HW Problem E35

[Angelica Dis 3k]

Hello friends,

The problem is: If all the hydroxyapatite is converted into fluorapatite by what percent does this conversion increase the mass of the enamel?

The molar mass of hydroxyapatite is 502 g/mol and fluorapatite is 504
I started this problem by finding the difference between these and got 2
Then I divided 2 by 502, but in the solutions guide it says to divide 2 by 504....

Why do we divide the difference in molar masses by the final compound's molars instead of the initial compound's molar mass?

[Leah Thomas 2E]

I think that the solutions manual made a mistake. I am sure that your are supposed to divide by the initial molar mass of the compound to find the percent change in mass. Either way the answer does remain the same.

[Naveed Zaman 1C]

Ya it most likely is a mistake because there is no reason that you should divide by the final mass since you are trying to compare to the initial mass. If you pretend that there is one mole of hydroxyapatite to start with, and it all turns to fluorapatite, then you would say that there is a 1.99g increase, or a 0.396% increase, which rounds to 0.4%. This is slightly different than the solutions manual has, which is 0.39% (rounded from 0.394% using the formula in this book) which makes it a little difficult to determine that the solutions manual probably has it wrong. However, there is no logical reason to calculate the percentage increase if you're not comparing it to the initial value (before the increase), so you had it right in the beginning.

[Grace Ramey 2K]

As a follow-up question for Problem E35, I was wondering how you know the conversion of 1 mol Ca5((PO4)3)OH is exactly 1 mol Ca5((PO4)3)F ? Is it assumed since the problem doesn't state otherwise?

[JamesAntonios 1E]

As a follow-up question for Problem E35, I was wondering how you know the conversion of 1 mol Ca5((PO4)3)OH is exactly 1 mol Ca5((PO4)3)F ? Is it assumed since the problem doesn't state otherwise?

That was the whole point of the percent increase problem. If you have a certain number of grams of Ca5((PO4)3)OH divided by the 502 g/mol, that has to be equal to the grams of Ca5((PO4)3)F/504 g/mol. Because of the law of the conservation of mass and the problem states that all of the Ca5((PO4)3)OH was converted, the moles have to the stay the same. However, that is not saying that grams have to stay the same. Think of it in terms of a balanced chemical equation.

[Angelica Dis 3k]

ahhh thank you guys so much, I really thought that I was making a mistake! Didn't consider it was just a textbook error!