)=m2c2(θθ2)
Thus, temperature of mixture can be written as,
θ=m1c1θ1+m2c2θ2m1c1+m2c2
32=m1×0.2×40+100×0.5×20m1×0.2+100×0.5
8m1+1000=6.4m1+1600
1.6m1=600
m1=375 g
Thus, option (d) is the correct answer.

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