[Asum]

I'm looking at older exam questions.

If you mix 5 kg of water, with temperature 20 celcius with 1kg of steam, with temperature 100 celcius. What's the total temperature?
(heat capacity(water)=4 kJ/kgK and r (steam heat) = 2 MJ/kg)

I dont know which formula to use to calculate it. Anyone know?

thx

[rabolisk]

You have two different substances (or rather, two different phases of the same compound) with different initial temperature. Without proof, I will say that the final temperature will be the same, which shouldn't be too surprising. Now there is a formula that relates change in temperature to something(s) that is given in the question. Also, you're assuming that the system (water + steam) is isolated.

[Asum]

But with intuition aside, how would you calculate the answer?

thx

[rabolisk]

Use the formula that relates heat to specific heat, mass, and temperature change.

[DrCMS]

You also need to consider the heat of vapourisation of water.  As well as a change in temperature there will also be a phase change.