During the electrolysis of water, hydrogen and oxygen are released in which ratio?

Question

During the electrolysis of water, hydrogen and oxygen are released in which ratio?

1. A. 4:1
2. B. 1:1
3. C. 3:1
4. D. 2:1 (Correct answer)

Correct Answer

Option D — 2:1

Detailed Solution & Explanation

The correct answer is 2:1.

Key Points

• Electrolysis of water splits it as $2H_2O \rightarrow 2H_2 + O_2$, so two volumes of hydrogen are released for one volume of oxygen — a ratio of 2 : 1.
• The ratio comes straight from the formula of water itself: each molecule contains two hydrogen atoms to one oxygen, and the gases are collected in the same proportion.
• Hydrogen is liberated at the cathode (negative electrode) and oxygen at the anode (positive electrode).

Additional Information

• This is a demonstration of Avogadro's law — equal volumes of gases at the same temperature and pressure contain equal numbers of molecules — which is why the *volume* ratio equals the *molar* ratio.
• By mass the ratio is inverted and quite different: 1 : 8, since oxygen's atomic mass is 16 against hydrogen's 1.
• Pure water conducts poorly, so a few drops of dilute sulphuric acid are added to supply mobile ions; the acid is not consumed.
• The classic apparatus is the Hofmann voltameter, where the two graduated tubes visibly fill in a 2 : 1 ratio — the experiment this question is drawn from.