TI-nspire calculator. I need to calculate the pH value for a solution with a (H+) of 7.65 X 10-5 The answer is pH = 4.11....but I can't get that answer. when I press the minus before log...it says "ans --" I have worked all day trying to get this right answer. HELP! I am using the pad that has little green buttons.

The calculator does not have a Computer Algebra System or CAS: It cannot handle non numerical quantities. Every letter symbol must have a value stored in it prior to being used. If no value is stored in a letter symbol, value 0 is assigned to it. Sorry.
As to calling Pi a variable, I would not got that far, even if the calculator would give me 2Pi.
TI calculators that have a CAS are the TI92 Plus, the Voyage 200 PLT, and the TI Nspire CAS, TI Nspire CX CAS.

Do you have the concentration? if YES, you CALCULATE the pH with the relation
pH=-log(c) where c is the value of concentration in mol/L.
ex: c= 3.57x10^(-6)
pH: (-) [LOG] 3.57 [2nd][EE] [(-)] 6 [ ) ]
Here you have 2 change sign key press [(-)]

If you have the pH, and want to calculate the concentration that yields that value of the pH you proceed as follows
c= 10^(-pH)
Ex pH=8.23
c: [2nd][LOG] [(-) 8.23 [)]

Here is a screen capture to show you both calculations

The TI Nspire has two calculation modes: Auto and Approximate. In Auto mode the calculator will attempt to do exact calculations. If that is not possible then the results are displayed as decimals. The non CAS version does not do symbolic manipulations, only the CAS version can do that.
However if you are actually using the CAS calculator, then go to the settings ( Settings>General ) and change the settings for the calculatoion mode to EXACT. The Non Cas does not have that third choice.

In OS 3.x Press 5:Settings>2;Settings>1:General.



In the screen that opens, use TAB repeatedly until you
reach the line : Calcul Mode. Use right arrow to open
that menu. Select the option APPROXIMATE and press
ENTER. Use the TAB to move the highlight on the OK
button and Press ENTER/OK to save the changes.
All large values will be displayed in Scientific Notation.
>

If you are doing a logistics type of problem, there is a function called LogisticD which will duplicate the processing method of the older calculators. It is a result of the way that the calculators process the problem.

You need the TINspire CAS (Computer Algebra System) to do symbolic manipulations. The Blue TINspire (with TI 84 keyboard) can only perform numerical computations.
If you use variables, the calculator will assign the numerical value stored in the variable.

The problem is simple. You're trying to get an [H+] concentration which is obviously going to have a value of some number times ten raised to a negative power. Therefore, you have to insert the negative value of the pH into the 10^(x). When you insert said negative number you will come out with the right answer.
i.e.
The pH of a sample of human blood was calculated to be 7.41. What is the [H+] concentration of the blood?
10^(-7.41) = [H+]
[H+] = 3.9 E-8  
(the answer should only have two sig. figs because the pH has two digits after the decimal.

I reccently had the same problem, but I have a Ti-84 Plus,and I am disgusted by how poorly this matter is covered by Texas Intruments. Esp. since the solution is rather simple.

The antilog key is [2nd] [log] or the 10^x. On your screen it should apear as 10^( and then you just input the value that you want to find the antilog of.

When used to solve pH problems remember to input the pH value as a negative number, since pH= -log[H+] then [H+]= antilog(-pH).

Hope that helped))))) ^^

You enter a pH value and change the sign to minus, then hit 2nd LOG which is 10^x. The x value is what you're looking for. To put the answer in scientific notation, press 2nd 5 to do so.

Example:

For a pH = 4.1

-4.1 2nd LOG will return 0.0000794

Hiting 2nd 5 will express it as 7.94 -5, giving you the [H+] concentration.

pH is minus (log to base 10) of the hydrogen ion activity of an aqueous solution, or (log to base 10) of (1/hydrogen ion activity)

To get the inverse log, i.e the hydrogen ion activity corresponding to a specified pH, simply enter the pH value and press

2nd
LOG
1/x

Answer 0.001