Calculating Water and Steam Temperatures

[Asum]

I'm looking at older exam questions.

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

I don't know which formula to use to calculate it. Anyone know?

thx

 

[danago]

Whatever the case is, the energy lost by the steam as it condenses and cools must be equal to the energy gained by the water as it heats up. You can use this idea to find the equilibrium temperature.

 

[Borek]

I don't know which formula to use to calculate it. Anyone know?

Am I right assuming you think each question has its own magical formula, that you just have to plug numbers into get the final result? Well, that's wrong. The sooner you will accept the fact, the better for you.

In the case of heat balance questions only formula you need is heat gained = heat lost. But it is not a simple universal formula - you must be able to calculate heat gained and heat lost. It is always some combination of mcΔT (when substance changes temperature) and mL (when there is a change of state - vaporizing, melting, subliming). You have to be able to recognize individual heat sources and sinks, describe them using simple formulas, and combine them to get the formula you need. So you have to build your own formula from smaller formulas.