Question
Sat December 08, 2012 By:

how to use log table to find log and anti-log to find pH and concentration???

Expert Reply
Tue December 11, 2012

The logarithm of a number has two parts the first of which is the exponent of 10, called the characteristic, and the second part is a decimal called the mantissa which is read from the log tables. The characteristic can be positive or negative. The negative characteristic is written with a bar above the number. The mantissa is always a positive decimal less than 1.

How to find log:

To look up a logarithm of say, 15.27, you would First work out the characteristic, in this case it is 1 Run your index finger down the left-hand column until it reaches 15 Now move it right until it is on column 2 (it should be over 1818) Using another finger, find the difference on column 7 of the differences (20) Add the difference.

So the logarithm of 15.27 is 1.1818 + 0.0020 = 1.1838

Antilog of x is equal to 10x

For example find the antilog of 2.6992

The number before the decimal point is 2, so the decimal point will be after the first 3 digits.

From the antilog table, read off the row for .69 and column of 9; the number given in the table is 5000. The mean difference in the same row and under the column 2 is 2. To get the inverse of mantissa add 5000 + 2 = 5002.

Now place a decimal point after the first 3 digits and you get the number 500.2

Thus antilog 2.6992 = 500.2
 

Calculation of concentration from pH:

The pH is equal to the base 10 logarithm of the H+ concentration, multiplied by -1. If you know the pH of a solution, you can use this formula in reverse to calculate the H+ concentration in that solution.

For ex: calculate [H+] of a solution with pH= 4.30

pH = - log [H+]

4.30 = - log [H+]

- 4.30 = log [H+]

Divide -4.30 into 2 parts so that the first part contains the decimal places and second part the whole number.

-4.30 = 0.70 - 5 = log [H+]

Find the antilog:

Antilog of 0.70 = 5.0

Antilog of -5 = 10-5

Now, multiply both the antilog to get [H+]:

5.0 x 10-5 = [H+]

So, [H+] = 5.0 x 10-5 M

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