# Tricky Geometric Question

1. Oct 18, 2009

### Suy

1. The problem statement, all variables and given/known data

If the ph of battery acid should be 0.8, what is the pH of the acid in a battery that is one-fifth as acidic as it should be?

2. Relevant equations

3. The attempt at a solution
We just learned how to comparing pH in Geometric unit, using the formula 10^(ph1-ph2).
But for this question, do we really need to use that formula, or we just 0.8/5=0.16?
ty!

Last edited: Oct 18, 2009
2. Oct 18, 2009

### Staff: Mentor

How is this a geometric question?

To answer your question, you need to find out what "one-fifth as acidic" means. I suspect that it means that the concentration of H ions is one-fifth what the concentration it should be, but that doesn't mean that the pH will be one-fifth of what it should be.

10^(pH1-pH2) is not a formula, since it is not an equation. Look up the equation you need.

3. Oct 18, 2009

### symbolipoint

"one fifth as acidic" seems to be directly related to concentration, not pH of concentration. You would focus on concentration of hydronium ions.

4. Oct 18, 2009

### Suy

so it would be 1/5=10^(0.8-ph2)?
Am i right?

5. Oct 18, 2009

### symbolipoint

No. You need to convert directly to concentration, perform your specified calculation, then convert back to pH.

6. Oct 18, 2009

### Suy

But isn't they are the same?
pH=-log[H+]
-0.8=log[H+]
10^(-0.8)=H+
H+=0.15848931924611134852021013733915

0.15848931924611134852021013733915/5
=0.03169786384922226970404202746783

-log[0.03169786384922226970404202746783]
=1.50
----------------------------------------------
The book teach us to do it this way
10^(ph1-ph2)
10^(0.8-ph2)=1/5
ph2=1.50

anyway.., is this right?
ty!

7. Oct 18, 2009

### symbolipoint

I myself don't follow what your book is teaching but neither am I trying to analyze it. No matter - just use fundamental principles and you should be able to think your way through effective steps.

One thing you should know is that your use of excessive accuracy does not give you better numeric value information. Usually, pH's are good to only about 2 or 3 significant figures.

8. Oct 18, 2009

### Suy

oh, because i use the computer calculator. I just copy the number and put it here. I won't do that in my assignment.
Thx for helping me!

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook