## Fundamentals E.35

Jacinda Wollenweber 1D
Posts: 22
Joined: Wed Nov 16, 2016 3:02 am

### Fundamentals E.35

The decay of teeth can be slowed by the use of fluoridated toothpaste. The fluoride ions convert the hydroxyapatite in tooth enamel into fluorapatite. If all the hydroxyapatite is converted into fluorapatite, by what percentage does this conversion increase the mass of enamel?

I found the molar mass of each:
a) hydroxyapatite: 502.31 g/mol
b) fluorapetite: 504.30 g/mol

I am confused about how to solve for what the question is asking for. I noticed that in the solution they subtracted the 2 molar masses and divided it by the molar mass of fluorapetite. Can someone explain why we do that in order to get our answer?

Sarah_Wilen
Posts: 62
Joined: Fri Jun 23, 2017 11:39 am
Been upvoted: 4 times

### Re: Fundamentals E.35

With the use of the fluoride toothpaste, the mass of the enamel increases when changed from fluorapatite to hydroxyapatite. The question is asking you what is the percent change of mass from hydroxyapatite to fluorapatite in order to see the increase of the mass of the enamel.

The percent change equation is $\frac{final-initial}{initial}\cdot 100$
Subtract the initial mass of hydroxyapatite from the final mass of fluorapatite (what the enamel becomes), then divide that number by the initial mass of hydroxyapatite. Finally, you multiply by 100% to get a percentage. That is your percent increase.

Pooja Nair 1C
Posts: 55
Joined: Thu Jul 13, 2017 3:00 am

### Re: Fundamentals E.35

in the solutions the manual, they actually did (final-initial)/final: I believe it was to show the percent of the final fluorapetite that is caused by the conversion? i'm not entirely sure but that's how i rationalized the solution manual