DAT Destroyer Gen Chem Questions problem 28

[jsk26]

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Im trying to understand the concept as well as math for problem 28 in the gen chem section of dat destroyer. If there is 1x10^-12 M HCl solution is that why water needs to be incorporated in the solution? Also, the solution says 1x10^-12 + 1x10-7 is around 1x10^-7. I don't understand how that makes sense?

 

[DAT Destroyer]

Im trying to understand the concept as well as math for problem 28 in the gen chem section of dat destroyer. If there is 1x10^-12 M HCl solution is that why water needs to be incorporated in the solution? Also, the solution says 1x10^-12 + 1x10-7 is around 1x10^-7. I don't understand how that makes sense?

The acid has a VERY VERY low concentration......thus we must consider the small amount given by the water,.,,,,the hydrolysis of water !!! Water will give us 1 x 10 exp -7 of hydronium ions......usually we dont need to count it.....but in this case we do, since the acid concentration is so SMALL. If you add 1 x 10 exp 12- plus 1 X 10 exp 7-...we get approx a value of 1 x10 exp -7. Thus the pH is about 7 .

Hope this helps.

Dr. Romano

 

[Paper2627]

The acid has a VERY VERY low concentration......thus we must consider the small amount given by the water,.,,,,the hydrolysis of water !!! Water will give us 1 x 10 exp -7 of hydronium ions......usually we dont need to count it.....but in this case we do, since the acid concentration is so SMALL. If you add 1 x 10 exp 12- plus 1 X 10 exp 7-...we get approx a value of 1 x10 exp -7. Thus the pH is about 7 .

Hope this helps.

Dr. Romano

I understand that it is low concentrations and why we add hydronium ions from water, but for some reason, I am having trouble figuring out how to add (or even subtract) numbers written in scientific notation that have exponents that are more than a few digits away. For example its easy to add 1.2x10 exp -7 plus 1.2x10 exp -8, but how would you quickly determine that 1 x 10 exp 12- plus 1 X 10 exp 7- is equal to about 1 x10 exp -7??

 

[DAT Destroyer]

I understand that it is low concentrations and why we add hydronium ions from water, but for some reason, I am having trouble figuring out how to add (or even subtract) numbers written in scientific notation that have exponents that are more than a few digits away. For example its easy to add 1.2x10 exp -7 plus 1.2x10 exp -8, but how would you quickly determine that 1 x 10 exp 12- plus 1 X 10 exp 7- is equal to about 1 x10 exp -7??

Let's keep life simple......if you add 1 x 10 exp-7 to 1.x10 exp-8. -9. -10, -11, -12, -13.....it would be approximately . 1 x10 exp-7 as well. Thus when you do a pH calculation, you get a value very close to 7. Take out a calculator and prove this to yourself.

Hope this helps.

Dr. Jim Romano

 

[Pearl E. White]

Basically, .0000007 + .00000000001 = .0000007
The other value is way too small when you add them and round up.

 

[Paper2627]

Let's keep life simple......if you add 1 x 10 exp-7 to 1.x10 exp-8. -9. -10, -11, -12, -13.....it would be approximately . 1 x10 exp-7 as well. Thus when you do a pH calculation, you get a value very close to 7. Take out a calculator and prove this to yourself.

Hope this helps.

Dr. Jim Romano

Ohh I see, I don't know why I didn't realize that earlier... Needed to take a break I guess haha. Thanks so much though!

 

[DAT Destroyer]

Ohh I see, I don't know why I didn't realize that earlier... Needed to take a break I guess haha. Thanks so much though!

Nothing wrong with taking a small break, it often does a lot of good.

Keep up the good work!

Dr. Romano