[Indranil (OP)]

''1 volume of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''

Now my questions are as follows:
1. How many hydrogen molecules are in 2 volume of hydrogen?
2. How many oxygen molecules are in 1 volume of oxygen?
3. How many water vapor molecules are in 2 volumes of water vapor?
please explain. I am confused.

[evan_au]

The question talks about "volumes" of a gas, eg liters, cubic meters or footballs.
- They don't need to specify how much volume is present because gases combine in certain ratios of volume, and as long as the temperature and pressure is the same when you take the measurements, you will get the same ratios.
- This rule of ratios was discovered before scientists discovered the nature of atoms and molecules
- But chemists like to talk about a particular volume called a molar volume of a gas, because you can tell how much mass is present by just looking at a periodic table.
- A molar volume occupies 22.4 liters at 25C and normal atmospheric pressure, independent of what type of gas is present, and has the same number of molecules = 6.022 × 10²³ (Avogadros's constant, abbreviated N_A).
See: https://en.wikipedia.org/wiki/Molar_volume
https://en.wikipedia.org/wiki/Avogadro_constant

So a "mole" of oxygen will occupy 22.4 liters (at 25C) and contains 6.022 × 10²³ molecules of O₂. If you look at a periodic table, you will see that the atomic mass is 16 (or 15.9994 if you want to be really precise). This mole of oxygen will have a mass of 2x 15.9994 = 31.988 grams.
- And a mole of hydrogen will occupy 22.4 liters (at 25C) and contains 6.022 × 10²³ molecules of H₂. If you look at a periodic table, you will see that the atomic mass is 1 (or 1.00794 if you want to be really pedantic). This mole of hydrogen will have a mass of 2x 1.00794= 2.01588 grams.

Now, to address your questions:

1. How many hydrogen molecules are in 2 volume of hydrogen?
2. How many oxygen molecules are in 1 volume of oxygen?
3. How many water vapor molecules are in 2 volumes of water vapor?
please explain. I am confused.

Since we don't know (and don't really care) how big this "volume" is, we can't say how many hydrogen and oxygen molecules are present. But we can say:
- There is the same number of molecules in 1 volume of hydrogen as there is in 1 volume of oxygen
- There are twice as many molecules in 2 volumes as there are in 1 volume
- Assuming that they are all gases, and all measured at the same temperature and pressure

''1 volume of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''

I'm afraid that this doesn't quite stack up.

Now that we do know about atoms and molecules, we can write an equation like:
2H₂ + O₂ → 2H₂O
Which can be read as ''2 volumes of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''.

This is a little tricky, because:
- If you conduct this reaction, it releases a lot of heat, and the temperature is far higher than 25C. So rather than causing a net reduction in volume, it causes a massive increase in volume, quite possibly causing glass to shatter and injuring the person conducting the experiment.
- You will know that at a temperature of 25C, water is not a gas, but is a liquid. So if you collect the hot gases and cool them down, it turns into a liquid which takes up far less space than 2 volumes of gas.
- This is why they have to specify water vapor. But to measure the volume of water vapor at 25C you have to do tricks like measure the volume at increased temperature and/or reduced pressure, and extrapolate to 25C and atmospheric pressure.

Warning: Do not try this experiment at home until you have done it safely, when supervised in a science class. And do wear safety goggles!

[Indranil (OP)]

The question talks about "volumes" of a gas, eg liters, cubic meters or footballs.
- They don't need to specify how much volume is present because gases combine in certain ratios of volume, and as long as the temperature and pressure is the same when you take the measurements, you will get the same ratios.
- This rule of ratios was discovered before scientists discovered the nature of atoms and molecules
- But chemists like to talk about a particular volume called a molar volume of a gas, because you can tell how much mass is present by just looking at a periodic table.
- A molar volume occupies 22.4 liters at 25C and normal atmospheric pressure, independent of what type of gas is present, and has the same number of molecules = 6.022 × 10²³ (Avogadros's constant, abbreviated N_A).
See: https://en.wikipedia.org/wiki/Molar_volume
https://en.wikipedia.org/wiki/Avogadro_constant

So a "mole" of oxygen will occupy 22.4 liters (at 25C) and contains 6.022 × 10²³ molecules of O₂. If you look at a periodic table, you will see that the atomic mass is 16 (or 15.9994 if you want to be really precise). This mole of oxygen will have a mass of 2x 15.9994 = 31.988 grams.
- And a mole of hydrogen will occupy 22.4 liters (at 25C) and contains 6.022 × 10²³ molecules of H₂. If you look at a periodic table, you will see that the atomic mass is 1 (or 1.00794 if you want to be really pedantic). This mole of hydrogen will have a mass of 2x 1.00794= 2.01588 grams.

Now, to address your questions:

1. How many hydrogen molecules are in 2 volume of hydrogen?
2. How many oxygen molecules are in 1 volume of oxygen?
3. How many water vapor molecules are in 2 volumes of water vapor?
please explain. I am confused.

Since we don't know (and don't really care) how big this "volume" is, we can't say how many hydrogen and oxygen molecules are present. But we can say:
- There is the same number of molecules in 1 volume of hydrogen as there is in 1 volume of oxygen
- There are twice as many molecules in 2 volumes as there are in 1 volume
- Assuming that they are all gases, and all measured at the same temperature and pressure

''1 volume of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''

I'm afraid that this doesn't quite stack up.

Now that we do know about atoms and molecules, we can write an equation like:
2H₂ + O₂ = 2H₂O
Which can be read as ''2 volumes of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''.

This is a little tricky, because:
- If you conduct this reaction, it releases a lot of heat, and the temperature is far higher than 25C. So rather than causing a net reduction in volume, it causes a massive increase in volume, quite possibly causing glass to shatter and injuring the person conducting the experiment.
- You will know that at a temperature of 25C, water is not a gas, but is a liquid. So if you collect the hot gases and cool them down, it turns into a liquid which takes up far less space than 2 volumes of gas.
- This is why they have to specify water vapor. But to measure the volume of water vapor at 25C you have to do tricks like measure the volume at increased temperature and/or reduced pressure, and extrapolate to 25C and atmospheric pressure.

Warning: Do not try this experiment at home until you have done it safely, when supervised in a science class. And do wear safety goggles!

Thank you for your kind effort. I got it. But I want to clear one concept here.
2 volumes of hydrogen vapor = 2 X 22.4 lit of hydrogen vapor = 2 X 6.023X10^23 hydrogen vapor molecules? Could you tell me I am right or wrong and please point out my mistakes where I am wrong.

[Bored chemist]

"Does 1 volume of hydrogen + 1 volume of oxygen make 2 volumes of water vapour?"
No.
It would make 1 volume of water and half a volume of  leftover oxygen.