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I just quickly want to ask 2 things about heat and doing problems involving it. I will use one of my questions as a template for my questions:

Chapter 18, Problem 041: (a) Two 41 g ice cubes are dropped into 290 g of water in a thermally insulated container. If the water is initially at 20°C, and the ice comes directly from a freezer at -15°C, what is the final temperature at thermal equilibrium? How much ice melts? (b) What is the final temperature if only one ice cube is used? The specific heat of water is 4186 J/kg·K. The specific heat of ice is 2220 J/kg·K. The latent heat of fusion is 333 kJ/kg.

So my questions are:

1.) Are you required to work in kg?

2.) Am I allowed to say that the ice mass is 82 g, instead of 2 seperate systems each with a mass of 41 g, or just say that $$\sum Q =2Q_{ice} - Q_{water}$$ where $Q_{ice} $ = the heat from a single 41 g ice block

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1) No, you're not required to work in kg. I probably would in this case, for the simple reason that I would rather convert two values to kg instead of converting 3 other values to use g. It doesn't really make a difference as long as your units are being used consistently.

2) It shouldn't make a difference here. In reality, it would effect the rate that the ice is able to melt (more surface area = faster heat transfer); but since we only care about the end equilibrium, the time it takes to reach that equilibrium is irrelevant. All that matters is the heat in the system, and that depends only on the mass and temperature of the ice; not how many pieces it is in.

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