You mix 1 glass of water at 20 C and 1 glass of water at 80 C. What will be temperature of mixture 30

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Explanation:

When Two Samples of Water are Mixed, what Final Temperature Results?

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Worksheet #2

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Example #1: Determine the final temperature when 32.2 g of water at 14.9 °C mixes with 32.2 grams of water at 46.8 °C.

This is problem 8a from Worksheet #2.

First some discussion, then the solution. Forgive me if the points seem obvious:

1) The colder water will warm up (heat energy "flows" into it). The warmer water will cool down (heat energy "flows" out of it).

2) The whole mixture will wind up at the SAME temperature. This is very, very important.

3) The energy which "flowed" out (of the warmer water) equals the energy which "flowed" in (to the colder water)

This problem type becomes slightly harder if a phase change is involved. For this example, no phase change. What that means is that only the specific heat equation will be involved

Solution Key Number One: We start by calling the final, ending temperature 'x.' Keep in mind that BOTH water samples will wind up at the temperature we are calling 'x.' Also, make sure you understand that the 'x' we are using IS NOT the Δt, but the FINAL temperature. This is what we are solving for.

The warmer water goes down from to 46.8 to x, so this means its Δt equals 46.8 − x. The colder water goes up in temperature, so its Δt equals x − 14.9.

That last paragraph may be a bit confusing, so let's compare it to a number line:



To compute the absolute distance, it's the larger value minus the smaller value, so 46.8 to x is 46.8 − x and the distance from x to 14.9 is x − 14.9.

These two distances on the number line represent our two Δt values:

a) the Δt of the warmer water is 46.8 minus x

b) the Δt of the cooler water is x minus 14.9

Solution Key Number Two: the energy amount going out of the warm water is equal to the energy amount going into the cool water. This means:

qlost = qgain

However:

q = (mass) (Δt) (Cp)

So:

(mass) (Δt) (Cp) = (mass) (Δt) (Cp)

With qlost on the left side and qgain on the right side.

Substituting values into the above, we then have:

(32.2) (46.8 − x)(4.184) = (32.2) (x − 14.9) (4.184)

Solve for x