Question

Calculate the pH for the following solutions of weak acids and bases, make sure you know if you are dealing with an acid or a base.

0.051 M H2CO3 (Ka = 4.5 * 10-7) pH

Answer 1

Answer on the question #61140, Chemistry / Other

Question:

Calculate the pH for the following solutions of weak acids and bases, make sure you know if you are dealing with an acid or a base.

0.051 M H₂CO₃ (Ka = 4.5 * 10⁻⁷) pH

Solution:

H₂CO₃ is an acid, it dissociates, producing proton:

$$H_2CO_3 \rightarrow H^+ + HCO_3^-$$

The equilibrium constant of this reaction is:

$$K_a = \frac{[H^+][HCO_3^-]}{[H_2CO_3]},$$

where the values in square brackets are concentrations of particles. As $[H^+]$ is equal to $[HCO_3^-]$, we can write:

$$[H^+]^2 = K_a \cdot [H_2CO_3] = 4.5 \cdot 10^{-7} \cdot 0.051 = 0.23 \cdot 10^{-7}$$

Then, pH of the solution is:

$$pH = -\log ([H^+]) = -0.5 \cdot \log (0.23 \cdot 10^{-7}) = 3.82$$

Answer: pH of the solution is 3.82

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