# Chemistry HELP

<p>I have a few questions from my homework that I don't understand. If anyone can help, it would be greatly apppreciated.</p>

<p>1) The heat lost or gained by a system is related to its temperature change by a property called its heat capacity. The molar heat capacity of metals at or above room temperature equals 24.92 J/K.mol. The temperature of a block of copper (MM 63.55) was raised by 10.3 degrees when 28.0 J of heat were added to it. What is the mass of the copper block? Enter your answer with three significant figures. </p>

<p>2) In this experiment, a hot metal block is added to 100 mL of cold water in a calorimeter. The heat lost by the metal block (originally at 100 degrees C) equals the heat gained by the cold water. If the mass of the metal block is 9.40 grams, the temperature of the cold water extrapolated forward to the time of mixing is 13.8 degrees C, and the temperature of the metal block and water mixture extrapolated back to the time of mixing is 34.4 degrees C, what is the specific heat of the metal? Enter your answer with three significant figures in units of J/K.g. </p>

<p>3) In this experiment you use the Law of Dulong and Petit to determine the mean molar mass of an unknown alloy. If careful measurements are made, it is possible to determine the composition of the alloy from the mean molar mass. For example, bronze is an alloy of tin (MM 118.7) and copper (MM 63.55). If a 90.0 gram block of a bronze alloy has a heat capacity of 26.8 J/K, calculate the mole fraction of copper in the alloy. The Dulong-Petit constant = 24.94 J/K.mol. Enter your answer as a number with three significant figures. </p>

<p>help, please :). I have no idea as to where to begin :-/</p>

<h1>1: Use dimensional analysis. You want the mass, so start with grams in the numerator and try to eliminate all the other units.</h1>

<p>? g = 63.55 g/mol * K-mol/24.92 J * 1/10.3 K * 28.0 J = 6.93 g</p>

<h1>2: Use dimensional analysis again. This time, though, there are two steps to the problem. First find the amount of heat gained by the water, and then use that to find the specific heat of the metal.</h1>

<p>Gotta go to class now. Gluck with the last problem.</p>

<p>I have to say, unless you'r ein college (in which case you wouldn't be posting on the high school life thing), you're not going to use those Laws of Dulong and Petit for the rest of high school. I took honors chem, and then AP chem and never heard of either. And I got a 5 on the AP. And knew what all teh equations on the equation sheet meant (maybe I just didnt' know tehir names?)</p>

<p>I've never seen it either, and I'm taking college chem. It looks like a lab-specific question, though, so I guess the OP has to know it just for that.</p>