Question

Phosphoric acid is a common ingredient in traditional cola drinks. Assuming that the concentration of phosphoric acid is 7.00 x 10^-3 M, calculate the pH in this solution. (Given: H3PO4(aq) + H2O(l) ⇌ H3O+(aq) + H2PO4-(aq), Ka1 = 7.5 x 10^-3)

Answer 1

The pH of a 7.00 x 10^-3 M phosphoric acid solution is calculated using the provided Ka value and the formula pH = -log[H3O+], resulting in a pH of approximately 2.15.

Step-by-step explanation:

Calculating the pH of a Phosphoric Acid Solution

To calculate the pH of a phosphoric acid solution with a concentration of 7.00 x 10^-3 M, we use the provided dissociation constant (Ka1) for phosphoric acid, which is 7.5 x 10^-3. Since the acid is weak and does not dissociate completely, we can assume that the concentration of hydronium ions (H3O+) will be close to the initial concentration of the acid because the Ka is similar in magnitude to the acid concentration. Therefore, the [H3O+] can be approximated as 7.00 x 10^-3 M.

The pH is then calculated using the formula:
pH = -log[H3O+]

Substituting the hydronium ion concentration:

pH = -log[7.00 x 10^-3] ≈ 2.15

This means the pH of the phosphoric acid solution in cola drinks is approximately 2.15, indicating it is quite acidic.