## Calculating pH and molarity

**O F A C I D S A N D B A S E S**

### CHEMath

Logarithms: the common logarithm of a number is the exponent to which ten must be raised to produce the given number: Log x = y, if x = 10 y .

So…. Log 0.01 = -2; because 0.01 = 10 -2

And log 10,000 = 4; because 10,000 = 10 4

### Antilog

The antilog of a number x is the number y whose logarithm is x. Because log y = x means y=10 x , the antilog of x is the same as 10 x

In calculator….2

nd function log (pH value) enter

Write your answer in scientific notation

Use this to calculate molarity when given pH

Example: pH = 7.0

Antilog (7.0) = 10,000,000

Molarity) = 1.0 x 10 7

### Calculating pH values

To calculate the pH of substance:

pH = -log [H + ]

For H + use the molarity concentration given

When calculating pH of a base, you must FIRST use the constant value of 1 x 10 -14 and divide it by the given molarity of OH – THEN use the log formula above

### Calculating molarity concentration

When given pH and asked to solve for molarity you will likely need to use the logarithm table in your textbook; especially if the pH is a decimal number

Example: pH is 3.70

To do this: FIRST rearrange the given information so that you have a negative pH

-pH = -3.70

### Calculating concentration (2)

Next: add and then subtract the whole number that is closer to and larger than the negative pH value

(that number would be 4)

= (-3.70 + 4) – 4

= 0.30 – 4

= 10 (0.30 -4)

= 10 0.30

x 10 -4

From the log table, find the number with a log of

0.30 (it’s 2.0); the antilog of 0.30 is thus 2.0

### Calculating concentration (3)

From the log table, find the number with a log of

0.30 (it’s 2.0); the antilog of 0.30 is thus 2.0

So……………….. 2.0 x ?

The number with a log of -4 is 10 -4

Therefore, [H+] = 2.0 x 10 -4 M

### Assignment

Complete calculations

SHOW ALL EQUATIONS you set up

Circle your final answers WITH units*

Textbook: page 586 – 589

Problems # 8 - # 15

*Remember concentration has the unit of measure

Molarity (M); pH does not have a unit