Negative exponents How do I calculate -log(4.27X10 to the power -90)?

If by power, you mean exponents, there are several rules.

Multiplication with the same base - add the exponents
Division with the same base - subtract the exponents
Exponent to an exponent - multiply the exponents
Negative exponent - 1 over the same number to the positive exponent
Zero exponent - anything to the exponent 0 is 1

Good luck,

Paul

I don't have this calculator, but let's give this a shot and see if it gives us the correct answer.

Let's try 10 to the exponent -2. The rule for negative exponents is to put 1 on the top and the term with a positive exponent on the bottom. In this case, it would be 1 on the top and 10 to the exponent 2 on the bottom, or 1 / 100 or 0.01.

In the calculator, enter 10, hit the x to the power of y key, left bracket (, the negative key (-), 2, the right bracket ), and finally the = key. You might not have to enter the left and right brackets, but I like to enter them to understand what I enter in brackets is the exponent.

Let me know if it works and if you have any other questions.

Good luck.

Paul

For integer powers of 10 you can use the EE shortcut. [2nd][,] displays E.
For non-integer values of the exponent (such as -5.34) you must use the key sequence [2nd][LOG] (10^x) then enter your negative non-integer exponent.

To enter an exponent, positive or negative, use the EE (Enter Exponent) key (the shifted function of the , key just above the 7 key). Do 5 2ND [EE] (-) 1 0 / 5 2ND [EE] (-) 1 0 ENTER and you'll see 1.

What I think you're doing (I can't be sure without seeing your keystrokes) is calculating
5*10^-10/5*10^-10 which is interpreted as (5*10^-10/5)*10^-10, which indeed is 10^-20.

You're not raising a negative number to an exponent, you're negating a number raised to an exponent.

If you want to raise -2 to the second power, you need to enter it as
( (-) 2 ) ^ 2

Without the parens, what you're getting is the equivalent of -(2^x)

Use the log function (common logarithm). See screen capture for markings on key.

To evaluate the negative of a logarithm
Enter the number (argument) of the logarithm
Press the = key to have it calculated.
While result is still displayed, press the sign change key or (+/-).

Beware that -log(number) is not necessarily a negative value.

Another way to calculate -log is to use the relation
log(1/a)=-log(a) .

Your example

3 [EXP] [(] [+/-] 2 [) ] [LOG][=] [+/-]

[EXP] is the key to enter exponent of a power of 10 :It is a shortcut, so no 10 is entered. Respect the key sequence above.
[ (] left parenthesis to prevent ambiguity due to the negaticve exponent. Just a security.
[)] is the enclosing right parenthesis
[+/-] is the Sign Change key

You need the [EE] key to enter powers of 10 exponents.
You need the change sign key [+/-] key.
You need the [LOG] key.

pKa: 6.5 [EE] [+/-] 5 [LOG] [+/-]
There are two change sign: The first (either before or after the exponent 5), the last is entered after you press the log key , so as to negate (make negative if result is positive, and make positive is result is negative) the value.
The result should be 4.1870866 or 4.19

You compute the negative of a logarithm the same way you compute the negative of anything else. However, do you really mean the antilog (inverse logarithm)? Use 2ND [e^x] for the natural antilog, 2ND [10^x] for the common antilog.

For negative exponents, as well as negative anything, use the (-) key next to the decimal point. For your example, 1 EE (-) 6

Uh yes it will allow you to enter a negative exponent.... press the numbers you want then hit EXP button, then hit the +/- button and then the exponent. 60 % of the time, it works...... everytime.

Hello,
Use the small (-) sign sometimes labelled [(-)] or [+/-] and put the whole exponent netween parentheses 18 [y to the x] [(] [(-)] 3 [)] . On some calculators you enter the small - signe after the value of exponent,
18 [y t x] 3 (-) [Enter] should give you 1.7146E-4
In summary the small minus sign; the whole negative exponent between parentheses.
Hope it helps.