How can we calculate the HCO3- (bicarbonate ion) concentration in water in 25C pH7.0?

I think we can use PV=nRT equation, but I am not sure.
If pH is changed from 7.5 to 6.5 ( around pka 6.3 of CO2), how can change the HCO3- concentration?

Thanks in advance.

To calculate the bicarbonate ion (HCO3-) concentration in water at 25°C and pH 7.0, you need to consider the bicarbonate equilibrium reaction in water:

H2CO3 ↔ H+ + HCO3-

In this equilibrium, H2CO3 is the carbonic acid, and HCO3- is the bicarbonate ion. The concentration of HCO3- can be calculated using the Henderson-Hasselbalch equation, which relates the pH of a solution to the ratio of the concentrations of the acid and its conjugate base:

pH = pKa + log([A-]/[HA])

In this equation, pKa is the acid dissociation constant of the acid (H2CO3), [A-] is the concentration of the conjugate base (HCO3-), and [HA] is the concentration of the acid (H2CO3).

To calculate the HCO3- concentration, you need to know the pKa value of CO2 (which is around 6.3), the pH of the solution (7.0), and the concentration of dissolved CO2 in water.

Using the Henderson-Hasselbalch equation, you can rewrite it as:

[HCO3-] = 10^(pH - pKa) * [H2CO3]

Now, let's address your second question about the change in HCO3- concentration when the pH is changed from 7.5 to 6.5, close to the pKa of CO2 (6.3).

As the pH decreases from 7.5 to 6.5, the solution becomes more acidic. This means the concentration of H+ ions increases, which will shift the bicarbonate equilibrium reaction to the left, favoring the formation of carbonic acid (H2CO3). Consequently, the concentration of HCO3- will decrease.

In summary, to calculate the HCO3- concentration in water, you can use the Henderson-Hasselbalch equation. When the pH is reduced from a value above the pKa of CO2 to a value close to it, the concentration of HCO3- decreases due to the shift in the bicarbonate equilibrium.