## Dumb_as_a_Duck 3 years ago Why does dry hydrogen gas have no effect on litmus? Is it because it's neutral?

1. Xishem

Hydrogen gases aren't acidic when they are dry, as there are no free hydrogen ions. Once a hydrogen gas is dissociated into water, it becomes acidic because the hydrogen ions are now present in the solution.

2. Dumb_as_a_Duck

OH. So hydrogen gas is neutral?

3. Dumb_as_a_Duck

i mean, as a gas, if one ignores the dissolving in water and ionising bit.

4. Xishem

Yes, but understand WHY it's neutral. Acidity is determined by the concentration of hydrogen ions in a solution. In the case of a hydrogen gas, the hydrogen atoms are bonded and therefore not ions in solution.

5. Dumb_as_a_Duck

Ah. Goddit. Thanks! :)

6. Xishem

In other words, something can't have a pH unless it's dissolved in aqueous solution.

7. Dumb_as_a_Duck

Yea, some gases are said to be acidic or basic. Like ammonia is said to be basic and sulphuric oxide is said to be acidic.

8. Dumb_as_a_Duck

but some *

9. Xishem

It can be acidic or basic, but it doesn't have a pH. The definition of pH is "A measure of the acidity or basicity of an aqueous solution."

10. Dumb_as_a_Duck

OH. Ok then.

11. Xishem

You can identify the acidity or basicity of a substance by looking at the acid dissociation constant or I think what is called "base association constant". They are both types of equilibrium constants.

12. Dumb_as_a_Duck

Where do you find such constants?

13. Xishem

If you know the concentrations of substances in an equilibrium reaction, more specifically the dissociation of a substance...$HA ⇌ A^- + H^+$Where HA is some acid, A is the conjugate base of the acid, and H is a hydrogen ion. You can use...$K_a=\frac{[A^-][H^+]}{[HA]}$Where the concentrations are of the concentrations of the substances at equilibrium.