AP Chemistry Question !!

Test Preparation AP Sciences
1

<p>Magnesium sulfate is often used in first-aid hot packs, giving offheat when dissolved in water. When 2.00g of MgS)4 dissolves in 15.0mL of water at 25C in a coffee-cup calorimeter, 1.51 kJ of heat is evolved.</p>

<p>What is the final temperature of the solution? (Specific heat of water is 4.18 J/g*C).</p>

<p>I was given two equations from the book in this chapter:</p>

<p>Heat flow equations (q=m<em>c</em>change in temp)
Coffee-cup calorimeter (q= -m of water * 4.18J/g * C * Change in t)</p>

<p>I used the Coffee-cup equation. but apparently I was wrong.</p>

<p>This is what I did.</p>

<p>1.51= -15g * 4.18J/gC * (tF - 25C)</p>

<p>but the answer book shows it as this</p>

<p>1.51= 15g * 4.18J/gC * (tF - 25C)</p>

<p>Why do you use the Heat flow equation instead of Coffee cup calorimeter equation for this problem?</p>

2

<p>If you haven’t realized already, those equations are the same.</p>

<p>Try to get a numerical value for your response. The result will be lower than the value you started off with, which doesn’t make sense because heat was given off.</p>

<p>Remember that in an exothermic reaction heat is given off, thus the change in heat is noted as negative. The negatives on both sides of the equation cancel out, resulting in what is given in the answer book.</p>

<p>Be careful - the heat given off in the problem is given in kilojoules and the second equation requires joules to be valid.</p>