## Dumb_as_a_Duck Why does dry hydrogen gas have no effect on litmus? Is it because it's neutral? 2 years ago 2 years ago

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.