To calculate the pKa of an acid, you can use the dissociation constant (Ka) by applying the formula:

pKa = -log₁₀(Ka)

For example, if you have a dissociation constant of 1.75 × 10^-5, you would input this value into your calculator to find the pKa. Start by entering the number:

1.75 × 10^-5

Then, take the negative logarithm of this value. On most calculators, you can do this by using the log function and ensuring to include the negative sign. The calculation would look like this:

pKa = -log₁₀(1.75 × 10^-5)

After performing the calculation, you would find that the pKa is approximately 4.75. This indicates that the acid in question is relatively strong, as a pKa value below 16 suggests a good acid. In this case, the acid is identified as a carboxylic acid, which are known for their acidity.

Understanding the relationship between pKa and acid strength is crucial in organic chemistry, as it helps in predicting the behavior of acids in various chemical reactions.