Kaplan practice test 6 physical science q 25 math

[2010premed]

---

Members don't see this ad.

what would be the pH if .28 g of fe reacted with 25 L of water, assuming rex 3 contributed to the pH?

rex 3: F2 + 3 H2o -->3H+ + Fe(OH)3

math: .38g Fe x 1mol Fe x 3 mol H+ / 25L x 56 g Fe x 1 mol Fe = x

then, pH = - log x

... what's an "easy" way to do this math?

 

[Rabolisk]

I assume you mean .28 grams of Fe. .28 / 56 is .005. Then you multiply by 3 to get .015. Now to divide that by 25 L, if you're mathematically talented, you can easily tell that 15/25 is .6. If you couldn't, you still know that it's a little bit over .5. Now convert .015 to 15 x 10E-3. The answer is then .6 x 10E-3 or 6 x 10E-4 M of H+. That is between 10E-3 and 10E-4, so you pick an answer roughly halfway between 3 and 4. Don't be panicked by math. Write it down, and scientific notation is your friend, so use it.

If yo

 

[2010premed]

thanks. the question was actually .38 g, but hey this is kaplan not aamc