# Homework Help: Heats of reaction lab

1. Nov 1, 2012

1. The problem statement, all variables and given/known data
Hi
Lets say I heat 50g of water to 80°C. I take another 50g of room temperature water and put it in a bowl.

Then I add this 50g of heat water to the bowl, the temperature in the bowl goes from 25°C to 48°C.

What is the amount of heat absorbed by the room temperature water?

2. Relevant equations

Is it
Q = (50g)(4.18 J/(g*k))(48-25)
or
Q = (100g)(4.18 J/(g*k))(48-25)

3. The attempt at a solution

Do you add the solution of water in the mass or just use 50g=mass in the equation?

2. Nov 1, 2012

### Staff: Mentor

What is the mass of the room temperature water? 50g or 100g?

3. Nov 1, 2012

I add the 50g of hot water to 50g of water in the bowl. Wondering when calculation the
Q=mc(dT)
should the mass be 100 or 50

4. Nov 1, 2012

### Staff: Mentor

You have 50 g of the room temperature water and 50 g of hot water. What is the mass of the room temperature water?

This is like one of those stupid joke questions kids ask - answer is so obvious nobody sees it.

Yes, it is THAT simple.

5. Nov 1, 2012

What is the mass of the room temperature water?

6. Nov 1, 2012

K. What I am asking to you is when I add hot water in the bowl to temperature of bowl changes from 25°C to 48°C. The total mass of water in the bowl is 100g.

What is the amount of heat absorbed by the room temperature water?
So do I use 50g or 100g as a mass?

7. Nov 2, 2012

### Staff: Mentor

There is 50 g of the room temperature water, so 50 g of the room temperature water can absorb a heat.

Imagine you have not mixed these samples of water, but they were isolated by a thin metal sheet so that they could not mix, but the heat could easily flow. Final temperature is identical in both cases, but it should be obvious why there is 50 g of the room temperature water absorbing heat.

Note that so far we were all the time discussing exactly the same problem, you just couldn't make a connection. If there is 50 g of the room temperature water there is 50 g of water that can absorb heat.