[bonz061583]

The problem states, that given 100 gram block of bronze with heat capacity 27.9 J/K, it asks to calculate the mole fraction of tin in the bronze alloy. It also gives the Dulong-Petit constant = 24.94 J/K*mol, I'm kind of lost on how to start the problem.  

[Borek]

Using D-P constant calculate heat capacity of the pure tin and pure copper (use molar masses). Assume linear dependence between heat capacity and molar fraction.

[bonz061583]

Well I found the heat capacity of tin is .210 J/K*g and that of copper is .392 J/K*g. I just can't figure out what exactly u need to do after that

[xiankai]

i heard that alloys do not have a definite melting/boiling point. will this also translate into non-definite heating curves? it doesnt seem to be so in this case, so i am asking now

[Borek]

Well I found the heat capacity of tin is .210 J/K*g and that of copper is .392 J/K*g. I just can't figure out what exactly u need to do after that

Assume that heat capacity is additive and can be caculated as linear combination of heat capacities of solid components. In case of most metal alloys it holds precisely enough.

[bonz061583]

how exactly do you use the 27.9 J/K heat capacity of bronze, and equat those with the heat capacity of tin and copper?

[Borek]

x - molar fraction of tin

heat capacity of bronze = x * het capacity of tin + (1-x) * heat capacity of copper.

For most alloys this approach is precise enough, it doesn't mean it can be safely used always.

[bonz061583]

my answer came out to be .379, i'm wondering if that is logically correct, given a 100 g block of bronze.

[bonz061583]

actually to correct that, i got x = 0.621

[Borek]

I would say every answer between 0 and 1 is logically correct when it comes to molar fractions

[bonz061583]

i put in, .621 and it said i was wrong, maybe someone can verify me

[Borek]

Seems to me you are wrong, show your calculations so that we will be able to look for your mistake.

But I just come with different, probably much easier approach to the problem. First - DP constants is nothing else than a specific heat of one mole of metal. If 100 g of alloy has specific heat of 27.9 J/K there is 27.9/24.94=1.119 mole of metal in the 100 g of alloy.

If tin molar fraction is x and there is a total of 1.119 mole of both metals, mass of the alloy can be expressed as

1.119*x*118.71 (mass of tin) + 1.119 * (1-x) * 63.55 (mass of copper) = 100

Solve for x.

[bonz061583]

  Thanks a lot Borek, i appreciate your help, and I don't think this will be the last time i need it.