what should be the initial mass?

2008-06-07 3:08 am

0.1 kg of milk at 20度is added into a cup of hot coffee at 80度. If the temperature of the mixture is 604, what is the initial mass of the hot coffee in the cup? The specific heat capacities of coffee and milk are 4000 J kg^-1 度^-1 and 3800 J kg^-1 度^-1 respectively.

The answer is 0.19kg,,
but ive never got it.

Please show steps.

回答 (1)

NCY

2008-06-07 3:19 am

✔ 最佳答案

According you answer,I think the temperature of the mixture should be 60度.
Let the the initial mass of the hot coffee in the cup be m kg,
By the law of conservation of energy,
energy released by hot coffee = energy absorbed by milk
(mc) (c)Δ(Tc﹣T) = (mm) (c)Δ(T ﹣Tm)
m(4000)(80﹣60) = (0.1)(3800)(60﹣20)
m(4000)(20) = (0.1)(3800)(40)
80000m = 15200
m = 0.19kg
∴ the the initial mass of the hot coffee in the cup is 0.19kg.

2008-06-06 19:21:25 補充：
According your answer

2008-06-06 19:22:18 補充：
Assume that no energy lost to the surrounding

2008-06-06 19:26:45 補充：
中文：你所說的混合溫度是604度是錯誤的，因為一杯80度的和20度的液體，混合的溫度是介乎20和80度之間。

2008-06-06 20:11:37 補充：
sorry for some errors,it should be
(m)(c)(Tc﹣T) = (m)(c)(T ﹣Tm)

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