How do I find the final temperature using specific heat?

What is the final temperature of water and iron if a $30~\mathrm{g}$ piece of iron at $144~\mathrm{°C}$ was dropped into a calorimeter with $40~\mathrm{g}$ of water at $20~\mathrm{°C}$?

Specific heat of water is $4.184~\mathrm{J/g°C}$

Specific heat of iron is $0.449~\mathrm{gJ/g°C}$

Here is my work:

$Q = mcΔT$

$Q_1 = -Q_2$

$Q_1 = (30)(.449~\mathrm{J})(x - 144)$ //Iron

$Q_2 = (40)(4.184)(x - 20)$ //Water

$13.47 (x-144) = - (167.36) (x-20)$

$13.47x - 1939.68 = -167.36x + 3347.20$

$5286.88 = 180.83x$

$x = 0.03420$

This is giving me an answer that wouldn't work. What did I do wrong, and how can I fix it? Thanks

• $\frac {5286.88}{180.83} \neq 0.03420$ – LDC3 Apr 20 '14 at 2:48
• Use Kelvin instead of Centigrade/ Celsius! It would not change in this calculation as they are on the same scale and you are using differences. Also try using units throughout the whole process, this will give you a hint if you transformed your equations correctly. Apart LDC3's comment, I cannot see anything wrong. – Martin - マーチン Apr 20 '14 at 9:23

All you did is essentially right, your only mistake is in the last step, as LDC3 already pointed out in the comments. However, I am encouraging you to use units all the way and when dealing with thermodynamics use Kelvin instead of Celsius. \begin{align} Q &= mc\Delta T\\ \end{align} Now you can form the equations for each of the problem, while substituting $\Delta T$ with a temperature range, being $x$ the final temperature the whole system will end up on. Also note, that the iron will be cooled down, while the water will be heated. (I am using a different approach than you. \begin{align} Q_\mathrm{loss} &= m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}[T(\ce{Fe})-x]\\ Q_\mathrm{gain} &= m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}[x-T(\ce{H2O})]\\ \end{align}
The transferred heat has to equal $$Q_\mathrm{gain} = Q_\mathrm{loss}$$
With this you can solve for $x$. \begin{align} m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}[T(\ce{Fe})-x] &= m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}[x-T(\ce{H2O})]\\ %m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}T(\ce{Fe})-m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}x &= m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}x-m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}T(\ce{H2O})\\ % m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}T(\ce{Fe}) +m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}T(\ce{H2O})&= m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}x+m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}x\\ % m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}T(\ce{Fe}) +m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}T(\ce{H2O})&= [m(\ce{H2O})\cdot{}c(\ce{H2O})+m(\ce{Fe})\cdot{}c(\ce{Fe})]\cdot{}x\\ x &=\frac{m(\ce{Fe})\cdot{}c(\ce{Fe})\cdot{}T(\ce{Fe}) +m(\ce{H2O})\cdot{}c(\ce{H2O})\cdot{}T(\ce{H2O})}{ m(\ce{H2O})\cdot{}c(\ce{H2O})+m(\ce{Fe})\cdot{}c(\ce{Fe})}\\ % x &=\frac{30~\mathrm{g}\cdot{}0.449~\mathrm{\frac{J}{gK}}\cdot{}417~\mathrm{K} +40~\mathrm{g}\cdot{}4.184~\mathrm{\frac{J}{gK}}\cdot{}293~\mathrm{K}}{ 40~\mathrm{g}\cdot{}4.184~\mathrm{\frac{J}{gK}}+30~\mathrm{g}\cdot{}0.449~\mathrm{\frac{J}{gK}}}\\ x &= \frac{5616.99~\mathrm{J}+49036.48~\mathrm{J}}{167.36~\mathrm{\frac{J}{K}}+13.47~\mathrm{\frac{J}{K}}}\\ x &= \frac{54653.47}{180.83}~\mathrm{K} =302.24~\mathrm{K}\\ x &\approx 29~\mathrm{^\circ{}C} \end{align}